PLKS 122 EC
BIODIVERSITY
Faculty of Natural and Agricultural Sciences
Study guide compiled by: Prof S Janse van Vuuren & Dr M Struwig
Copyright © 2021 edition. Review date 2021.
North-West University
No part of this study guide may be reproduced in any form or in any way without the written permission of the publishers.
It all starts here
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Ranked in the top 5% of universities globally by the QS-rankings
Contributes the second largest number of graduates annually to the labour market
Dit begin alles hier
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As een van die top 5% universiteite wêreldwyd deur die QS-ranglys aangewys
Lewer jaarliks die tweede meeste graduandi aan die arbeidsmark
Gotlhe go simolola fano
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Re beilwe mo gare ga diyunibesiti tse 5% tse di kwa godimo go ya ka peo ya
maemo ya QS
Ngwaga le ngwaga go abelwa palo ya bobedi ka bogolo ya badiri mo maketeng
ya badiri
MODULE CONTENTS
Module information ......................................................................................................................... iii
A word of welcome.......................................................................................................................... iii
Presumed learning .......................................................................................................................... iii
Contact sessions............................................................................................................................. iii
Study material ................................................................................................................................. iii
How to use this study guide ............................................................................................................ iii
How to study ...................................................................................................................................iv
Evaluation ....................................................................................................................................... v
Absence .......................................................................................................................................... v
Action words ................................................................................................................................... v
Module outcomes.......................................................................................................................... viii
Module plan and working programme ........................................................................................... viii
Icons ...............................................................................................................................................ix
Warning against plagiarism .............................................................................................................ix
Study unit 1
Evolution ....................................................................................................... 1
Study unit 2
Classification................................................................................................. 2
Study section 2.1 Development of binomial nomenclature .......................................................... 3
Study section 2.2 Development of the kingdom concept ............................................................. 4
Study section 2.3 Classification of major groups ......................................................................... 5
Study section 2.4 The future of plant classification ...................................................................... 6
Study unit 3
Prokaryotes ................................................................................................... 7
Study section 3.1 Classification of prokaryotic organisms ........................................................... 9
Study section 3.2 The characteristics of cyanobacteria ............................................................. 10
Study section 3.3 The structure of cyanobacteria ...................................................................... 11
Study section 3.4 Reproduction of cyanobacteria ...................................................................... 13
Study section 3.5 The biological importance of cyanobacteria................................................... 16
Study section 3.6 Practical: Representative cyanobacteria genera .......................................... 17
Study unit 4
Algae ............................................................................................................ 22
Study section 4.1 The evolution of algae ................................................................................... 24
Study section 4.2 The structure of flagella in eukaryotic cells .................................................... 31
Study section 4.3 Phylum Euglenophyta (euglenoids) ............................................................... 34
Study section 4.4 Phylum Dinophyta (dinoflagellates) ............................................................... 41
Study section 4.5 Phylum Bacillariophyta (diatoms) .................................................................. 49
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Study section 4.6 Phylum Chlorophyta (green algae) ................................................................ 60
Study section 4.7 Other phyla of algae ...................................................................................... 85
Study section 4.8 The biological importance of algae ................................................................ 89
Study unit 5
Mosses......................................................................................................... 92
Study section 5.1 An introduction to the Bryophytes (mosses) .................................................. 94
Study section 5.2 The classification of mosses ......................................................................... 97
Study unit 6
Seedless vascular plants.......................................................................... 112
Study section 6.1 An introduction to the seedless vascular plants ........................................... 113
Study section 6.2 Types of living seedless vascular plants...................................................... 114
Study unit 7
Gymnosperms ........................................................................................... 128
Study section 7.1 An introduction to the gymnosperms ........................................................... 129
Study section 7.2 Types of living gymnosperms ...................................................................... 131
Study unit 8
Angiosperms ............................................................................................. 153
Study section 8.1 Introduction to the Angiosperms .................................................................. 154
Study unit 9
Fungi .......................................................................................................... 160
Study section 9.1 An introduction to Fungi .............................................................................. 161
Study section 9.2 Zygomycetes............................................................................................... 164
Study section 9.3 Basidiomycetes ........................................................................................... 167
Study section 9.4 Lichens ....................................................................................................... 170
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Module information
Module code
PLKS 122
Module credits
12
Module name
Biodiversity
Name of
lecturer(s)
Prof S Janse van Vuuren
Dr Madeleen Struwig
Office telephone
(018) 299 2514
(018) 389 2288
Email address
sanet.jansevanvuuren@nwu.ac.za
madeleen.struwig@nwu.ac.za
Building and
Office nr
Room 134, JS van der Merwe
building (Potchefstroom campus)
Office G10, New Science Building
(A12) (Mafikeng campus)
Consulting
hours
A word of welcome
The section Botany would like to welcome you to the second semester. We hope that
you will find your studies stimulating, rewarding and pleasant. We would try our best to
give you the best academic and scientific education possible in Botany.
The aim of this module is to give you an overview of the biodiversity that we find in the
plant kingdom as well as organisms from other kingdoms of life.
Presumed learning
See admission to the Faculty of Natural and Agricultural Science, North-West
University (2020 yearbook).
Contact sessions
According to the class timetable
Study material
Study guide for PLKS122
How to use this study guide
As indicated by the name, the purpose of this study guide is to guide you through the
work. In some study units you will find assignments that you must do. Ensure therefore
that you always have the study guide at hand and always bring it with you to the
contact sessions.
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How to study
In this module, it is expected from you to study the material in the study guide. When it
is expected from you to consult additional books, it will be specified as such.
The contents of this module deal with the diversity of life. The study guide is divided
into a number of study units with outcomes. Each study unit deals with a specific
subject and you must make sure that you’ve reached the study unit outcomes at the
end of each study unit and that you can answer the self-evaluation questions.
The information in this study guide will support and help you to study the contents. It is
expected from you to rely very strongly on the study guide when you study the
individual study units.
During the contact sessions, the theory and practical will be presented (sometimes in
an integrated manner). A number of hours are reserved for each study unit. The first
few of these in each study unit will be used for theory to give you the necessary
background information required and to tell you exactly what will be expected from you.
The rest of the time allocated to a specific study unit will be used to study the material
yourself (and to make fully labelled drawings of the material). When the time allocated
per study unit has expired, you will have to study the rest of the material in your own
time. Thus, it can be expected from you to do some self study. The theory and
practical work cannot be separated from one another – they form an inseparable unit.
The importance of drawings and figures cannot be overemphasised, since most of the
contents of this module can be explained better with the aid of representations such as
drawings, diagrams or schemes. A fully labelled drawing to illustrate the relevant
structures is sometimes worth more than pages full of memorised facts. It is also
important during preparation for the examination.
A few important aspects that should be kept in mind when you make drawings are the
following:
(1)
Plan the placement of your drawing in your practical workbook.
(2)
Use a sharp drawing pencil (HB or B).
(3)
Keep both eyes open when you do microscope work. In the beginning, it may be
difficult, but with some exercise it will be possible to see the object through the
microscope and to draw it at the same time.
(4)
Each drawing should have a complete, explanatory caption.
(5)
Each cell has a specific form and its own cell wall. Cells are attached to other cells in
a specific way and have specific borderlines. Indicate these carefully.
(6)
Make sure that you draw different cells or structures in the correct proportion to one
another.
(7)
Draw large and neat, but only draw what you see through the microscope or on the
material.
It is extremely important that self-study assignments must not be neglected, because it
forms an integral part of the module and it will be examinable.
It is very important to study the material in such a way that you will be able, after
completion of the module, to integrate all the different components and to relate them to
one another.
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Evaluation
Participation mark:
Theory:
Semester test
Class tests
Other assessments (small tests, bookmarks, reports, etc.)
40%
10%
20%
Practical:
Practical exam
30%
Module mark:
Participation mark
Examination mark
(100)
(100)
In order to gain admission to the exam, the student must:
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have a participation mark of 40% or more; and
not miss 25% or more of the theoretical or practical lectures without acceptable
reasons.
The participation and examination marks are equal – in order to pass the module, a student
must have an average of 50%.
A student passes the module with distinction if an average (participation mark and
examination mark) of 75% and higher is obtained.
Absence
If you are absent due to illness (or another valid reason) and you missed a test or due date
for an assignment, you must provide the lecturer with a medical certificate (or other valid
documentation) as soon as possible. Medical certificates and other valid documentation
must be submitted to the lecturer within 10 days after the date when the student was
absent. If you neglect to do this in time, you will receive zero marks for the test(s) or
assignment(s).
Action words
Questions in outcomes, exercises, assignments, tests and examinations are put in a
specific way. The following verbs (with their meanings) may be used:
Compare
Give the similarities and differences between structures. This is usually done in a table.
Define
Give the essential characteristics of the term.
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Describe
To give the characteristics or properties of something in a logical, well-structured
manner.
Discuss
Write in detail about the subject in the question.
Distinguish
Discern/show the differences between two structures, terms etc.
Explain
Make something clear or known in detail by using a description, example or illustrations
Give labels, functions, genus names, descriptions, differences, reasons, one or two
words, or make labelled drawings
Write down the information that is required.
Make a line diagram or line drawing
Use a sharp pencil. If there is lots of detail in an object, it can become tedious to draw
every single cell or organelle. Instead, a line drawing consists mainly of lines. Only a
few cells of each kind can be drawn to represent all the other cells. Cells and objects
must be drawn in the correct relation (size) to each other. A representative part of the
total object may be drawn. An example of a line drawing is given below:
Make a schematic representation
Give a scheme (words and lines/arrows) of the different steps or phases in the life cycle
e.g. the scheme at the end of page 93.
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Make a diagrammatic representation
Give a scheme of the different steps or phases in the life cycle and illustrate the
processes by means of drawings.
Identify
Establish who and what the given/shown structure or organism is. This means that you
must write down the name of the structure, e.g. flagellum, strobilus. When it is required
to identify an organism you must give the scientific name of the organism, e.g. Volvox,
Pinus.
Indicate
Give the answer in only a few words, e.g. indicate whether rhizoids of the Bryophyta is
haploid or diploid. Answer: haploid.
List
Provide the required information in the form of a list.
Make a drawing
Make a drawing, with labels, that shows the different parts of the organism/object.
Name/List
Give the information point by point and list the most important points. No discussion is
needed.
Study the line diagram, photograph, diagram
Spend some time looking at the diagram or photograph to familiarize yourself with what
is presented. This statement is always followed by questions that relate to the diagram
or photograph.
Tabulate/in table form
Compare in a table. Make sure that similar characteristics are compared DIRECTLY
OPPOSITE each other, e.g.:
Dinophyta
Cell covering
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Bacillariophyta
Amphiesmal vesicles with Cell wall impregnated by
or without cellulose
silica dioxide, known as a
frustule
Lecturers will also provide further elucidation on these (and other relevant terms) in the
class where necessary. However, for your own purposes, it is important to know in
advance what is expected of you if such an action verb appears in a paper.
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Module outcomes
After completion of the PLKS122 module, the student should demonstrate:
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Knowledge and informed understanding of key terms, principles, concepts, facts,
rules and theories used in the field of Biodiversity;
Basic knowledge and informed understanding of taxonomic principals, including
evolution and classification, in order to distinguish and compare different
prokaryotic organisms, algae, fungi and plants;
The ability to apply standard methods, procedures and techniques commonly
used in botanical studies to study characteristics of living organisms in order to
identify, name, illustrate and classify them;
The ability to communicate verbally and in writing and gather information reliably
and accurately from a range of sources in order to do elemental research in a
scientific way by using conventional methods and basic technologies;
The ability to monitor his/her own learning progress individually and in groups,
and implement relevant learning strategies in Biodiversity;
Ethical and professional behaviour within the academic environment, inclusive of
adherence to rules on plagiarism and copyright principles.
Module plan and working programme
A work program will be uploaded on efundi before the lectures commence
Study unit (SU)
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Time allocation
(Total number of contact
sessions –theory + practicals)
SU 1.
Evolution
±1
SU 2.
Classification
±1
SU 3.
Prokaryotes
±2
SU 4
Algae
± 14
SU 5.
Mosses
±6
SU 6.
Seedless vascular plants
±6
SU 7.
Gymnosperms
±6
SU 8.
Angiosperms
±2
SU 9.
Fungi
±4
Icons
Time allocation
Learning outcomes
Study material
Assessment /
Assignments
Individual exercise
Group Activity
Example
Reflection
Warning against plagiarism
ASSIGNMENTS ARE INDIVIDUAL TASKS AND NOT GROUP ACTIVITIES.
(UNLESS EXPLICITLY INDICATED AS GROUP ACTIVITIES)
Copying of text from other learners or from other sources (for instance the study guide,
prescribed material or directly from the internet) is not allowed – only brief quotations
are allowed and then only if indicated as such.
You should reformulate existing text and use your own words to explain what you
have read. It is not acceptable to retype existing text and just acknowledge the source
in a footnote – you should be able to relate the idea or concept, without repeating the
original author to the letter.
The aim of the assignments is not the reproduction of existing material, but to ascertain
whether you have the ability to integrate existing texts, add your own interpretation
and/or critique of the texts and offer a creative solution to existing problems.
Be warned: students who submit copied text will obtain a mark of zero for the
assignment and disciplinary steps may be taken by the Faculty and/or
University. It is also unacceptable to do somebody else’s work, to lend your work
to them or to make your work available to them to copy – be careful and do not
make your work available to anyone!
Plagiarism is a serious offence and you should familiarise yourself with the
plagiarism policy of the NWU. http://library.nwu.ac.za/copyright-and-plagiarism
Please refer to the Policy on Academic Integrity which is found on the following
website: http://www.nwu.ac.za/sites/www.nwu.ac.za/files/files/i-governancemanagement/policy/2P-2.4.3.2_Academic%20integrity_e.pdf
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Study unit 1
Study unit 1
EVOLUTION
Study hours
This study unit is only introductory and you will be able to complete it within 3 hours.
Learning outcomes
Once you have completed the study unit, you should be able to:
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Know that evolution is changes over long periods of time;
Know who proposed the theory of evolution of species through natural selection;
Understand the concept of natural selection;
Describe the meaning of the ‟Origin of Species”;
Describe the roles of each of the following on evolution:

extinction
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catastrophic events
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continental drift

habitat destruction;
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Understand that evolution explains both the unity of life, by studying common
ancestors, and the diversity of life, by considering adaptive change.
Study material
Use your study guide in conjunction with the PowerPoint slides uploaded on eFundi.
This will give you an overview of evolution that would be enough to serve as
background information.
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Study unit 2
Study unit 2
CLASSIFICATION
Study hours
This study unit is only introductory and you will be able to complete it within 3 hours.
Learning outcomes
On completion of this study unit, you should have mastered the outcomes listed
under each study section.
Study material
Use your study guide in conjunction with the PowerPoint slides uploaded on eFundi.
This will give you an overview of classification that would be enough to serve as
background information.
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Study unit 2
Study section 2.1
Development of binomial nomenclature
Study outcomes
Once you have completed this study section, you should be able to write notes on:
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The ‟Doctrines of the Signatures”;
Binomial nomenclature;
The role that Linnaeus played in the development of binomial nomenclature;
The correct way to write scientific names, according to the binomial
nomenclature system;
The hierarchical classification system and the major categories (taxa) of
classification (in sequence).
Study material
Use your study guide in conjunction with the PowerPoint slides on eFundi.
Individual activity
Write notes on Linnaeus and the development of binomial nomenclature. Explain
exactly what is meant by binomial nomenclature.
Feedback
The scientific names of species consist of two parts (binomial). The first word is the genus
name and always starts with a capital letter. The second word is the specific epithet and
always starts with a lower-case letter. Both words are either italicised or, if written by hand,
underlined. For example: in Lobostemon fruticosus, Lobostemon is the name of the genus.
The specific epithet is fruticosus. The species name is a combination of the genus name
and the specific epithet e.g. Lobostemon fruticosus.
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Study unit 2
Study section 2.2
Development of the kingdom concept
This study section is very important (also for exam purposes).
Study outcomes
Once you have completed this study section, you should be able to write notes on:
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The history of how development took place from a 2 Kingdom system to a 6
Kingdom system;
The type of organisms placed in each kingdom, how they differ from each other,
and how they were shuffled during the establishment of new kingdoms;
Problems encountered with the 5 Kingdom system;
The current classification system.
Study material
Notes given in the following paragraph and discussions in class serve as study material.
In the beginning, there were only 2 Kingdoms, namely Plantae (photosynthetic green
plants) and Animalia (animals). After this, a 3 Kingdom system was developed,
consisting of Plantae, Animalia and Protoctist (all organisms with characteristics not
satisfying those set for plants and animals. This name was later shortened to Protista.
After this, the 4 Kingdom system was developed, consisting of Plantae, Animalia,
Protoctista and Monera (prokaryotic organisms that were removed from the kingdom
Protoctista). Then came the 5 Kingdom system, namely Plantae, Animalia, Protoctista,
Monera and Fungi. In this system, there were three kingdoms clearly different from the
rest regarding their nutrition methods – photosynthesis (Plantae), ingestion (Animalia)
and absorption (Fungi) as well as 2 Kingdoms that were distinguished on account of
differences between their cellular structure (prokaryotic Monera and eukaryotic Protista).
The 5 kingdom system was accepted for a long period of time until the 1980s. It was
then proposed that the Monera must be divided into two and this was accepted in 1990.
Currently, a 6 Kingdom system is used, that is placed under three higher levels
(domains). Domain Archaea (Kingdom Archaea) and Domain Bacteria (Kingdom
Bacteria) consist of prokaryotic organisms. They are distinguished from another on
account of differences in RNA and cell wall composition. The other 4 Kingdoms (Protista,
Fungi, Plantae and Animalia) fall under the Domain Eukarya.
Individual activity
Self-evaluation:
Explain how classification progressed from a 2 Kingdom system to the system in
which organisms are currently classified.
Feedback:
The answer can be found in the paragraph above in conjunction with information given
in the class.
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Study unit 2
Study section 2.3
Classification of major groups
This study section is very important (also for exam purposes).
Study outcomes
After you have completed this study section, you must be able to write notes on:
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The major groups (taxa) in the hierarchical system of classification that are
currently found under the Domain level;
The erection of superkingdoms (domains) and where they fit into the
classification system;
The types of organisms – with their characteristics – that belong to each domain
and kingdom.
Study material
Current hierarchy: Domain – Kingdom – Division (Phylum) – Class – Order – Family
– Genus – Species.
Self-evaluation
Explain how classification developed from a 2 Kingdom system to how organisms are
currently classified. Also name the different types of organisms, with their
distinguishing, characteristics, found in each domain and kingdom.
Feedback
The answer will be discussed during lectures in class.
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Study unit 2
Study section 2.4
The future of plant classification
Study outcomes
After you have completed this study section, you must be able to write notes on:
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The role that DNA analyses will have on the future of plant classification.
Study material
Notes in class.
Self-evaluation:
Explain how new techniques and molecular analysis can influence classification
systems used up to now.
Feedback:
The answer to this question will be discussed in class.
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Study unit 3
Study unit 3
PROKARYOTES
Study hours
You should spend at least 6 hours on this study unit.
Study material
Most of the learning content of this study unit is contained in this study guide.
Study outcomes
Once you have completed the study unit, you should be able to
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write notes on the classification of prokaryotic organisms;
describe the characteristics of cyanobacteria (with regard to pigments, storage
products, motility/locomotion and cell covering);
discuss the internal structure of a cyanobacterium cell and make a fully annotated
drawing of it;
name the functions of different cytoplasmic structures;
describe different types of reproduction found in cyanobacteria;
give an overview of the biological importance of cyanobacteria;
describe the environmental impacts often associated with cyanobacteria; and
identify, classify and make fully labelled drawings of microscope slides, as well as
live material, of representative cyanobacterial genera.
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Study unit 3
Introduction
Prokaryotic organisms dominate the biosphere. Their joint biological mass (biomass)
exceeds that of all the eukaryotic organisms together more than tenfold. More prokaryotes
live in a handful of fertile soil or in the mouth or on the skin of a human than the total
number of people who have ever lived! Prokaryotes are found everywhere where life is
possible – they thrive in habitats which are far too cold, warm, salty, acidic or alkaline for
eukaryotic organisms. In 1999, biologists even discovered prokaryotes growing on the walls
of a gold mine two miles below the earth’s surface. Therefore, although individual
prokaryotes are very small organisms, their impact on the earth and on life on earth is
phenomenal.
Prokaryotes often live in close association (symbiosis) with other prokaryotes as well as
eukaryotic organisms. According to the (endo)symbiosis theory, mitochondria and
chloroplasts originated from prokaryotes that were housed in larger host cells.
Modern prokaryotes have diverse structures and metabolisms. Approximately 4,000
species of prokaryotes are currently known.
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Study unit 3
Study section 3.1
Classification of prokaryotic organisms
In the traditional five-kingdom system, prokaryotes were classified under the kingdom
Monera (the four eukaryotic kingdoms being Protista, Plantae, Fungi and Animalia).
However, during the past two decades, systematists realised that a single kingdom that
includes all prokaryotes is not sufficient. By analysing ribosomal RNA and the genome
sequences of various species, two main branches were identified within prokaryotic
evolution. The common names of these two groups are bacteria and archaea. Species
belonging to the archaea group usually occur in extreme environmental conditions,
such as geothermal (hot) springs and saltwater. Very few organisms can withstand
such extreme conditions.
Subsequently, a six-kingdom taxonomy, consisting of two prokaryotic kingdoms and
four eukaryotic kingdoms, was proposed. Nowadays life is generally divided into three
domains, domains being the taxonomic level above that of kingdom. According to this
view, prokaryotic organisms comprise two domains (the domain Bacteria and the
domain Archaea), while the eukaryotic organisms comprise the third (domain Eukarya).
In this study unit, we shall pay particular attention to the domain Bacteria.
In order to study the structure of bacteria, we shall take one group of photosynthesising
bacteria namely the cyanobacteria (also called blue-green bacteria, or incorrectly bluegreen algae) as an example of prokaryotic organisms. The reason why we use this
group as an example is because they cause huge problems, especially in our
freshwater supplies.
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Study unit 3
Study section 3.2
The characteristics of cyanobacteria
Important information
When studying this study section, it is important to continuously compare the
prokaryotic cyanobacteria with the eukaryotic algae that you will study later on.
Cyanobacteria differ from algae because they are prokaryotic (algae are eukaryotic). They
are more closely related to bacteria than to algae and therefore it is scientifically incorrect
to refer to them as blue-green “algae”. Cyanobacteria differ from normal bacteria regarding
the following aspects:
They have chlorophyll a (like plants) and produce oxygen during photosynthesis.
They contain blue and red pigments (phycobiliproteins) – more about this later...
Certain cyanobacteria can fix atmospheric nitrogen (other photosynthetic bacteria cannot
fix nitrogen).
Cyanobacteria may occur as single cells or may form colonies or filaments (filamentous
cyanobacteria). The individual cells are usually small (< 0,5 µm) and morphologically
simple. The cells of eukaryotes are usually > 2 µm. Unlike other bacteria, cyanobacteria do
not possess any flagella, and if movement does take place, it is usually a sliding movement
effected by means of mucilage secretions. Only vegetative reproduction (binary fission or
fragmentation) or asexual reproduction (spore formation) takes place. There is no sexual
reproduction. Cyanobacteria occur widely – from the coldest to the warmest places.
The most important characteristics which we shall now discuss are pigments, storage
products, the cell covering and motility/locomotion.
Pigments
The main pigments in cyanobacteria are chlorophyll a and carotene pigments ( carotene).
Cyanobacteria also possess two unique and very important pigments which are called
phycobiliproteins or phycobilins. The two types of phycobiliproteins that occur are
phycocyanin (blue pigment) and phycoerythrin (red pigment). Phycocyanin is responsible
for the typical blue-green colour of these organisms (hence the name blue-green bacteria).
The phycocyanin and phycoerythrin occur in special granules: the phycobilisomes (see Fig
3.1 below). These phycobilisomes occur between the thylakoids (sometimes attached to
them). The thylakoids are arranged separately and equidistant from one another. Under
conditions of low light intensity, more phycobiliproteins are manufactured.
Storage products
In the case of cyanobacteria, the products of photosynthesis are stored in the form of
glycogen (sometimes called cyanophycin starch).
Motility/locomotion
Many representatives of the cyanobacteria are immotile. Sometimes the filaments are
enveloped by a mucilage sheath on the outside of the cell walls, and the organisms can
make sliding movements with the aid of this mucous sheath. The sheath attaches to the
substrate and the filament slides inside the sheath. Many members of the cyanobacteria
have gas vacuoles which enable them to move up and down in the water
Cell covering
Cyanobacteria are enveloped by a thick, firm cell wall consisting of four layers. (The layers
are often numbered Li, Lii, Liii and Liv.) The outer layer (Liv) has villi (see Fig 3.1 below).
Outside the cell wall, there is a thin or very thick layer of mucilage.
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Study unit 3
Study section 3.3
The structure of cyanobacteria
Cyanobacteria may occur as single-celled organisms, as colonies (where collections
of cells are grouped together) or as filaments. A filament consists of a chain of cells
(the trichome) and a surrounding mucilage sheath. You will study examples of
unicellular organisms, colonies and filaments later on in this study unit.
Important information
Drawings like that in Fig. 3.1, which illustrate the internal structures of cells, are VERY
important because they supplement the text and are, therefore, also important for
examination purposes. Such a drawing constitutes a very good synopsis of the learning
content that you have to know. During the exam, a similar drawing may be given to
you, and a variety of questions may be asked on it.
Mucilage sheath
Thylacoids
Cell wall
Gas vacuoles
Polyhedral
body
Polyphosphate
body
Ribosomes (dot)
DNA-fibrils
Centroplasm
Phycobilisome
Chromatoplasm
Glycogen granules
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 69, Fig 2.2.
Figure 3.1: Diagrammatic representation of the internal structure of a
cyanobacterium cell.
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Study unit 3
Two types of cytoplasm, separated from each other by thylakoids, can be
distinguished:
Chromatoplasm (the peripheral cytoplasm): The thylakoids with the associated
pigments occur in this cytoplasm. In Figure 3.1 you can see that the thylakoids are
separate from each other and that they carry phycobilisomes.
Centroplasm (the central cytoplasm): The centroplasm is the clear central cytoplasm
that contains the DNA (without histone compounds). A variety of granules also occur in
the centroplasm:
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Glycogen granules: The storage product of photosynthesis is stored in these.
Polyhedral bodies: They play a role in fixing/binding carbon during
photosynthesis.
Ribosomes: The ribosomes are of the 70S type, and they differ from the
ribosomes of eukaryotic cells (80S type).
•
•
•
https://biology.stackexchange.com/questions/60450/why-do-60s-40s-ribosomal-subunits-makean-80s-ribosome-not-100s
Figure 3.2: Difference between 70S (prokaryotic) and 80S (eukaryotic)
ribosomes.
•
•
12
Polyphosphate bodies: These granules contribute to the survival of this group
of organisms, because when there is an excess of phosphates in the water, the
organisms can take up considerable quantities of the available phosphate and
store it in these polyphosphate bodies. Under unfavourable conditions, when the
phosphate in the water is depleted, phosphates can then be retrieved from this
store until the ambient conditions become favourable again.
Gas vacuoles: These are groups of hollow, cylindrical vesicles which make it
possible for the cell to move up or down in the water. They also enable the cells
to float on the surface, and cyanobacteria therefore often form a layer of foam on
the surface of the water.
Study unit 3
Study section 3.4
Reproduction of cyanobacteria
Cyanobacteria can only reproduce vegetatively and asexually. No sexual reproduction
takes place. A brief summary of different kinds of reproduction is provided below:
•
Cell division (Fig 3.3)
Cell division is common among unicellular organisms, and this is often the only method
of reproduction they have. A new cell wall is then formed centripetally.
https://www.flickr.com/photos/126377022@N07/14763390882/
Figure 3.3: Diagrammatic representation of cell division in Chroococcus.
•
Fragmentation
The process whereby a piece of a filament breaks off (often because of a cell dying)
and the fragments continue to grow is called fragmentation. This is very common in
filaments. During fragmentation, the short piece of filament that breaks off is called a
hormogone. A hormogone can usually execute sliding movements.
•
Spore formation

Hormospore:
Sometimes a hormogone is surrounded by a thick, resistant wall and then it
is known as a hormospore. Because of the thick wall, a hormospore can
survive unfavourable conditions.

Exospores and endospores (Fig 3.4):
Exospores are abscised to the outside, while endospores are formed in the
cell and are released when the cell bursts open.
13
Study unit 3
A
B
Source: Van den Hoek, C, Mann, DG & Jahns, HM. 1995. Algae: An introduction to phycology.
Cambridge: Cambridge University Press: 21, Fig 2.4r, s, t, Fig 2.5c.
Figure 3.4: Diagrammatic representation of endospores (A) en exospores
(B).
•
Akinetes (Fig. 3.5):
During unfavourable conditions, a vegetative cell enlarges and a thick, threelayered resistant wall is formed around the cell. Considerable nutritional reserves
are also stored in such cells. Akinetes may be smooth or have small protrusions
on their surfaces. In the case of Anabaena, akinetes can survive unfavourable
conditions for up to 60 years! Once more favourable conditions return, the
akinete may germinate and divide through mitosis to form a new filament.
•
Heterocysts (Fig. 3.5):
Heterocysts are specialised cells that have the primary function of nitrogen
fixation or nitrogen binding. They are somewhat larger than the vegetative cells,
are enveloped by a very thick wall (can you think of a reason for this?) and they
do not store nutritional reserves (like akinetes do). Although their primary function
is, therefore, not survival and reproduction, they can survive unfavourable
conditions for a limited period of time because of the presence of the thick,
resistant cell wall. Because they do not have food reserves, they cannot survive
as long as akinetes. Once favourable conditions return, heterocysts can also
germinate and divide through mitosis to form a new filament (thus they play a role
in asexual reproduction).
Incidentally, the reason why heterocysts are surrounded by a thick wall is
because nitrogen fixation can only take place in anaerobic conditions (that is in
the absence of oxygen). The thick wall, therefore, prevents oxygen in the
surrounding water from penetrating the cells.
14
Study unit 3
Akinete
Heterocyst
http://cfb.unh.edu/phycokey/Choices/Cyanobacteria/cyano_filaments/cyano_unbranched_fil/unta
pered_filaments/heterocysts/no_visible_sheath/ANABAENA/Anabaena_Image_page.html
(modified from image at nifty.com). Image used with permission from A.L. Baker
Figure 3.5: Diagrammatic representation of akinete and heterocyst
formation by Anabaena.
15
Study unit 3
Study section 3.5
The biological importance of cyanobacteria
Cyanobacteria are a highly successful group of organisms that thrive under favourable
environmental conditions (sufficient nutrients, favourable temperatures, et cetera). In
such conditions they may occur in extremely high concentrations in water – so high that
they may discolour the water (this is called a "bloom"). These blooms may have several
disadvantages, such as the following:
•
Certain cyanobacteria are poisonous and release toxins that poison crustaceans and
fish (as well as birds and humans who eat the fish). These toxins may be even more
poisonous than the poison of a cobra! Genera such as Microcystis, Oscillatoria and
Anabaena, which you will study in your practical module, are all potentially poisonous.
•
•
Cyanobacteria may be responsible for skin irritations, skin rashes and allergies.
Cyanobacterial blooms may cause tastes and smells (think of the blooms of
Microcystis in the Hartbeespoort Dam).
When a cyanobacterial bloom comes to an end (for instance, when the nutrients are
depleted), the dead cells sink to the bottom where they are broken down by
decomposing bacteria. This decomposition process requires oxygen, resulting in the
depletion of oxygen and the death of fish.
Cyanobacterial blooms interfere with water purification (the filaments block sand filters
and cause foam), increasing the cost of water purification significantly.
Cyanobacterial blooms interfere with recreational activities.
•
•
•
The above are only a few of the adverse effects – there a many more! However, not all
cyanobacteria are harmful and some may have advantages – consider the following:
•
•
•
•
•
•
•
•
16
They form the basis of the aquatic food chain – shellfish and other invertebrates
and fish feed on them (although fish will usually avoid cyanobacteria if other algae
are available). Many of these organisms, in turn, serve as food for other aquatic
organisms or humankind.
Oxygen is produced during the process of photosynthesis.
Nitrogen-fixing cyanobacteria are very important in the production of rice.
Certain cyanobacteria are used as food, especially in the oriental countries (Japan
and China). Spirulina is becoming a very popular health food, even locally,
because of its high protein and vitamin content.
Spirulina is, however, mainly used for the extraction of phycocyanin (blue
pigment). This pigment is used commercially as food colouring and in the
cosmetics industry (for instance, in the manufacturing of eyeshadow).
Some cyanobacteria (Nostoc) produce antibiotics that are used in medicine.
Cyanobacteria are often added to sewerage water (which is rich in nitrogen and
phosphates). Because nitrogen and phosphates are their main nutrients, they
remove these from the sewerage. It has been discovered that 90 to 99 percent of
dissolved inorganic nitrogen and phosphate can be removed by cyanobacteria.
The quality of the water improves and, at the same time, the blue-green bacteria
serve as food for fish.
It has been proven that the same toxin that may cause a skin rash (see adverse
effects above) may also suppress leukaemia and other types of cancer.
Study unit 3
Study section 3.6
Practical: Representative cyanobacteria genera
Practical
Study the microscope slides of Microcystis, a colonial cyanobacterium on the
monitors and make fully labelled drawings of the colonies. Identify the following:
colony, broad mucous sheath, vegetative cells and gas vacuoles.
Figure 3.6: Diagrammatic illustration of Microcystis, a problem organism that
secretes toxins.
17
Study unit 3
Study the microscope slide of Anabaena, a filamentous blue-green bacterium, with
the microscope. Make a fully labelled drawing of the filament and identify the
following: vegetative cells, gas vacuoles trichome, thin mucous sheath (difficult to
see), filament, akinete, heterocyst, hormogonia
Figure 3.7: Diagrammatic illustration of Anabaena, an unbranched filament with
akinetes and heterocysts.
Study the microscope slides as well as live material of Oscillatoria (filamentous bluegreen bacterium). Make a fully labelled drawing of the filament and identify the
following: vegetative cells, trichome, thin mucous sheath (difficult to see), filament,
hormogonia.
Figure 3.8: Diagrammatic representation of Oscillatoria, an unbranched filament
without any akinetes or heterocysts.
18
Study unit 3
Study the video clip illustrating problems caused by blue-green bacteria. Make a
summary of these problems. It is important for exam purposes.
Individual activity (optional)
If possible, access the Internet and visit the following interesting webpages. Read
them as revision.
https://www.toledoblade.com/local/2014/08/18/Here-s-toxic-tale-of-Annie-FannieMike/stories/20140818049
http://www.microscopy-uk.org.uk/mag/wimsmall/bacdr.html
Answer the following questions based on the articles on these webpages:
1.
Who are “Anny”, “Fanny” and “Mike”?
2.
Make a short summary of factors that contribute to cyanobacterial blooms.
3.
How can cyanobacterial blooms be limited?
19
Study unit 3
Summary of key points
•
Cyanobacteria are prokaryotic organisms and, therefore, not essentially algae,
although they are sometimes called blue-green algae.
Phycobiliproteins are important pigments that occur in cyanobacteria.
Two types of phycobiliproteins are distinguished, namely phycocyanin (blue
pigment – gives blue colour) and phycoerythrin (red pigment).
The phycobiliproteins occur in phycobilisomes (which are situated between the
thylakoids).
The thylakoids are arranged singly.
The storage product of cyanobacteria is glycogen.
The cell covering is a thick, four-layered cell wall enveloped by a mucous sheath.
Some cyanobacteria execute sliding movements (apart from passive upward and
downward movements in the water column by means of gas vacuoles). Others
are immobile.
•
•
•
•
•
•
•
•
•
Thylakoids divide the cytoplasm in two parts (chromatoplasm and centroplasm).
A variety of cytoplasmic structures with important functions occur in the
centroplasm.
Reproduction occurs only vegetatively and asexually by means of cell division,
fragmentation (the formation of hormogones) and a variety of spores (for
example, hormospores, exospores and endospores, akinetes and heterocysts).
Heterocysts are the locations of nitrogen fixation.
Cyanobacteria are of very great importance and may be detrimental or
advantageous.
•
•
•
Group activity
Form small groups and make sure you can answer the following questions:
1.
2.
Discuss the characteristics of cyanobacteria under the following headings:
•
pigments
•
storage products
•
motility/locomotion
•
cell covering.
Draw a fully annotated diagram of the internal structure of a cyanobacterium cell.
Indicate the functions of each annotated structure in brackets.
3. Discuss the different methods of reproduction found in cyanobacteria.
4. Evaluate the validity or applicability of the terms “blue-green algae” and “bluegreen bacteria”.
5. Write brief notes on the detrimental effects of cyanobacterial blooms in fresh
water.
6. Write brief notes on advantageous cyanobacteria.
7. Give brief explanations of the following terms:
•
20
akinete
Study unit 3
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
bacillus
chromatoplasm
coccus
exospores
endospores
filament
fragmentation
phycobiliprotein
phycobilisome
phycoerythrin
phycocyanin
gas vacuole
glycogen
heterocyst
hormogone
hormospore
Monera
polyphosphate body
polyhedral body
centroplasm
spirillum
trichome
Reflection
Make sure that you have fully reached the outcomes set at the beginning of
this study unit.
21
Study unit 4
Study unit 4
ALGAE
Study hours
This study unit is comprehensive and you have to spend at least 40 hours to master
the content thereof.
Study material
Most of the learning content of this study unit is contained in this study guide.
Study outcomes
Once you have completed this study unit, you should be able to:
•
•
•
•
•
•
•
•
•
•
22
Give a complete description of the various theories on the origins of organelles
such as nuclei, chloroplasts, mitochondria, dictyosomes and the endoplasmic
reticulum (ER), as well as the possible origin of flagella;
describe the endosymbiotic theory;
describe the endomembrane-system theory (or autogenetic theory);
give examples of different symbiotic relationships in nature;
discuss the number, location, internal and external structure of eukaryotic
flagella;
deliver a detailed discussion of all the characteristics (such as pigments, storage
products, motility/locomotion and cell covering) of the Euglenophyta, Dinophyta,
Bacillariophyta and Chlorophyta;
give detailed descriptions of the internal structure of Euglena, Chlamydomonas
and dinoflagellate cells with the aid of fully annotated drawings;
give detailed descriptions of the external structure of dinoflagellate cells and
diatoms with the aid of fully annotated drawings;
describe the structure and functions of a variety of structures and organelles of
algae of different phyla (for example, the flagellum/flagella, eyespot, nucleus,
cytosome, pharyngeal rods, contractile vacuole, ejectisomes, pusule);
describe the structure (and composition, where applicable) of different types of
cell coverings (for example, pellicle, theca, frustule, cell wall, lorica);
Study unit 4
•
•
•
•
•
•
•
•
•
•
•
•
cite the characteristics of representatives of the Euglenophyta, Dinophyta,
Bacillariophyta and Chlorophyta;
discuss different types of protrusions/outgrowths that occur in diatoms;
compare phylogenetic lines (from primitive to advanced) in representatives of the
group Volvocaceae, making use of specific examples;
discuss the asexual and sexual reproduction of Volvox;
discuss the five most important plant body types (unicellular, colonial, aggregate,
conjugative and filamentous forms), giving examples of each;
describe the morphology and reproduction of Spirogyra and Cosmarium;
discuss the structure and reproductive organs of Chara;
explain why the Prymnesiophyta and Bacillariophyta, which were previously
classified under the phylum Chrysophyta, were placed into independent phyla;
discuss the similarities between the Prymnesiophyta, Bacillariophyta and
Chrysophyta;
describe the importance of algae;
explain a number of terms pertaining to algae (see self-evaluation questions);
and
identify, classify and make fully labelled drawings of microscope slides, as well
as live material, of representative genera of the Euglenophyta, Dinophyta,
Bacillariophyta and Chlorophyta.
Introduction
The earth probably originated between four and five billion (4-5 x 109) years ago.
Prokaryotes are most likely the first organisms that lived (approximately three billion
years ago). The eukaryotic cell developed about 1,5 billion years ago.
Until the early 1970s, scientists have postulated that prokaryotes have gradually
become increasingly complex by developing an endomembrane system (see study
section 4.1.1 below), whereby organelles such as nuclei, ER, dictyosomes and
vacuoles came about. They speculated that cyanobacteria might possibly be the link
between prokaryotes and eukaryotes.
In 1976, unicellular prokaryotes were found in Mexico, and although these prokaryotes
(Prochloron) were structurally similar to cyanobacteria, the cells contained no
phycobiliproteins but chlorophyll a and b, similar to the chlorophyll pigments of
eukaryotes. As far as is known, Procholoron is the only prokaryote with chlorophyll a
and b and can, therefore, be viewed as a possible link between prokaryotes
(cyanobacteria) and eukaryotic algal cells. Unlike the prokaryotic organisms you
studied in Study Unit 3, all true algae are, therefore, eukaryotic.
23
Study unit 4
Study section 4.1
The evolution of algae
In the following paragraphs, you will learn more of theories about the possible origin of
eukaryotic cell structures.
4.1.1
The endomembrane-system theory (autogenetic theory)
This theory explains how a prokaryotic cell might have gradually become increasingly
complex, ultimately developing into a eukaryotic cell with a nucleus, ER, dictyosomes,
mitochondria, chloroplasts, et cetera.
According to this theory, invagination (and later abscission) of the plasma membrane
(cell membrane) of a prokaryotic cell might have led to the development of a nuclear
membrane and a variety of cell structures (such as the ER, vacuoles and dictyosomes).
Electron-microscopic investigations made it possible to observe the development of
mitochondria from mitochondrion initials. A mitochondrion initial is a small vesicle,
enveloped by a double membrane, which appears in the cytoplasm of cells. These
initials may develop through budding and abscission of the nuclear membrane, or from
the double membrane that envelops existing mitochondria (Fig 4.1 below). The
mitochondrion initials enlarge, and the inner membrane develops invaginations
perpendicular to itself. Thus a promitochondrion is formed, which will eventually
develop further into a mitochondrion. Mitochondria may also multiply by dividing
(through the formation of a partition membrane or through abscission).
24
Study unit 4
Nucleus
Vesicle
Mitochondrioninisial
Budding
Promitochondrion
Dividing
by
abscission
Mitochondrion
Dividing
by
partition membrane
Source: Robards, AW. 1970. Electron microscopy and plant ultrastructure. London: McGraw-Hill.
Figure 4.1: Diagrammatic representation of the origin and division of
mitochondria.
Many of the arguments about the origin of mitochondria also apply to chloroplasts. Just
like mitochondria, chloroplast initials may also arise through the abscission of vesicles
from the nuclear membrane or through abscission from existing chloroplast
membranes (see Fig 4.2 below). The double membrane of the abscised chloroplast
initial surrounds an area that contains the plastid stroma. The initial (at this stage not
distinguishable from a mitochondrion initial) enlarges, and invaginations form on the
inside of the inner membrane. The invaginations are parallel to the inner membrane
and, therefore, differ from those of mitochondria, which are perpendicular to the inner
membrane. The inner membrane develops further, and a proplastid is formed. Further
development of the proplastid depends on the cell in which it occurs and on the
environment in which the plant is growing. Just like mitochondria, chloroplasts may also
divide by forming a partition membrane or through abscission.
25
Study unit 4
Amiloplast
Nucleus
Budding
Kernmembraan
Darkness
Etioplast
Mature
chloroplast
Light
Dividing by
abscission
Dividing
by
partition membrane
Degeneration
Chromoplast
Source: Robards, AW. 1970. Electron microscopy and plant ultrastructure. London: McGraw-Hill.
Figure 4.2: Diagrammatic representation of the origin and division of
chloroplasts.
4.1.2
The endosymbiotic theory
In 1905, a Russian botanist (Mereschkowski) postulated that chloroplasts and
mitochondria probably originated from symbiotic relationships between prokaryotic
organisms (blue-green and other bacteria) and other larger prokaryotic cells. Lynn
Margulis, a biologist and professor at the University of Massachusetts Amherst,
hypothesised that if organelles were indeed prokaryotic symbionts, they would have
their own DNA and that it would differ from the DNA of the host cell in which they occur.
In the 1980s, this hypothesis was proven for mitochondria, centrioles and chloroplasts.
After the discovery of DNA in these organelles, this hypothesis was readily accepted.
According to the endosymbiotic theory, primitive organisms (prokaryotes such as
bacteria and blue-green bacteria) started living symbiotically with other cells to such an
extent that they eventually became part of the cells and these cells could no longer
function without the prokaryotes.
26
Study unit 4
The symbiosis theory explains the origin of organelles (such as mitochondria and
chloroplasts) and flagella as follows:
•
•
•
Aerobic, heterotrophic bacteria were most likely ingested by a larger cell and
started living symbiotically with the cell. The host cell benefited from the
bacterium’s ability to use oxygen, and the host cell in turn provided organic
nutrients to the bacterial cell. These bacteria probably gave rise to mitochondria.
Chloroplasts most probably developed in a similar manner from a blue-green
bacterium that had been ingested through endocytosis. The host cell benefited
from the blue-green bacterium’s ability to photosynthesise and produce organic
substances.
Because all eukaryotic cells (with the exception of Mixotricha – see Fig 4.4.
below) contain mitochondria while only some contain chloroplasts, mitochondria
are thought to have developed first.
The following factors support the theory that mitochondria and chloroplasts might have
originated from prokaryotes (heterotrophic bacteria and blue-green bacteria
respectively):
•
•
•
•
•
The size and structure of mitochondria and chloroplasts concur with those of
bacteria and blue-green bacteria respectively.
Mitochondria and chloroplasts are surrounded by a double membrane. The
external membrane is probably derived from the enveloping vesicle formed during
endocytosis, and the internal membrane may represent the plasma membrane of
the original prokaryote.
The DNA of mitochondria and chloroplasts is like that of bacteria, occurring in the
shape of a looped (DNA) molecule.
Although most of the proteins in mitochondria and chloroplasts are produced by
the eukaryotic host cell, mitochondria and chloroplasts have their own ribosomes
that can manufacture proteins. These ribosomes are of the 70S type, similar to the
ribosomes of prokaryotic organisms.
Both mitochondria and chloroplasts divide by means of cleavage (independent
from the rest of the cell) and, like prokaryotes, they do not have microtubules.
There are, however, also factors that contradict this theory:
•
•
Mitochondria and chloroplasts cannot live independently when isolated from the
eukaryotic cell.
Mitochondrion and chloroplast DNA have introns – a phenomenon never
observed in prokaryotes.
If the endosymbiotic theory is true, one must consider what the original cell was
(without mitochondria or chloroplasts) and how it survived (glycolysis?). Why have
primitive cells that can live without these organelles never been found?
There is another possibility: That the flagella of eukaryotic cells derive from a
spirochaete (prokaryote) that attached to the host cell and enabled it to move through
water. The mobile spirochaete-like organism is believed to have become more
intimately associated with the host.
27
Study unit 4
https://d2jmvrsizmvf4x.cloudfront.net/rHqxVR5TCGlWNYf788oq_endosymbiosis.jpg
Figure 4.3: Diagrammatic representation of the evolution of a eukaryotic
cell.
Individual activity
Watch the youtube video at the following link:
https://www.youtube.com/watch?v=KtLE0Ok9FKA
Various other symbiotic relationships can be observed in nature. Such relationships may
represent transitional forms. In some instances, organisms live so closely together that they
can no longer function separately. The following are examples:
Mixotricha paradoxa (Fig 4.4) is a flagellated organism that occurs in the digestive tract of
termites. This organism has four flagella. Spirochaete cells are, however, also attached to
the surface of the cell. Bacteria occur in close association with the spirochaete cells (that is
externally). The function of the spirochaetes is to enable the organism to move
(spirochaetes move in phase, like cilia). In spite of the fact that the organism occurs in
oxygen-poor conditions in the digestive tract of termites, it does not have any mitochondria.
Yet, endosymbiotic bacteria occur inside the organism, probably fulfilling the role of
mitochondria together with the external bacterial cells.
Source: Margulis, L & Sagan, D. 1997. Slanted truths. New York: Springer-Verlag: 120, Fig 9.4.
Figure 4.4: Diagrammatic representation of Mixotricha paradoxa.
28
Study unit 4
Cyanophora paradoxa is a freshwater organism with typical blue-green pigments; as a
result it was classified as a cyanobacterium. Later on, it was found to be a protozoon
which harbours cyanobacteria as endosymbionts. The cyanobacteria (Cyanocyta) in
Cyanophora have lost their cell walls as a result of adaptation.
Source: Starr, C & Taggard, R. 1998. Biology: The unity and diversity of life. Belmont:
Wadsworth Publishing Company, Belmont: p. 341, Fig 21.12.
Figure 4.5: Diagrammatic representation of Cyanophora.
Some dinoflagellates have two nuclei. Research has shown that the second nucleus
belongs to an algal cell which is completely enclosed by the dinoflagellate cell. They
undergo mitosis and cell division simultaneously. More information on this is provided
in study section 4.4 (Phylum Dinophyta) below.
Summary of key points
The endomembrane-system theory (autogenetic theory) maintains the following:
•
Invaginations of the cell membrane gave rise to the nuclear membrane, ER,
dictyosomes, vacuoles, et cetera.
•
The first mitochondria and chloroplasts developed from vesicles (mitochondrion
initials and plastid initials, respectively) that were abscised from the nuclear
membrane. These vesicles then developed into promitochondria (or proplastids),
which further increased in size to become mitochondria (or plastids).
•
Existing mitochondria and chloroplasts gave rise to new mitochondria in that

The existing mitochondria/chloroplasts abscised a vesicle (mitochondrion
initial/plastid initial) which enlarged and eventually formed a
mitochondrion/plastid, and

The existing mitochondria or chloroplasts divided into two, either by means
of the formation of a partition membrane or by invagination.
The endosymbiotic theory maintains the following:
Organisms started living together symbiotically until one organism could not do without
the other. The ingested organism underwent changes and gave rise to
chloroplasts/mitochondria/flagella as follows:
•
•
•
aerobic, heterotrophic bacteria may have given rise to mitochondria,
blue-green bacteria may have given rise to chloroplasts, and
spirochaetes may have given rise to flagella.
29
Study unit 4
Various factors support the endosymbiotic theory, but there are also factors which
contradict it.
Various similar symbiotic relationships can be observed in nature.
Group activity
Form small groups and make sure you can answer the following questions:
30
1.
Describe the different theories on the origins of each of the following:
2.
•
Nuclei
•
Mitochondria
•
Chloroplasts
•
Flagella.
Discuss in detail what you understand under the endosymbiotic theory.
3.
Describe some examples of symbiotic relationships found in nature.
4.
Discuss arguments that support and contradict the endosymbiotic theory.
Study unit 4
Study section 4.2
The structure of flagella in eukaryotic cells
Before you start to study individual groups of algae, it is important to understand certain
aspects of algae that apply to more than one group of algae. You have already heard
of flagella – appendages to the cell that enable algal cells to swim through water – and
how they might have originated. Now we shall look briefly at the structure of flagella in
eukaryotic organisms.
The number of flagella per cell, their length, position of implantation and so forth differ
in different types of algae. Flagella may be implanted apically (in front in terms of the
direction of swimming) (Fig 4.6A below), subapically (Fig 4.6B), laterally (Fig 4.6C) or
even posteriorly (Fig 4.6D).
A: Anterior/apically
B: Subapically
C: Laterally
D: Posteriorly
Figure 4.6: Diagrammatic representation showing the different positions
of flagellum implantation.
Flagella may be smooth on the outside, in which case they are described as
acronematic. In other instances, flagella may be covered with hair-like structures
(mastigonemes), in which case they are described as pleuronematic (see Fig 4.7). In
the case of pleuronematic flagella, the mastigonemes may be arranged in a single row
only (the flagellum is stigonematic, or in two rows (the flagellum is pantenomatic), or
the mastigonemes may project in all directions (the flagellum is amphitrichous).
A: Acronematic
B: Stigonematic
C: Pantenomatic
D: Amphitrichous
Figure 4.7: Diagrammatic representation of acronematic (A) and pleuronematic
(B, C and D) flagella to show the arrangement of mastigonemes.
31
Study unit 4
Mastigonemes arise in the perinuclear space (the space between the double nuclear
membranes) and are transported to the flagellum by means of dictyosome vesicles. In
some instances, flagella may also have scales on their external surface. Flagella are
surrounded by the cell membrane on the outside, and all eukaryotic flagella display a
typical arrangement of microtubules known as a 9+2 axoneme (portrayed in Fig 4.8).
The axoneme consists of nine pairs of peripheral microtubules (doublets) and two free,
central microtubules. The central pair of microtubules ends in a thick plate at the
entrance of the cell, and the peripheral microtubules increase to sets of three inside the
cell (still nine groups, Fig 4.9B) and end in the basal body. Attached to the basal body
are microtubule roots or striated fibril roots, which anchor the flagellum firmly in the cell
(Fig 4.9C).
Source: Moore, R, Clark, WD & Stern, KR. 1995. Botany. Dubuque, Iowa: Wm C Brown: 68, Fig
3.28.
Figure 4.8: Internal structure of a eukaryotic flagellum (outside the cell).
Flagella
Flagella
A
B
C
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 5, Fig 1.2.
Figure 4.9: Structure of flagellum apparatus in Chlamydomonas.
A: Diagram of longitudinal section through flagellum; B: Cross-sections of flagellum at
various levels; C: Flagellum apparatus with roots
32
Study unit 4
Now that you know how eukaryotic cells (such as algae) might have originated and
understand the structure of flagella, you can start studying different groups of algae. In
this study unit, we shall concentrate on groups of algae belonging to four phyla only,
but we shall briefly refer to members of other phyla as well.
Summary of key points
•
The number, length, position of implantation and external structure of flagella
may vary.
The eukaryotic flagellum has a specific internal structure consisting of nine pairs
of peripheral microtubules and two central microtubules. This arrangement of
microtubules is known as the 9+2 axoneme.
The two central microtubules end where the flagellum enters the algal cell, and
the arrangement changes to nine peripheral triplets.
The flagellum is anchored in the cell by means of microtubule roots.
•
•
•
Group activity
Form small groups and make sure you can answer the following questions:
1.
2.
3.
Describe the structure of the eukaryotic flagellum under the following headings:
•
Position of implantation
•
External structure
•
Internal structure
•
Function.
Make a fully labelled drawing to illustrate the internal structure of an algal
flagellum.
Define the following terms:
•
•
•
•
•
•
•
acronematic
axoneme
amphitrichous
mastigoneme
pantenomatic
pleuronematic
stigonematic
33
Study unit 4
Study section 4.3
Phylum Euglenophyta (euglenoids)
This phylum consists of more than 750 species, of which the Euglena sp is probably
best known. Organisms belonging to the phylum Euglenophyta are usually unicellular
and occur mostly in freshwater. It is a very interesting group of organisms which
exhibits characteristics of both plants and animals. The fact that the cells actively swim
through water and change their shape makes them an interesting and popular group to
study microscopically.
Important information:
When studying the Euglenophyta, it is important to continuously compare their
characteristics and structure with those of other groups of algae that you are studying,
will study or have already studied.
4.3.1
Characteristics of the Euglenophyta
•
Pigments
A large number of chloroplasts are found in the cell (the shape of which may vary), and
the thylakoids of the plastids are arranged in groups of two to six (but mostly three). A
very prominent red eyespot plays a role in the detection of light intensity. The pigments
in the Euglenophyta include chlorophyll a and b (note that chlorophyll b is present in
very few groups of algae and that it is considered to be an advanced characteristic,
since chlorophyll b also occurs in vascular plants). The main carotenoid pigments
present are  and  carotene, while the main xanthophyll is astaxanthin.
•
Storage products
The product of photosynthesis, namely paramylon, is stored in large granules
(paramylon granules) in the cell. A pyrenoid is sometimes present with paramylon
surrounding it.
•
Motility/locomotion
Cells have two flagella which are implanted in a groove on the anterior side of the cell.
Although both flagella might protrude (extend past the opening of the groove), one
flagellum is usually short and does not protrude so that only the other flagellum is
visible. The protruding flagellum bears a single row of mastigonemes (stigonematic)
and it enables the cell to swim through the water.
Apart from swimming, certain Euglenophyta representatives may also change their
shape; this type of movement is known as metabolic or euglenoid motion and is unique
to this group. During this type of movement, the cell may contract to form a small,
round ball and then again extend itself to form a long cell. Metabolic motion enables
algae such as Euglena to “wiggle” through the sand filters of water purification plants,
and they therefore often end up in the final stages of water purification. However, not
all Euglenophyta can change shape – this depends on the flexibility/mobility of the cell
covering.
34
Study unit 4
•
Cell covering
There is usually no cell wall outside the cell membrane. On the inside of the cell membrane
there is, however, a protective cell covering called a pellicle. In the Euglenophyta, the
pellicle occurs in the form of protein strips which are wrapped helically around the cell. In
some representatives (for example, Euglena), the pellicle strips may slide over one another
and these cells are able to execute metabolic motion. In others (for example, Phacus), the
pellicle strips cannot move across each other and such cells cannot change shape.
An additional cell covering, called a lorica, is present outside the cell membrane in some
representatives. A lorica is an incomplete cell wall which is separated from the cell
membrane by a space. In some instances, the lorica may be impregnated with iron and
manganese and appear very dark. Loricas are present in Trachelomonas and
Strombomonas.
4.3.2
The structure of the Euglenophyta
The internal structure of the cell is illustrated in Figure 4.10.
Mastigoneme
Eyespot
Long flagellum
Short flagellum
Contractile vacuole
Cell membrane
Pellicle strip
Pellicle groove
Paramylon
Mucilage bodies
Microtubules
Microtubules
Nucleus
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 286, Fig 6.1.
Figure 4.10: Diagrammatic representation of the ultrastructure of the
anterior of a Euglena cell.
The structure and functions of the following structures, most aspects of which have
been discussed in the paragraphs above, are important:
The pellicle, flagella, paramylon granules and eyespot
Contractile vacuole: It is the function of the contractile vacuole to get rid of excess water
that enters the cell (osmoregulation and excretion). Dictyosome vesicles, filled with fluid,
fuse with the contractile vacuole and release their contents in it. The contractile vacuole
expands until it reaches a maximum size, whereupon it moves to the cell membrane and
fuses with it in order to release its contents to the outside. A new contractile vacuole is
formed immediately. Contractile vacuoles are, therefore, temporary structures.
35
Study unit 4
Cytosome (Fig 4.11): Entosiphon (a heterotrophic representative) has a primitive
digestive tract (the cytosome). Algae and bacteria in various stages of decomposition
are often found in the cytosome.
Pit/groove in
which the
flagella occur
Cytosome
Pellicle strips
Figure 4.11: Diagrammatic representation of Entosiphon to show the
position of the cytosome.
Pharyngeal rods (Fig 4.12): Pharyngeal rods are pushed out to “catch” prey (for
example, diatom cells) and draw them into the cytosome, whereupon they are digested
in the food vacuole, and the indigestible parts of the prey are pushed out/ejected again.
Cytosome
Two emergent flagella
Cytosome
Pellicle strips
Pharyngeal
rods
Figure 4.12: Diagrammatic representation of Jenningsia showing the
pharyngeal rods in the cytosome.
36
Study unit 4
4.3.3
Reproduction in the Euglenophyta
Only asexual reproduction (binary fission, through the process of mitosis) occurs.
Sexual reproduction does not occur in Euglenophyta at all.
4.3.4
Practical: Representative Euglenophyta genera
Three of the four genera in the table below will be studied with the microscope during
the practical. The difference between the genera are summarised in the following table:
Euglena
Phacus
Strombomonas
No lorica
No lorica
Lorica present
Pellicle strips can
move – in other
words, cell can
execute metabolic
motion
Pellicle strips are Spout
fixed/immobile and present
cannot slide over
each other – cells
are, therefore, nonmetabolic
Trachelomonas
Lorica present
usually Spout present or absent
Lorica may have a No tail-like structure
tail-like
appendage
Spout of lorica Spout of lorica forms a 90
never forms a 90 angle with the rest of the
angle with the rest lorica
of the lorica
37
Study unit 4
Practical: Phylum Euglenophyta, Genera Euglena, Phacus &
Trachelomonas
Study the microscope slide and live material of Euglena. Note the type of swimming
movement that Euglena performs and make a fully labelled drawing of a Euglena cell
in which you distinguish the following: Cell membrane, nucleus, chloroplasts,
paramylon, position of flagellum, eyespot (if visible).
Figure 4.13: Diagrammatic representation of Euglena sp.
Study the permanent microscope slides of Phacus sp. Make a fully labelled drawing
of a Phacus cell to illustrate its structure. Labels: Cell membrane, pellicle strips,
nucleus, paramylon, position of flagellum.
Figure 4.14: Diagrammatic representation of Phacus sp.
38
Study unit 4
Study the permanent microscope slides of Trachelomonas sp. Make a fully labelled
drawing of a Trachelomonas cell to illustrate its structure. Labels: Cell membrane,
lorica, nucleus, chloroplasts, spout of lorica, flagellum.
Figure 4.15: Diagrammatic representation of Trachelomonas sp.
Study the photos, as well as the video clip about the Euglenophyta that will be shown
to you in class. Note the swimming movements as well as metabolic movements
performed by the alga.
Summary of key points
•
•
•
•
•
•
The Euglenophyta are considered to be a highly advanced group of algae which
also exhibit animal characteristics (such as movement, a heterotrophic way of life
in some, a cytosome representing a primitive digestive tract in some, a prominent
eyespot for light detection, et cetera).
The storage product is paramylon, which occurs in the form of granules in the
cell.
Euglenophyta are usually unicellular organisms with one or (seldom) two
protruding flagella (the second flagellum is often short and hidden in the
pit/groove in which the flagella are implanted).
The cell covering is just under the cell membrane in the form of pellicle strips
consisting of proteins. In some species, the pellicle strips can slide over one
another, enabling the cell to change its shape (metabolic motion), while the
pellicle strips are fixed in others and cannot slide over one another.
Some cells have loricas (Strombomonas and Trachelomonas), and the shape of
the lorica is often used for classification purposes.
Advanced, heterotrophic forms have a cytosome which may be regarded as a
primitive digestive tract. Pharyngeal rods are used to catch and draw prey into
the cytosome. Indigestible parts are ejected to the outside with the aid of the
pharyngeal rods.
39
Study unit 4
Group activity
Form small groups and make sure you can answer the following questions:
1.
Discuss the characteristics of the Euglenophyta under the following headings:
2.
•
Pigments
•
Storage products
•
Motility/locomotion
•
Cell covering.
Draw a fully annotated diagram of the structure of a Euglenophyta cell.
3.
Give a complete description of the cell covering of Euglenophyta.
4.
Tabulate the differences between Strombomonas and Trachelomonas, as well as
those between Phacus and Euglena.
5.
Briefly explain what you understand under the following terms:
•
•
•
•
•
•
•
•
•
40
astaxanthin
pharyngeal rods
contractile vacuole
lorica
metabolic/euglenoid motion
eyespot
paramylon
pellicle
cytosome.
Study unit 4
Study section 4.4
Phylum Dinophyta (dinoflagellates)
This phylum consists of approximately 3,000 species and representatives are very
common in seawater, although a limited number of freshwater species also occur. Both
autotrophic and heterotrophic species occur. Members of the Dinophyta are usually
unicellular, flagellated organisms and they are probably the fastest swimmers of all the
algae. Dinophyta are of great ecological importance because they are considered to be
problem algae which cause the phenomenon known as “red tide” in oceans. During red
tide, high concentrations of dinoflagellate cells release toxins that poison filter-feeders
(such as mussels). Toxins accumulate in the mussels, and if sea birds or even humans
were to eat the mussels, they are also poisoned. During the practical module, we shall
show you a video to illustrate how serious this problem really is.
Individual activity (optional)
Read the information contained in the following webpages carefully
https://oceanadventures.co.za/red-tide/
http://www.saeon.ac.za/enewsletter/archives/2014/february2014/doc04
http://www.afrol.com/News2002/sa026_red_tide.htm
Make a summary on the phenomenon of red tide, with specific reference to
conditions in South Africa. You can expect a question about red tides in the exam.
Important information:
When studying the Dinophyta, it is important to continuously compare their
characteristics and structure with those of other groups of algae that you are studying,
will study or have already studied.
4.4.1
Characteristics of the Dinophyta
•
Pigments:
In autotrophic types, a number of chloroplasts occur in the cell, and the thylakoids of these
chloroplasts are arranged in groups of three. The most important pigments in this group are
chlorophyll a and c,  carotene and the xanthophylls peridinin and dinoxanthin. Some cells
have an additional pigment, namely fucoxanthin. Cells that contain fucoxanthin are also
characterised by two nuclei in the cell. We shall discuss this phenomenon in more depth
when we come to the internal structure of the cells. The pigment fucoxanthin gives the
algae a brown colour in freshwater and a red colour in seawater.
•
Storage products:
The storage products are in the form of real starch (which may be deposited in the
chloroplast or outside it) and oils.
41
Study unit 4
•
Motility/locomotion:
The cells move with the aid of two flagella which are situated in grooves on the surface
of the cell. The structures of the two flagella differ from each other. The cingulum
groove is a groove that runs transversely across the cell, encircling it and dividing the
cell into two halves. The cingulum flagellum is located in the cingulum groove. It bears
a single row of mastigonemes and is responsible for the forward motion and rotation of
the cell. The second groove on the cell surface is known as the sulcus groove. The
sulcus is present on one side of the cell only (the ventral side) and is usually
perpendicular to the cingulum groove. The sulcus flagellum is located in the sulcus.
The sulcus flagellum trails behind the cell and is mainly responsible for steering the
cell. The sulcus flagellum is acronematic (in other words, it has no mastigonemes).
•
Cell covering:
The cell covering of the Dinophyta is unique and characteristic of this group. Just like
the pellicle of the Euglenophyta, the cell covering of the Dinophyta is located on the
inside of the cell membrane. Vesicles, called amphiesmal vesicles, occur on the inside
of the cell membrane (see Fig 4.16).
Cell membrane
Pore
Cellulose
Thecal plate
Amphiesma vesicle
Amphiesma vesicle
A: Unarmoured (naked)
B: Armoured
Drawing: S Janse v Vuuren
Figure 4.16: Diagrammatic representation of the amphiesma.
As illustrated in Fig 4.16A, these amphiesmal vesicles may be empty, and then the cell
is said to be unarmoured. Because unarmoured cells are enclosed by the cell
membrane only, they are very fragile. In other cells, the amphiesmal vesicles may be
filled with varying quantities of cellulose (Fig 4.16B). These cellulose-filled vesicles
form plates, the thecal plates, which are visible on the surface of the cell and which
make the cells very resistant. Cells with thecal plates (that is cellulose deposits in the
amphiesmal vesicles) are known as armoured cells. If little cellulose is deposited in the
amphiesmal vesicles, the thecal plates appear thin and the cells are lightly armoured.
The arrangement of thecal plates is species-specific and may, therefore, be compared
to the fingerprints of humans. By numbering these plates in a specific manner, a plate
formula, which is unique to a particular species, can be compiled.
Individual activity
Study the photograph of an armoured cell on the following website:
https://www.sciencephoto.com/media/798066/view/armoured-dinoflagellateperidinium-sp-sem
Take particular notice of the thecal plates on the surface and the positions of the
cingulum and sulcus grooves.
42
Study unit 4
4.4.2
The structure of the Dinophyta
4.4.2.1
External structure
The structures of unarmoured and armoured cells are to a large extent similar. In both,
the cells are divided into two halves by the transverse groove (cingulum).
In the case of unarmoured cells, these two halves are called the
•
epicone: situated anterior (in front, in terms of the direction of movement), and
•
hypocone: situated posterior (at the back, in terms of the direction of movement).
In the case of armoured cells, the epicone and hypocone consist of thecal plates of
cellulose (which may vary from thin to thick), and then the two halves are known as the
•
epitheca, and
•
hypotheca
The external structure of an armoured dinoflagellate cell is depicted in Figure 4.17. The
sulcus groove is always located in the hypocone or hypotheca. The cingulum groove is
usually approximately in the centre of the cell but may also be located nearer to one
pole. The view of the cell where the sulcus is visible is known as the ventral view. The
view which becomes visible when the cell is rotated through 180 degrees is called the
dorsal view, and here no sulcus is visible.
Epitheca
50 µm
Cingulum flagellum
Hypotheca
Nucleus
Cingulum
Sulcus
Sulcus flagellum
A: Ventral view
Thecal plate
B: Dorsal view
Source: Van den Hoek, C, Mann, DG & Jahns, HM. 1995. Algae: An introduction to phycology.
Cambridge: Cambridge University Press: 245, Fig 16.1.
Figure 4.17: External structure of an armoured dinoflagellate cell as seen
in ventral (A) and dorsal (B) view.
In some cases (for example in Ceratium), certain thecal plates may be elongated to
form horns. More information on this follows in paragraph 4.4.4, where we discuss
representative Dinophyta genera.
43
Study unit 4
4.4.2.2
Internal structure
The internal structure of the cell is illustrated in Figure 4.18.
Epitheca
Pore
Trichocyst
Mitochondrion
Sulcus
flagellum
Hypotheca
Chloroplast
Thylacoids
Cell membrane
Thecal plate
Dictyosome
Cingulum
Nucleolus
Dinoflagellate nucleus
Chromosome
Lipid body
Starch granule
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 308, Fig
7.1.
Figure 4.18: Diagrammatic representation of the internal structure of a
dinoflagellate cell.
The external layer of the cell (Fig 4.16) is known as the amphiesma and it consists of
the
•
•
•
•
cell membrane
amphiesmal vesicles with or without cellulose deposits (thecal plates)
associated microtubules
pores in the thecal plates under which ejectisomes, the trichocysts, are found.
The nucleus of Dinophyta is unique and is known as a dinokaryon. The dinokaryon
differs from normal eukaryotic nuclei in the following respects:
44
•
•
the chromosomes are very condensed and have transverse bands (see Fig 4.18),
the chromosomes contain very little nucleohistone (similar to the DNA of
prokaryotes), and
•
the synthesis of DNA is not interrupted (also similar to prokaryotes).
Study unit 4
Most Dinophyta cells have one nucleus, in which case the nucleus is a dinokaryon. In
some instances, however, two nuclei are present in the cell, namely a dinokaryon and
a normal eukaryotic nucleus. In types with a second eukaryotic nucleus, this nucleus is
most probably that of an endosymbiont, considering that the nucleus, together with a
full set of other organelles (for example, chloroplasts), is surrounded by a membrane.
Cells with only one nucleus (a dinokaryon) contain the xanthophyll pigments peridinin
and dinoxanthin. Those with a second nucleus also contain the pigment fucoxanthin in
addition to these pigments. Fucoxanthin occurs commonly in representatives of the
Phaeophyta, Chrysophyta and Bacillariophyta. It is, therefore, believed that the
endosymbiont might belong to one of these three groups of algae.
Instead of contractile vacuoles, a permanent structure occurs that plays a role in
excretion and osmoregulation. This structure is called a pusule (Fig 4.19). It is a saclike structure with a firm wall that opens into the sulcus groove (and sometimes the
cingulum) by means of pores. The contents of the pusule are released directly into the
groove and from there to the outside, when the pusule contracts.
Flagellum
Flagellum
Pusule vesicle
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 314, Fig
7.10.
Figure 4.19: Diagrammatic representation of a pusule.
The Dinophyta possess a number of different types of ejectisomes. The type that
occurs most commonly is the trichocyst. The structure of trichocysts is complex and
their true function is not yet known. These ejectisomes are ejected when the cell is
irritated, causing a quick movement of the cell in the opposite direction (that is the
direction opposite to the direction into which the ejectisome was ejected). This reaction
may, therefore, act as an escape mechanism or serve to defend the cell (acting like a
spear) against other organisms.
4.4.3
Reproduction in the Dinophyta
Both asexual and sexual reproduction occurs.
Asexual reproduction occurs by means of cell division. In some types, the thecal plates
are shared by the daughter cells, and some plates are formed anew. In other types,
thecal plates are shed, and a whole new set of plates is formed by the daughter cells.
During sexual reproduction, the gametes fuse to form a motile, diploid zygote, which
will divide again by means of meiosis.
45
Study unit 4
4.4.4
Practical: Representative Dinophyta genera
In the practical module, you will be shown photos, videos and permanent microscopic
preparations.
Individual activity
Study the permanent microscope slide of Peridinium sp and make a fully labelled
drawing to illustrate the general shape and structure of the cell. Distinguish the
following: dorsal/ventral view, cell membrane, epitheca, hypotheca, thecal plates,
cingulum, sulcus.
Figure 4.20: Diagrammatic representation to illustrate the structure of Peridinium,
an armoured dinoflagellate, in ventral view.
In Ceratium (an armoured dinophyte), certain thecal plates are elongated to form
horns. The names of the horns are derived from the names of the particular thecal
plates that are elongated.
Study the permanent microscope slide of Ceratium sp and make a fully labelled
drawing to illustrate the general shape and structure of the cell. Distinguish the
following: dorsal/ventral view, cell membrane, epitheca, hypotheca, thecal plates,
cingulum, sulcus, apical horn, postcingular horn(s) and antapical horn
Figure 4.21: Diagrammatic representation to illustrate the structure of Ceratium, an
armoured dinoflagellate, in ventral view.
46
Study unit 4
Individual activity
Go to the webpage provided below and make sure you go to "more on morphology"
(green block at the bottom of the page). Read the part on the morphology of
dinoflagellates:
http://www.ucmp.berkeley.edu/protista/dinoflagellata.html
Summary of key points
•
•
•
•
•
•
•
•
•
•
•
•
•
Marine representatives of the Dinophyta are often problem algae – certain species
cause toxic red tides in the ocean.
Dinophyta are often unicellular, flagellated organisms.
The flagella are situated in grooves on the surface of the cell.
The cingulum groove divides the cell into two halves (epitheca/epicone and
hypotheca/hypocone).
The sulcus groove is only situated on the hypotheca/hypocone and only on the
one side (the ventral side) of the cell.
The cingulum flagellum enables the cell to swim forward and rotate, while the
sulcus flagellum guides/steers the cell.
The nucleus of the Dinophyta is a dinokaryon, which differs from normal
eukaryotic nuclei.
Sometimes, a second nucleus (possibly that of an endosymbiont) is present in the
cells. If a second nucleus is present, the cells also contain the xanthophyll
fucoxanthin in addition to peridinin and dinoxanthin.
The most common cell covering consists of thecal plates of cellulose under the
cell membrane (cells armoured). Sometimes, very thin cellulose deposits are
present in amphiesma vesicles under the cell membrane (cells lightly armoured).
The amphiesma vesicles may also be empty, and then the cells are only enclosed
by the cell membrane (cells unarmoured).
The thecal plates of armoured cells are numbered for taxonomic purposes, and
plate formulae (which are species-specific) are compiled using this numbering.
In Ceratium, some of the thecal plates are elongated to form horns.
Instead of the usual contractile vacuoles, a permanent structure (the pusule) is
responsible for osmoregulation and excretion.
Ejectisomes (trichocysts) occur.
47
Study unit 4
Group activity
Form small groups and make sure you can answer the following questions:
1.
Discuss the characteristics of the Dinophyta under the following headings:
2.
•
Pigments
•
Storage products
•
Motility/locomotion
•
Cell covering.
Write brief notes on the Dinophyta as problem algae.
3.
Draw fully annotated diagrams of the external and internal structures of dinophyte
cells.
4.
Describe the differences between a dinokaryon and a normal eukaryotic nucleus.
5.
Describe the structure and function of the pusule.
6.
Explain what you understand under the following terms:
•
•
•
•
•
•
•
•
•
•
•
•
•
48
amphiesma
dinokaryon
ejectisome
epicone/epitheca
fucoxanthin
hypocone/hypotheca
plate formula
pusule
red tide
cingulum
sulcus
theca
trichocyst.
Study unit 4
Study section 4.5
Phylum Bacillariophyta (diatoms)
Representatives of this phylum are popularly known as “diatoms”. Diatoms are very
common and can be found in practically any water, where they float freely or are
attached to a substrate. There is a huge variety of species (the exact number of
species is unknown, but it may be as many as 100,000). Diatoms are mostly unicellular
but may also be found in colonies or as filaments. Most diatoms are autotrophic
organisms with a yellow-brown colouring due to pigments in the chloroplast that
overshadow the chlorophyll. High concentrations of diatoms may even colour the water
brown. Diatoms are probably the most important primary producers (especially in the
sea). The most salient characteristic of diatoms is their unique cell covering, which is
called a frustule. This cell covering is very resistant and may, therefore, remain
preserved for a long period of time – hence fossils of diatoms are abundant.
Diatoms are of great economic importance for reasons such as the following:
•
•
•
•
•
They are used in a variety of products (for example, polishes and toothpaste) as
an abrasive because of their hard cell covering.
They prevent the formation of ice crystals in ice cream.
Oil may be derived from diatoms (oil is an important storage product of diatoms).
They play an important role in forensic tests (murders have been solved by
looking at the diatom composition in the lungs of a person who has drowned).
The use of diatoms as indicators of water quality (especially of freshwater) is
increasing in popularity every day.
Individual activity
If you have Internet access, we STRONGLY recommend that you visit the following
webpage:
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopyuk.org.uk/mag/wimsmall/bacdr.html
Go to the bottom of this webpage and click on the link “Diatoms”. Carefully read the
section on diatoms. It will serve as a good introduction to the topic.
Important information:
When studying the Bacillariophyta, it is important to continuously compare their
characteristics and structure with those of other groups of algae that you are studying,
will study or have already studied.
49
Study unit 4
4.5.1
Characteristics of the Bacillariophyta
•
Pigments
The number of chloroplasts differs from one kind of diatom to the next, but they usually
vary from two to many. The chloroplasts have a typical brown colour because of the
presence of fucoxanthin, which overshadows the chlorophyll pigments. Other important
pigments that occur include chlorophyll a and c, and  and  carotene. The thylakoids
are arranged in groups of three.
•
Storage products
The two most important storage products are droplets of oil and chrysolaminarin (a
glucose compound) which are stored in a large vesicle in the cell. The oil droplets do
not only play an important role as storage product but also serve to maintain the
position of the cells in the water column (the cells are heavy due to the nature of the
frustule and, therefore, tend to sink toward the bottom). Sedimentation of cells would
result due to a lack of light for photosynthesis and must, therefore, be avoided at all
costs.
•
Motility/locomotion
Based on their structure, four main types of diatoms can be distinguished (see the
paragraph further down on representative genera). The two most common forms are
the centric and pennate diatoms. Centric diatoms are immotile (they do, however, form
flagellated male gametes, the sperm cells). Pennate diatoms may also be immotile, but
some have a longitudinal groove in the frustule through which mucous is secreted. This
groove is called a raphe. The mucous enables such pennate diatoms with raphes to
make sliding movements when in contact with a substrate. Pennate diatoms produce
immotile sperm cells. No flagella are found in vegetative diatom cells.
•
Cell covering
The cell covering of diatoms is unique, providing them with beautiful shapes and forms.
It consists of a cell wall (outside the cell membrane) which is impregnated with silica in
the form of silicon dioxide (SiO2). This silicate cell covering is called a frustule and is
extremely resistant. As you will be able to see in Figure 4.22 below, frustules boast all
kinds of decorative patterns which are used in the classification of these organisms.
The structure of the frustule is discussed in more detail in the next paragraph.
4.5.2
The structure of the Bacillariophyta
The frustule of diatoms consists of two halves that are nearly equal in size. The one
half is just slightly smaller and fits under the other (like a petri dish or like a box under
its lid). The larger half is called the epitheca, and the smaller half is called the
hypotheca. The epitheca may be subdivided into the epivalve (comparable to the flat
surface of half a petri dish or the surface of the lid of a box) and the epipleura
(comparable to the side/rim of half a petri dish or the side of the lid of a box). Similarly,
the hypotheca is subdivided into a hypovalve and a hypopleura. The area where the
epipleura and hypopleura overlap is known as the girdle (cingulum).
The valve view of the cell is round (in the case of centric diatoms; Fig 4.22A) or
elongated (in the case of pennate diatoms; Fig 4.22B). When seen in girdle view, both
centric and pennate diatoms appear rectangular (Fig 4.23).
50
Study unit 4
A
Sources:
B
A: Smith, G.M. 1950. Freshwater algae of the United States. Second Edition. McGraw-Hill Book
Company, New York: p. 463, Fig. 372
B: Morris, I. 1968. An introduction to the algae. Hutchinson & Co. Ltd, London: p. 100, Fig. 18B.
Figure 4.22: Diagrammatic representation of the valve view of a centric
diatom (A) and a pennate diatom (B).
Epivalve
Epipleura
Girdle
Hypopleura
Hypovalve
Figure 4.23: Diagrammatic representation of the girdle view of a diatom
cell.
The frustule is decorated with pores (puncta; singular: punctum). These puncta are
usually arranged in rows, and a row of puncta is known as a stria. In the case of centric
diatoms, the striae are arranged radially symmetrically (Fig 4.22A), while they are
arranged bilaterally symmetrically in the case of pennate diatoms (Fig 4.22B). On the
inside of a punctum, there is usually a hexagonal chamber, called the areola. Air may
be trapped in this chamber, making the diatom more buoyant so that it can maintain its
position in the water column.
Some pennate diatoms have a longitudinal slit in the valva. This slit is called the raphe
and it secretes mucilage which makes it possible for the diatoms to execute sliding
movements. The raphe may be present on both valves (the epivalve and the
hypovalve), or sometimes only on one. The raphe is interrupted in the middle, where
there is a thickening of the frustule called the central nodule. Similar thickenings, the
polar nodules, occur on each pole of the cell (Fig. 4.24 and 4.25A).
51
Study unit 4
Polar nodule
Valve
Raphe
Stria
Pleura
Central nodule
Cell lumen
Figure 4.24: Three-dimensional diagrammatic representation of a section
of the frustule of a pennate diatom.
Some pennate diatoms have no raphe in the centre of the valve but do have a smooth,
central section (lacking striae) called a pseudoraphe (Fig 4.25B).
Pseudeoraphe
Stria
Central nodule
Raphe
Polar nodule
A
B
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 417, Fig
13.3.
Figure 4.25: Diagrammatic representation of pennate diatoms with a raphe
(A) and a pseudoraphe. (B)
52
Study unit 4
Another adaptation of diatoms that enables them to maintain their position in the water
column is a number of protrusions/outgrowths that occur on the frustule. These
protrusions increase the surface : volume ratio and may also attach different cells to
one another (for instance, in the case of filaments or single cells that are linked to each
other in chains). The following two main types of protrusions/outgrowths occur:
•
Spines: short, solid, thorn-like protrusions (often horn-shaped)
Figure 4.26: Diagrammatic representation of a spine.
•
Setae: elongated, hollow protrusions. Two types of setae can be distinguished:

Labiate processes – tube-like outgrowths that represent an opening that
runs through the frustule and is flattened inside. They have been implicated
in mucilage secretion.
Figure 4.27: Diagrammatic representations of two labiate processes.

Strutted processes: - reinforced, hollow, tubular protrusions with strutted
structures at the bottom
Figure 4.28: Diagrammatic representation of a strutted process.
4.5.3
Reproduction in the Bacillariophyta
The most common method of reproduction is by means of cell division (asexual
reproduction), but sexual reproduction, through the formation of special spores, may
also take place.
.
53
Study unit 4
Individual activity
Carefully study the line drawings below that illustrate the reduction in cell size of one
daughter cell during each division. Make sure that you understand why the size of
daughter cells decreases with each new generation.
Source:
https://biology.stackexchange.com/questions/14753/reduction-of-size-in-diatomsdue-to-asexual-reproduction
Because of the decrease in size during cell division, diatoms have to have a
mechanism to readjust their cell size when the cells become too small. This occurs
during sexual reproduction with the formation of an auxospore. Sexual reproduction
can only take place if the following requirements are met:
•
The cells must have reached a minimum size (30-40% of the maximum size).
•
The environmental conditions must be favourable.
If the above requirements are met, the small (minimum size) vegetative cells form
gametes. The fertilisation of gametes leads to the formation of an auxospore, which
develops and grows and will ultimately produce a large cell. Thus auxospore formation
makes it possible for diatom cells to readjust their cell size so that vegetative growth
and reproduction can be continued. Unlike other groups of algae, sexual reproduction
in the Bacillariophyta is principally a mechanism to correct cell size.
4.5.4
Practical: Representative Bacillariophyta
Based on their structure and ornamentation patterns, the following four main groups of
diatoms can be distinguished:
•
•
Centric forms (Fig 4.29A): valve round, striae radially symmetrical (radiating
from a central point);
Trellisoid forms (Fig 4.29B): valve elongated, striae arranged uniformly, from
side to side, across the surface;
•
Gonoid forms (Fig 4.29C): valve triangular, sculpture determined by angles; and
•
Pennate forms (Fig 4.29D): valve elongated, striae bilaterally symmetrical.
Of these four groups, the centric and pennate diatoms are most abundant, and you will
have the opportunity to study them microscopically in your practical module. You will
also be shown a video in the practical module to illustrate the sliding movements of
pennate diatoms and the regulation of the cell’s oil content.
54
Study unit 4
A
B
C
D
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 417, Fig
13.2.
Figure 4.29: Diagrammatic representations showing the different forms of
diatoms (A = centric form; B = trellisoid form; C = gonoid form; and D =
pennate form).
Study the example of a pennate and centric diatom on the microscope slide
labelled “Diatoms”. Make fully labelled drawings of the pennate and centric
diatoms and supply the drawings with the following labels:
A: Valve view of pennate diatom: Frustule, valve, raphe, central node, polar nodes,
stria
B: Valve view of a centric diatom: Frustule, valve, stria, costa, punctum
A
B
Figure 4.30: Diagrammatic representation of the valve view of a centric (A) and a
pennate diatom (B).
55
Study unit 4
Group activity
Study the slide of Navicula lyra shown to you on the monitors. This slide is
magnified 1000x and immersion oil is used in order to see that the stria consists of a
row of puncta. Add the label punctum to the drawing made in Fig. 4.30.
Study the permanent microscope slide of Tabellaria sp, a pennate diatom with a
very thin valve and an elongated girdle. The shape of the cells (thin valve and
broad girdle) causes these cells often to be seen in girdle view, rather than valve
view. Identify a valve of Tabellaria and then study and draw the girdle view of a cell
carefully. Distinguish the following: Frustule, epivalve, epipleura, hypovalve,
hypopleura, girdle.
Figure 4.31: Diagrammatic representation of the girdle view of Tabellaria sp.
56
Study unit 4
Study the living material of a culture of pennate diatoms. Note the yellow colour
of the chloroplasts (due to the yellow pigment fucoxanthin that overshadows the
chlorophyll). Also, note the gliding movements performed by these diatoms in
contrast to centric diatoms which are non-motile.
Study the photos as well as the video-clip about the diatoms that will be shown to
you. Write notes on how especially centric diatoms (with a heavy silica frustule
and no structures for movement) are adapted to keep their position in the water
column for photosynthesis.
NOTES:
Individual activity
Complete the following table, showing the differences between centric and pennate
diatoms.
CENTRIC DIATOMS
PENNATE DIATOMS
Form of valve
Symmetry of ornamentations
Motility of vegetative cells
Motility of sperm cells
57
Study unit 4
Summary of key points
•
•
•
•
•
•
•
•
•
58
The most significant characteristic distinguishing diatoms is the presence of a
specialised cell wall, the frustule, which consists mainly of silica.
The two most important groups (types) of diatoms that are distinguished are
centric and pennate diatoms. These diatoms differ from each other in respect of
structure and motility.
Some pennate diatoms can execute sliding movements because the raphe
secretes mucilage.
The frustule is divided into an epitheca and a hypotheca. The epitheca comprises
an epivalve and an epipleura, and the hypotheca consists of a hypovalve and
hypopleura. The girdle is the area where the epipleura and hypopleura overlaps.
Sometimes, the raphe is absent but a smooth, central area (called a
pseudoraphe) is visible.
Under a microscope, striae can be seen on the surface of the frustule. Each stria
consists of a row of puncta.
Protrusions/outgrowths (spines and setae) occur on the surface of the frustule.
Amongst other things, they add to the buoyancy of the cell. Two types of setae
are distinguished, namely labiate processes and strutted processes.
During cell division, the size of one daughter cell is reduced with each new
generation, because the hypotheca of the parent cell acts as the epitheca of the
daughter cell.
To overcome the above problem, special sexual spores (the auxospores) are
formed during sexual reproduction. When these spores germinate, the cell size is
corrected.
Study unit 4
Group activity
Form small groups and make sure you can answer the following questions:
1. Discuss the characteristics of the Bacillariophyta (diatoms) under the following
headings:
•
Pigments
•
Storage products
•
Motility/locomotion
•
Cell covering
2. Describe the reproduction of diatoms.
3. Compare centric and pennate diatoms with each other, making use of a table.
4. Make a fully annotated drawing showing the structure of a pennate diatom.
5. Briefly describe how diatoms maintain their position in water so that
photosynthesis can take place maximally.
6. Briefly explain the following terms:
•
areola
•
chrysolaminarin
•
epipleura
•
epitheca
•
epivalve
•
frustule
•
girdle/cingulum
•
hypopleura
•
hypotheca
•
hypovalve
•
labiate process
•
auxospore
•
polar nodule
•
pseudoraphe
•
punctum
•
raphe
•
central nodule
•
setum (plural: seta/e)
•
spine
•
stria
•
strutted process
•
valve.
59
Study unit 4
Study section 4.6
Phylum Chlorophyta (green algae)
Of all the phyla of algae that you have studied thus far, the phylum Chlorophyta is
probably the most diverse. Organisms in this phylum vary from microscopically small (a
few micrometres) to large macroscopic organisms that are visible with the naked eye.
Most organisms belonging to this phylum are coloured bright green (hence the general
term “green” algae), unlike, for example, the diatoms and dinoflagellates which are
yellow-brown or brown. Chlorophyta occur in freshwater, brackish water and seawater,
and some kinds may even live in damp areas on land. Chlorophyta are probably the
best known of all the groups of algae and have a cosmopolitan distribution.
Approximately 7,500 species occur and they belong to a wide variety of genera and
classes. We shall only study representatives of the class Chlorophyceae and shall refer
only briefly to the class Charophyceae.
4.6.1
Class Chlorophyceae
The organisms belonging to this class are diverse and differ greatly in structure. They
may be unicellular or occur in colonies, or the cells may be arranged end-to-end to
form filaments.
Individual activity
Visit the following webpage.
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopyuk.org.uk/mag/wimsmall/bacdr.html
Go the bottom of the page and click on “green algae”. Read the section on green
algae. This should give you an indication of the diversity within the class.
Important information:
When studying the Chlorophyta, it is important to continuously compare their
characteristics and structure with those of other groups of algae that you are studying,
will study or have already studied.
4.6.1.1
Characteristics of the Chlorophyceae
•
Pigments
Just like plants, the Chlorophyceae contain the pigments chlorophyll a and b. This,
together with other similarities between the Chlorophyceae and plants, indicates that
green algae and plants might have had a common ancestor or that green algae might
be predecessors of plants. The number and shape of chloroplasts vary between
species and are often used in classification. As in plants, the thylakoids in the
chloroplasts are arranged in groups of two to six and form stacks, the grana (singular:
granum), in places.
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Study unit 4
•
Storage products
Green algae are also similar to plants in terms of their storage product – the most
important storage product being starch. The starch is often stored around a pyrenoid.
•
Motility/locomotion
If the cells are motile (many types are not), they usually move with the aid of flagella.
The number of flagella varies from two to many per cell. Some types may also execute
sliding movements with the aid of mucous secretions.
•
Cell covering
Some kinds are naked (that is surrounded only by the cell membrane). In most cells, a
cell wall occurs outside the cell membrane. This cell wall may consist of cellulose or
glycoproteins. A few representatives also have a lorica surrounding the cell membrane.
4.6.1.2 The structure and reproduction of Chlorophyceae
Because of the huge diversity within this group, the structure of representatives vary
considerably and different types can be distinguished, amongst others unicellular,
colonial, aggregate, conjugational and filamentous forms. A short overview of most
of these forms will be provided below:
Unicellular forms:
Unicellular algae may be immotile (in other words, there are no flagella or other
mechanisms of locomotion) or motile (mostly by means of flagella). Probably the most
well-known motile, unicellular alga is Chlamydomonas. The internal structure of
Chlamydomonas (Fig. 4.32) will, therefore, be used as an example of the general
structure of Chlorophyceae representatives (but bear in mind that the structure differs
considerably between different genera).
Each Chlamydomonas cell is motile thanks to two equally long flagella (implanted
anteriorly). The cell is surrounded by a cell wall of glycoprotein, and the greater part of
the interior of the cell is filled with a bright green, cup-shaped chloroplast. The storage
product (starch) is deposited freely inside this chloroplast and also around the
pyrenoid. Note the arrangement of the thylakoids within the chloroplast in Figure 4.32.
Contractile vacuoles may be present, the purpose of which is to get rid of excess water.
The nucleus of the cells is located in the cytoplasm. A bright red eyespot, which
orientates the cell with regard to light, can be observed in the chloroplast.
Chlamydomonas may reproduce asexually (the most common form of reproduction) or
sexually.
61
Study unit 4
Flagellum
Contrac tile vacuole
Cell wall
Mitochondrion
Nucleus
Nucleolus
ER
Dictyo some
Cell membrane
Eyespot
Cytoplasm
Chloroplast
Starch gr anule
Pyrenoid
Thylacoid
10 µ m
Starch
Source: Van den Hoek, C, Mann, DG & Jahns, HM. 1995. Algae: An introduction to phycology.
Cambridge: Cambridge University Press: 300, Fig 19.1.
Figure 4.32: Diagrammatic representation of the ultrastructure of a
Chlamydomonas cell.
Practical: Non-motile, unicellular forms
Individual activity:
Study the living cells of Monoraphidium sp. grown in culture as an example of a nonmotile unicellular form. Note the shape of the cells and the fact that they do not
move around. Make a drawing to illustrate the shape of the cell and indicate that no
flagella are present. Labels: Cell wall, chloroplast
Figure 4.33:
Diagrammatic representation to illustrate the structure of
Monoraphidium.
62
Study unit 4
Practical: Motile, unicellular forms
Individual activity:
Study the living cells of Chlamydomonas sp. grown in culture as an example of a
motile unicellular form. Note the swimming movements performed with the aid of 2
flagella. Also, study the permanent microscope slides "Chlamydomonas flagella" and
"Chlamydomonas w.m." The slide labelled Chlamydomonas flagella was stained to
show the flagella, while the slide labelled Chlamydomonas w.m. was stained to show
the inner contents of the cell but not the flagella. Make only one composite drawing to
illustrate the structure of a Chlamydomonas cell by studying the living material and
microscope slides. Labels: Cell wall, flagella, chloroplast, pyrenoid, starch, nucleus.
Figure 4.34:
Diagrammatic representation to illustrate the structure of
Chlamydomonas.
63
Study unit 4
Colonial forms
A large variety of non-motile, as well as motile green algal colonies, is found. In this
section, emphasis will be placed on motile colonies, but Scenedesmus will be studied
as an example of a non-motile colony.
Non-motile colonies
Practical: Non-motile colonies
Study the living culture of Scenedesmus sp as well as the permanent microscope
slide. Note the number of cells in a colony and make a labelled drawing to illustrate
the structure of the colony. Labels: Individual cell of colony, cell wall, chloroplast,
pyrenoid, starch, spine
Figure 4.35:
Scenedesmus.
64
Diagrammatic representation to illustrate the structure of
Study unit 4
Motile colonies
Within the Chlorophyceae, there is a series of colonies (belonging to the family
Volvocaceae) of which the structure of the cells is similar to that of Chlamydomonas.
These colonies were probably formed through the aggregation of Chlamydomonas-like
cells. The cells of these colonies are kept together by an extracellular matrix of
glycoproteins. Evolution within the family Volvocaceae gave rise to colonies of
increasing complexity. The basic cell unit of these organisms is that of
Chlamydomonas. Changes in the complexity of the colonies (phylogenetic lines) are
evident in certain characteristics. These characteristics are summarised in Table 1.
Table 1: Characteristics used to distinguish primitive and advanced colonies of
Volvocaceae
Characteristic
Primitive
Advanced
Number of cells
Few
Many
Polarity
Absent
Present
Functional differentiation
Absent
Present
Cytoplasmic connections
Absent
Present
Type of sexual reproduction
Isogamy
Oogamy
Based on these characters, a range of colonies can be distinguished, namely Gonium,
Pandorina, Platydorina, Eudorina, Pleodorina and Volvox (in order of increasing
complexity from primitive to advanced). It is generally accepted that Gonium is the
most primitive of this series of colonies, while Volvox is the most advanced. The
increasing complexity of these colonies (citing their characteristics) is summarised in
Figure 4.36 below.
.
65
Study unit 4
1
Gonium
3
Platydorina
4-32 Cells
No polarity
No functional differentiation
No cytoplasmic
connections
Sexual reproduction isogamy
2
16-32 Cells
Polarity: rounded in front,
lobed posteriorly
4
Pandorina
Eudorina
No functional differentiation
No cytoplasmic
connections
Sexual reproduction isogamy
5
Pleodorina
Sources:
± 128 Cells
Polarity: Anterior cells small,
posterior cells large;
eyespots of anterior cells
large, eyespots of posterior
cells small
Functional differentiation:
anterior cells reproduce,
posterior cells assimilate
No cytoplasmic connections
Sexual reproduction isogamy
6
Volvox
16-32 Cells
No polarity
No functional differentiation
No cytoplasmic connections
Sexual reproduction –
isogamy
16-32 Cells
Polarity: Elongated colony;
eyespots of anterior cells
large, eyespots of posterior
cells small
No functional differentiation
No cytoplasmic connections
Sexual reproduction isogamy
1,000-50,000 Cells
Polarity: Eyespots of anterior
cells large, eyespots of
posterior cells small
Functional differentiation:
special cells for reproduction
– gonidium, egg cell, sperm
cell
Cytoplasmic connections
Sexual reproduction oogamy
Smith, GM. 1950. Freshwater algae of the United States. 2nd ed. New York: McGrawHill: p 96, Fig 40; p 100, Fig 44, p 101, Fig 45.
Morris, I. 1968. An introduction to the algae. London: Hutchinson: 44, Fig 5B.
Miller, RN. 1982. Plant types 1. Algae, fungi and lichens. London: Hutchinson: 19.
Institute of Terrestrial Ecology. 1979. An illustrated guide to river plankton. Cambridge:
Crown: 14, Fig 12.
Figure 4.36: Increasing complexity of Volvocaceae colonies, based on
primitive and advanced characteristics.
66
Study unit 4
Individual activity (optional)
It is highly recommended that you visit the following webpage and read more about
the structure of Volvox in the paragraph “Volvox, one of the 7 wonders of the
microworld”:
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopyuk.org.uk/mag/artdec03/volvox.html
Although all the Volvocaceae colonies can reproduce asexually and sexually, Volvox is the
only one that reproduces sexually by means of oogamy. Sexual reproduction in all other
genera is by means of isogamy. We shall now discuss the reproduction of Volvox:
Asexual reproduction in Volvox (Fig 4.32 below) occurs by means of specialised cells,
the gonidia (singular: gonidium), which divide mitotically. The gonidia are larger than
vegetative cells, do not have flagella and are slightly deeper encased in the mucous layer.
Through repeated mitotic divisions, the gonidia divide to give rise to 2-, 4-, 8-, 16-, 32-cell
stages, and so forth. In this manner, a flat layer of cells (plakea) is formed. Following this
stage, mitotic division continues until a spherical daughter colony has been formed inside
the parent colony. The flagella of the cells of the daughter colony are initially directed
inward, and the colony must turn inside out (through the process of inversion) so that the
flagella are directed outward. Daughter colonies are released when the parent colony
breaks open to free the daughter colonies.
Gonidium
Daughter colony
(flagella towards
inside)
Source:
2-cell
stage
4-cellstage
Inversion
After inversion
(flagella toward
outside)
Van den Hoek, C, Mann, DG & Jahns, HM. 1995. Algae: An introduction to phycology.
Cambridge: Cambridge University Press: 360, Fig 21.6.
Figure 4.37: Asexual reproduction in Volvox.
67
Study unit 4
In the case of sexual reproduction (Fig 4.33), a cell—the antheridium (which is similar
to the gonidium)—undergoes repeated mitotic divisions (as in asexual reproduction)
until the 32-64-cell stage has been reached and a packet of sperm cells has been
formed in the process. In female colonies, a cell which is similar to a gonidium acts as
an oogonium, giving rise to a single egg cell. The packet of sperm cells is released as a
whole into the water and swims toward a female colony. At the entrance to the egg cell,
the packet of sperm cells disintegrates, and one sperm cell fertilises the egg cell to
form a diploid zygote. The zygote enlarges, often turning orange, and develops a
spinous wall for protection and survival in unfavourable conditions.
Sperm cells
Antheridium
Oogonium
Source:
Egg celll
Van den Hoek, C, Mann, DG & Jahns, HM. 1995. Algae: An introduction to phycology.
Cambridge: Cambridge University Press: 360, Fig 21.6.
Figure 4.38: Sexual reproduction in Volvox.
68
Zygote (2n)
Study unit 4
Practical: Motile colonies
Study the microscope slides of the Volvocaceae colonies showing primitive to
advanced characteristics and supply the photos below with labels:
•
Gonium and Gonium flagella: Distinguish between individual Gonium cell,
cell wall, pyrenoid with starch, colony (coenobium), flagella
https://www.flickr.com/photos/microagua/27736037969
Figure 4.39: Light microscope photo of Gonium.
•
Pandorina: Distinguish between individual Pandorina cell, cell wall, flagella,
pyrenoid with starch, colony (coenobium), mucous sheath
https://live.staticflickr.com/7550/15334855043_1518b76ce5_b.jpg
Figure 4.40: Light microscope photo of Pandorina.
69
Study unit 4
•
Platydorina: Distinguish between individual Platydorina cell, cell wall, flagella,
colony (coenobium), mucous sheath, lobed pole, rounded pole, pyrenoid with
starch
https://alchetron.com/Platydorina
Figure 4.41: Light microscope photo of Platydorina.
•
Eudorina: Distinguish between individual Eudorina cell, cell wall, flagella
(difficult to observe), pyrenoid with starch, colony (coenobium), mucous sheath
https://www.inaturalist.org/guide_taxa/823347
Figure 4.42 Light microscope photo of Eudorina.
70
Study unit 4
•
Pleodorina: Distinguish between individual Pleodorina cell, cell wall, flagella
(difficult to observe), pyrenoid with starch, colony (coenobium), mucous sheath,
smaller cells at anterior pole, larger cells at posterior pole.
https://www.inaturalist.org/guide_taxa/1125891
https://freethoughtblogs.com/fierceroller/?p=2591
Figure 4.43: Light microscope photos of Pleodorina.
71
Study unit 4
•
Volvox: Distinguish between individual Volvox cell, colony (coenobium),
mucous sheath, daughter colonies.
https://upload.wikimedia.org/wikipedia/commons/thumb/1/1c/Mikrofoto.de-volvox-4.jpg/320pxMikrofoto.de-volvox-4.jpg
•
•
Volvox, asexual reproduction: Distinguish between gonidium and inversion.
Photos by S Janse v Vuuren
•
Volvox, sexual reproduction: Distinguish the zygotes.
Photo by S Janse v Vuuren
Figure 4.44: Light microscope photos of Volvox.
72
Study unit 4
Aggregate forms:
Aggregate forms are colonial forms which reproduce asexually in a specific manner.
During asexual reproduction, the cell contents divide repeatedly and give rise to a
number of motile zoospores. For a while, these zoospores swim around in the parent
cell with the aid of flagella. Then they lose their flagella and arrange themselves
(aggregate) into a miniature colony within the parent cell. Examples of aggregate forms
are Pediastrum and Hydrodictyon..
Practical: Aggregate forms
Study the permanent microscope slides of Pediastrum sp. Compare your slides with
the photograph below and add the following labels to the colony on the photo.
Labels: Individual cell of colony, cell wall, chloroplast, pyrenoid, starch
https://www.flickr.com/photos/microagua/28466362720
Figure 4.45:
Pediastrum.
Diagrammatic representation to illustrate the structure of
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Study unit 4
Study the permanent microscope slides of Hydrodictyon sp, also known as the water
net. Make a labelled drawing to illustrate the structure of the colony.
Labels:
Individual cell of the colony, cell wall, chloroplast with pyrenoids and nuclei
Figure 4.46: Diagrammatic representation to illustrate the structure of
Hydrodictyon.
Conjugation forms:
The term “conjugation” refers to the type of sexual reproduction that these organisms
undergo. Conjugational forms may be unicellular or unbranched filaments. The most
well-known representative is probably Spirogyra, an unbranched filament (Fig 4.34
below). Outside the cell wall, there is usually a thick layer of mucous (which imparts a
characteristic slimy feel to the alga). A thin layer of peripheral cytoplasm is situated
inside the cell wall. From this cytoplasm, cytoplasmic strands extend to the centre of
the cell. Where these cytoplasmic strands join, a mass of cytoplasm is formed in which
the nucleus is suspended. The interior of the cell consists of a large, central vacuole.
The most salient characteristic of Spirogyra is the helical chloroplast which contains a
large number of pyrenoids.
During the sexual reproduction of Spirogyra, scalariform and lateral conjugation can
take place. In scalariform conjugation (Fig. 4.47 below), two filaments lie next two each
other, encased in mucous. Papillae are extended that finally connect to form a
conjugation tube. The contents of opposite cells round off to form gametes. At this
point, the conjugation tube opens, and by means of amoeboid movement, one gamete
moves through the conjugation tube to the other gamete, where fertilisation takes place
(anisogamy), or both gametes move toward each other and fertilisation takes place in
the conjugation tube (isogamy). A diploid zygote is formed and it develops a thick,
resistant wall. The zygote with the thick wall is called a zygospore. The zygospore
undergoes meiosis, and a cell with four haploid nuclei is formed. Three of these nuclei
disintegrate so that the cell ends up with only one haploid nucleus. The filament grows
further through the process of mitosis.
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Study unit 4
Source: Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 196, Fig 5.17.
Figure 4.47: Scalariform conjugation in Spirogyra.
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Study unit 4
Individual activity
Visit the webpage below:
http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopyuk.org.uk/mag/wimsmall/bacdr.html
To see the structure of Spirogyra, go to the bottom of this webpage and click on
“green algae”. At the bottom of that page, you will find a colour photograph and
description of Spirogyra. Below the description, there is a link: “Click to see how this
conjugation occurs in Spirogyra”. If you click on this link, you will see beautiful colour
photographs of scalariform conjugation.
In lateral conjugation, only one filament is involved and two adjacent cells in the same
filament conjugate (Fig. 4.48).
Cross wall
Zygospore
(2n)
Source:
Dekker, JG (Ed). 1989. Advanced biology. Kenwyn: Juta: 364, Fig 8.18.
Figure 4.48: Lateral conjugation in Spirogyra.
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Study unit 4
Practical: Conjugation forms (Spirogyra and Cosmarium)
Structure of Spirogyra
Study the permanent microscope slides labelled Spirogyra scalariform conjugation.
Focus on the single filament on the right-hand side of the microscope field.
Distinguish the following: cell wall, cytoplasm (three types: peripheral cytoplasm,
cytoplasm mass around the nucleus, cytoplasm threads), large central vacuole,
nucleus, helical chloroplast, pyrenoids, starch, cross wall
Figure 4.49: Diagrammatic representation to illustrate the structure of Spirogyra.
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Study unit 4
Sexual reproduction (conjugation) of Spirogyra
Scalariform conjugation in Spirogyra
Study the permanent microscope slides labelled Spirogyra scalariform conjugation.
Distinguish the following: - thallus, + thallus, conjugation tube, + gamete, +
gametangium, - gamete, - gametangium, zygote (zygospore).
Figure 4.50:
Spirogyra.
Diagrammatic representation of scalariform conjugation in
Lateral conjugation in Spirogyra
Study the process of lateral conjugation on the permanent slide provided. Make a
series of fully labelled drawings to illustrate this process. Labels: conjugation tube,
gametangia, gametes and zygote
Figure 4.51: Diagrammatic representation of lateral conjugation in Spirogyra.
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Study unit 4
Individual activity
Study the fresh material of Spirogyra provided and distinguish as many as possible of
the above on the live material. Note the spirally twisted chloroplast.
Structure of Cosmarium
Cosmarium provides us with an example of unicellular conjugation form. Each cell
consists of two halves that are mirror images of one another. Each semi-cell is filled
with a chloroplast, and one or two pyrenoids (which store starch) may occur in the
chloroplast. The nucleus occurs in the area between the two semi-cells (the isthmus).
The invaginations where the cell narrows are called the sinuses. Asexual reproduction
occurs when the nucleus divides mitotically: The semi-cells move away from each
other and each semi-cell forms a new half (mirror image). Sexual reproduction occurs
by means of conjugation, as in Spirogyra. Usually, both gametes move into the
conjugation tube where fertilisation takes place and the zygote is formed (isogamy).
Study the permanent microscope slide and living material of Cosmarium sp. Make a
labelled drawing to illustrate the structure of the cell. Labels: Semi-cells, cell wall,
chloroplast, pyrenoids, sinus, isthmus, nucleus
Figure 4.52: Diagrammatic representation of the structure of Cosmarium.
Sexual reproduction (conjugation) in Cosmarium
Conjugation is very similar to scalariform conjugation in Spirogyra and can be
illustrated by the following line drawing:
Note that fertilization takes place inside the conjugation tube in other words BOTH
gametes are motile and meet each other in the conjugation tube (differ from Spirogyra).
The zygote is formed in the conjugation tube and will develop a thick spiny wall to
survive unfavourable conditions. Such a zygote with a thick resistant wall is known as
a zygospore.
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Study unit 4
Individual activity
Study the photographs below of different stages after conjugation. Label the
following: Cosmarium cell, conjugation tube, young zygote, thick-walled zygospore.
http://www.desmids.nl/info/reproductie/desmids_sexual_reproduction.html
Figure 4.53: Photographs illustrating conjugation in Cosmarium.
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Study unit 4
4.6.2
Class Charophyceae
We shall not discuss the class Charophyceae in detail, but it is important that you
should know about their existence (and complexity). Charophyceae are said to have
possibly given rise to land plants.
Chara
(Fig. 4.53 below) will be used as a representative example of the
Charophyceae. Chara is a macroscopic fresh water alga (up to 30 cm high) which
occurs in clean, hard water (rich in CaCO3). CaCO3 is deposited in the cell walls;
consequently, most species are considerably calcified. Chara has a complex plant
body which exhibits many similarities to land plants. It consists of a central axis that is
divided into internodes and nodes, and garlands of lateral branches are borne on the
nodes. The axis is anchored in the substrate by means of branched rhizoids. The cells
have one nucleus and many chloroplasts. In the middle of the cells, there are large,
central vacuoles.
Chara usually reproduces asexually, but sexual reproduction also occurs and is very
complex. The male reproductive structure is called a globule. The globule is round and
orange when mature. Sperm cells are formed in the globule. The female reproductive
structure is called a nucule; it is oval and houses a single egg cell.
Side brances
Axis
Nucule
Rhizoids
A
Source:
Egg cell
Globule
B
C
Lee, RE. 1999. Phycology. 3rd ed. Cambridge: Cambridge University Press: 204, Fig 5.23.
Figure 4.53: Structure and sexual reproductive organs of Chara.
(A: Structure of Chara; B: Nucule and globule on node; C: Structure of the nucule)
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Study unit 4
Practical: Chara
Individual activity
Study the photo below of the nucule and globule of Chara.
https://d3i71xaburhd42.cloudfront.net/66dbfd2429ad537615159823aa04fef52ff81171/179Figure27-1.png
Published in Cellular Origin, Life in Extreme Habitats and Astrobiology 2015. The
Algae World
Figure 4.54: Nucule and globule (reproductive organs) of Chara.
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Study unit 4
Summary of key points:
•
•
•
•
•
•
•
•
•
•
The bright green colouring of Chlorophyta is due to the presence of the pigment
chlorophyll a and b.
Chlorophyta occur in nearly all kinds of water and have a cosmopolitan
distribution.
Chlorophyta usually store starch around a pyrenoid.
Chlorophyta include a wide variety of organisms – some are immotile; others are
motile with the aid of flagella, amoeboid movements or sliding movements. They
may occur as unicellular, colonial, aggregate, conjugational or (branched or
unbranched) filamentous forms.
The cell covering is mostly in the form of a cell wall.
Chlamydomonas is a very well-known example of a unicellular green alga. It is
encased in a cell wall and has two flagella. Chlamydomonas can reproduce
asexually or sexually.
A series of green algal colonies (classified under the family Volvocaceae) occurs.
The structure of the cells of these colonies is exactly like that of Chlamydomonas,
and the colonies reproduce either asexually or sexually. Depending on their
characteristics, the colonies range from primitive to advanced forms. These
characteristics include the number of cells, the presence or absence of polarity,
the presence or absence of functional differentiation, the presence or absence of
cytoplasmic connections and the type of sexual reproduction that takes place. The
most advanced colony in the series we have discussed is Volvox (there are many
cells; polarity is present; functional differentiation occurs; the cells are connected
via their cytoplasm, and sexual reproduction occurs through oogamy).
During the asexual reproduction of Volvox, a gonidium divides to give rise to a
daughter colony. The daughter colony undergoes inversion so that the flagella are
directed outward. During sexual reproduction, a packet of sperm is formed
through repeated mitotic divisions of an antheridium. One sperm cell will fertilise
an egg cell of a female colony, forming a diploid zygote with a spinous wall. This
zygote can survive unfavourable conditions.
Types like Spirogyra and Cosmarium reproduce asexually or sexually. Sexual
reproduction occurs by means of conjugation. In the event of scalariform
conjugation, two filaments are involved, while two adjacent cells of the same
filament conjugate in the case of lateral conjugation.
Chara has a multicellular, upright-growing body which exhibits many similarities to
higher plants. The upright-growing axis is divided into internodes and nodes. On
the nodes, lateral branches, as well as the organs responsible for sexual
reproduction (the nucule and globule), are borne.
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Study unit 4
Group activity
Form small groups and make sure you can answer the following questions:
1.
2.
Discuss the characteristics of the Chlorophyta (green algae) under the following
headings:
•
Pigments
•
Storage products
•
Motility/locomotion
•
Cell covering
Briefly discuss the five most important plant body types found in the
Chlorophyta and. Give examples of each.
3.
Make a fully annotated drawing showing the structure of Chlamydomonas.
4.
Discuss the different types of reproduction found in Chlamydomonas.
5.
Discuss the phylogenetic lines, from primitive to advanced, within the
Volvocaceae. In your discussion, use examples of specific genera with primitive
and advanced characteristics.
6.
Describe how asexual and sexual reproduction takes place in the Volvocaceae.
7.
Describe the type of reproduction that Spirogyra undergoes.
8.
Briefly describe the structure and the reproductive structures of Chara.
9.
Describe in detail what you understand under the following terms:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
84
globule
gonidium
inversion
isthmus
conjugation
contractile vacuole
scalariform conjugation
nucule
eyespot
pyrenoid
plakea
zygospore
sinus
lateral conjugation.
Study unit 4
Study section 4.7
Other phyla of algae
Apart from the Euglenophyta, Dinophyta, Bacillariophyta and Chlorophyta (which you
have studied in some detail), there is a large variety of other phyla of algae that we
shall discuss only briefly for the sake of completeness.
4.7.1
Phylum Xanthophyta (= Tribophyta)
•
•
•
•
•
•
•
•
4.7.2
These organisms are also generally known as yellow-green algae.
Organisms belonging to these groups are typically unicellular, but colonial and
filamentous forms also occur.
The cells may be immotile, but if they are motile, locomotion is brought about by
two flagella of unequal lengths.
The Xanthophyta are distinguished from the Chlorophyta mainly on the basis of
pigments and storage products, rather than on morphological characteristics.
Differences between the Xanthophyta and Chlorophyta include the following:
The Xanthophyta have yellow, yellow-green or yellow-brown chloroplasts (as
opposed to the bright green colour of the Chlorophyta).
The storage products are oil droplets or chrysolaminarin – starch and pyrenoids
are absent.
The cell walls of the Xanthophyta are of pectin, rather than cellulose or
glycoproteins.
Xanthophyta lack chlorophyll b but contain chlorophyll a and c.
Phylum Chrysophyta
•
The Chrysophyta include unicellular, colonial or (less commonly) filamentous
organisms.
•
The Chrysophyta (sometimes also called the golden or golden-brown algae) are
a group of organisms under which the diatoms were previously classified. There
are therefore many similarities between the diatoms (Bacillariophyta) and
representatives of the Chrysophyta.
Similarities include the following:
•
•
•
Silica plays an important role in the formation of cell coverings. In the case of
diatoms, the frustule consists of silica; in the case of Chrysophyta the cell
covering often consists of silica scales. The Chrysophyta also form resistant
spores (statospores) that consist of silica and are capable of surviving
unfavourable conditions.
The storage product of the Chrysophyta and diatoms is the same, namely
chrysolaminarin.
Apart from chlorophyll a and c, the pigment fucoxanthin is present in both
groups and it overshadows the chlorophyll, imparting to the algae belonging to
both groups a yellow to gold colour rather than green.
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Study unit 4
Differences between the Bacillariophyta and the Chrysophyta) include the following:
•
•
4.7.3
Cells belonging to the Chrysophyta can move with the aid of two flagella of
unequal lengths – the one flagellum is short and directed backwards, while the
other is long. Some are also capable of executing amoeboid movements. The
vegetative cells of diatoms, on the other hand, are immotile.
The silica frustule of diatoms is unique, and this was one of the main reasons
why this group was elevated to a separate phylum (Bacillariophyta).
Phylum Prymnesiophyta (= Haptophyta)
•
•
The Prymnesiophyta are primarily unicellular organisms.
Just like the diatoms, the Prymnesiophyta were previously classified under the
Chrysophyta but were also elevated to an independent phylum.
Similarities with the Chrysophyta include the following:
•
The Chrysophyta and Prymnesiophyta (and the Bacillariophyta) have the same
storage product, namely chrysolaminarin.
•
The Prymnesiophyta also possess the pigment fucoxanthin, which overshadows
chlorophyll pigments and gives the cells a yellow-brown colour.
•
As in the Chrysophyta, scales are the most common cell covering found in the
Prymnesiophyta. However, in the case of Prymnesiophyta, the scales consist
mostly of calcium carbonate (calcite) and are called coccoliths. This is, therefore,
an important difference between the scales of the Chrysophyta (silica) and the
Prymnesiophyta (calcite).
Differences (and thus the main reasons why the Prymnesiophyta were elevated to an
independent phylum) are the following:
•
•
•
4.7.4
Phylum Cryptophyta
•
•
•
86
The presence of a haptonema. The haptonema is an appendage to the cell that
exhibits similarities to flagella (but also great differences). It occurs between the
two equally long flagella and can roll up or extend and, therefore, appears shorter
or longer than the flagella. The internal structure of the haptonema differs from
that of flagella – it does not have a 9+2 arrangement of microtubules (like
flagella). Haptonemas occur ONLY in Prymnesiophyta and play a role in rapidly
moving away, steering the swimming cell, attaching the cell to a substrate and
gathering prey.
As mentioned previously, the Prymnesiophyta also have two equally long
flagella, unlike the unequal flagella of the Chrysophyta.
Another reason is the presence of coccoliths, as described above.
This is a relatively small group of unicellular organisms.
The colour of the cells may vary considerably: from blue-green to olive-brown, or
even red. The blue-green and/or red colour is due to the presence of
phycobiliproteins (phycocyanin and phycoerythrin), which also occur in the
cyanobacteria. It is the only flagellated group of algae that contains phycobiliproteins.
Unlike in the cyanobacteria, the phycobiliproteins of the Cryptophyta are not found in
phycobilisomes but occur freely within the thylakoids.
All the Cryptophyta cells can move with the aid of flagella of slightly unequal lengths.
Study unit 4
4.7.5
Phylum Phaeophyta
•
•
•
•
4.7.6
The Phaeophyta are also known as the brown algae.
They are nearly exclusively marine organisms, although a few freshwater types
also occur.
The organisms may vary from microscopic unicellular and filamentous forms to
giant thallus forms (for example, kelp) which may be several metres long.
In the larger forms, there is a considerable degree of organ, tissue and cell
specialisation.
Phylum Rhodophyta
•
•
•
•
The Rhodophyta are mainly multicellular marine organisms.
They are generally known as red algae (the colour ranges from red or brown to
olive green). The red colour comes from phycoerythrin which overshadows
chlorophyll. Phycoerythrin and phycocyanin occur (as in the cyanobacteria) in
phycobilisomes, which are associated with the thylakoids.
The Rhodophyta include unicellular and filamentous forms, as well as complex
filamentous aggregations.
There are no motile cells in this phylum, and their sexual reproduction is very
complex. Many of these algae are used as human food (see the section on their
biological importance further on).
Apart from the above 10 phyla of algae which you have studied to a greater or lesser
extent, other phyla are also sometimes distinguished (for example, the
Eustigmatophyta, Prasinophyta and Glaucophyta). These are rather small groups of
organisms, and we shall not study them for the purposes of this module. Classification
(taxonomic) systems also vary greatly, and some of these phyla are closely related to,
and sometimes included in, those that you have already studied.
Summary of key points
•
The names of different algal phyla and their popular names are given in the
following table:
PHYLUM
POPULAR NAME
Cyanophyta
Blue-green "algae"
Chlorophyta
Green algae
Bacillariophyta
Diatoms
Dinophyta
Dinoflagellates
Xanthophyta
Yellow-green algae
Chrysophyta
Golden or golden-brown algae
Phaeophyta
Brown algae
Rhodophyta
Red algae
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Study unit 4
The colour of most of these algae comes from additional photosynthetic pigments that
overshadow the chlorophyll pigments, and the colour of the algae is often reflected in
the names of the phyla (“Chloro” = green; “Rhodo” = red, et cetera). Although the
Cryptophyta and Prymnesiophyta are not named after their colours in the spoken
language, they often also assume specific colours. The Cryptophyta often appear bluegreen, olive-brown or red due to the presence of phycobiliproteins (as in the
Cyanobacteria), while the Prymnesiophyta are usually brownish because of the
presence of fucoxanthin.
•
•
•
•
•
Long ago, the Bacillariophyta and Prymnesiophyta were classified under the
Chrysophyta because of the marked similarities between these groups. There
are, however, also important differences (for instance, the silica frustule of the
diatoms and the haptonema of the Prymnesiophyta). For this reason, they were
elevated to independent phyla.
The Xanthophyta are structurally very similar to the Chlorophyta but differ from
them mainly in respect of pigments and storage products.
A haptonema occurs in the Prymnesiophyta, and although it is an appendage of
the cells which, like flagella, is used for locomotion, its internal structure differs
from that of flagella.
The phycobiliproteins of the Cryptophyta are, unlike those of the cyanobacteria,
not found in phycobilisomes. The pigments (phycocyanin and phycoerythrin)
occur freely in the thylakoids.
The Phaeophyta and Rhodophyta are mainly marine organisms, and some are
macroscopic with very complex structures and lifecycles.
Individual activity
Self-evaluation:
1.
Discuss the colours that algae belonging to different phyla exhibit as a result of
their unique photosynthetic pigments. Name the pigments in your discussion.
2.
Explain why the Bacillariophyta and Prymnesiophyta were previously classified
under the Chrysophyta and also give the reasons why they were elevated to
independent phyla.
3.
Discuss, in the form of a table, the similarities and differences between the
Prymnesiophyta and the Chrysophyta.
4.
Discuss, in the form of a table, the similarities and differences between the
Bacillariophyta and the Chrysophyta.
5.
Explain briefly what a haptonema is and how it differs from flagella. Also, name
the functions of the haptonema.
6.
Briefly explain each of the following terms:
•
•
•
88
haptonema
coccoliths
statospores
Study unit 4
Study section 4.8
The biological importance of algae
It is practically impossible to summarise the importance of algae in one paragraph (or
even a whole number of pages). As you have seen in the previous study unit, their
roles vary from acting as “the basis of the food chain” to being the source from which
the most advanced of chemical products is extracted. We shall now attempt to give a
very concise overview of the importance of algae, but you must realise that their true
importance extends far wider than this.
The advantages of algae include the following:
•
•
•
•
•
•
•
As we have mentioned in the previous study unit, cyanobacteria form the basis of
the aquatic food chain, but all algae fulfil this function too. Diatoms are, for
instance, an important source of food for fish.
Up to 40 percent of the mass of a diatom consists of oil which can be converted
into cod-liver oil, a rich source of vitamins for humans.
Oil from algae has in the past also contributed to petroleum deposits. There are
also currently projects underway to process oil obtained from diatoms (in culture)
and Chlamydomonas into a substitute fuel for diesel.
Diatoms also have other, more direct, industrial applications. In the past, the
frustules of millions of diatoms that died accumulated on the floor of the ocean,
forming so-called diatomaceous earth. These deposits have accumulated to form
a layer that may be hundreds of metres deep and are mined nowadays. The
mined product consists of a light, porous, powdery material that contains
approximately six billion diatom frustules per litre. It has an exceptionally high
melting point (1,750°C) and is insoluble in most acids and other fluids. These
characteristics make it ideal for a variety of industrial and domestic applications,
including various types of filters. Diatomaceous earth is used in the sugar
industry to refine sugar, in swimming pool filters, in silver and metal polishes, in
toothpaste and in the manufacturing of light-reflecting paint. This paint is used on
the number plates of motor vehicles and to mark roads. Diatomaceous earth is
also used in a variety of isolation materials.
Many sea algae are edible and are rich in vitamins and iodine. In Eastern
countries, the use of sea algae as food is as common as barbeque (“braaivleis”)
is in South Africa. Until such time as more people have developed a taste for sea
algae, they will, in Western countries, be used mainly for the production of
alginate, carrageen and beta carotene, which act as stabilisers, thickeners and
food colorants. In these forms, they do end up on our dinner tables!
Dunaliella salina (a green alga) is being cultivated as a source of beta carotene.
Beta carotene is used as an orange colorant and also as a vitamin A supplement.
Apart from vitamin C, Chlorella (a green alga) contains most of the vitamins
necessary for human nutrition. It is also an important protein source, and
because Chlorella is so easy to cultivate, it may become an important source of
proteins in many parts of the world, especially in view of the food shortage we are
facing. Chlorella has also been investigated as a potential source of oxygen for
submarines.
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Study unit 4
•
•
•
•
•
•
•
•
A variety of commercially produced products (such as ice cream, salad dressing,
beer, jelly beans, paints, paper, textiles, toothpaste, ceramic ware and floor
polish) contain alginate which is produced by kelp and other brown algae.
Carrageen (produced by red algae) is used mainly to create viscosity and is used
in the pharmaceutical industry and cosmetics as well as in substances like paint
and ink. Carrageen is also used in ice cream and puddings.
One of the most important substances produced by algae (red algae) is agar. It is
used all over the world in laboratories and medical institutions as a culture
medium for bacterial cultures. If various nutritional substances are added, it can
also be used as a culture medium in which to grow plant and animal cells. It is
also used in baked products to retain moisture and as a basis for cosmetics.
Brown algae also yield other useful substances. Many marine algae, and
especially kelp, contain iodine concentrations of up to 20,000 times that of the
surrounding seawater. Dried kelp has, therefore, been used in the treatment of
thyroid problems that are due to an iodine deficiency.
Kelp is rich in nitrogen and sodium and is used as fertiliser.
Kelp is also used as feed for livestock.
Red algae contain a number of substances that are of great medical value. More
than 20 types are used in de-worming agents and medication for diarrhoea as
well as in the treatment of cancer.
Lichens are formed through a symbiotic relationship between cyanobacteria (or
green algae) and fungi. They are sensitive to air pollution, and the presence of
lichens is usually an indicator that pollutants (especially SO2) are absent.
As in the case of cyanobacteria, eukaryotic algae may however also be detrimental,
and many of the arguments listed under the cyanobacteria also apply to algae:
The disadvantages of algae include the following:
•
Fish farmers often lose large numbers of salmon and cod if there are high
concentrations of the diatom Chaetoceros in the water. This diatom has long,
hollow spines that break off and penetrate the gills of the fish. This causes
bleeding and disrupts gas exchange.
•
•
•
Blooms may occur, imparting bad smells and tastes to the water.
Algae interfere with water purification and increase the cost thereof.
Dinoflagellates are responsible for red tides in the ocean. The toxins released
during red tide poison shellfish that feed on algal cells, and if humans or animals
then eat the shellfish (say mussels), they are also poisoned.
If such a bloom of dinoflagellates comes to an end and dies off, the oxygen in the
water is consumed by the decomposition process, and crayfish (which cannot
swim away to oxygen-rich water) move to shallower water where the waves add
a little oxygen to the water. Then, when the water subsides during low tide, they
are exposed and die on the beaches.
•
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Study unit 4
Group activity
Form small groups and make sure you can answer the following questions:
1.
Discuss the biological importance of algae under the following headings:
2.
•
Algae as source of nutrition
•
Algae as source of commercial products
•
Toxic algae
Discuss the following statement: “Algae are essential in our daily existence.”
3.
Name a few advantages of algae.
4.
Name a few disadvantages of algae.
5.
Discuss the following statement: “Algae may be beneficial or detrimental.”
Reflection
Make sure that you have fully reached the outcomes set at the beginning of this
study unit.
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Study unit 5
Study unit 5
MOSSES
Study hours
You will have to spend at least 17 hours to master the contents of this study unit.
Study material
Most of the study material can be found in this study guide. Additional information will be
uploaded on eFundi as part of the PowerPoint presentations.
Reflection / Overview of the study unit
In this study unit, we shall study the similarities between the more advanced algae and
mosses, as well as those between mosses and vascular plants. We shall concentrate on
adaptations of mosses to survive on land, and we shall study representatives of two of the
three major groups of mosses in detail.
Study outcomes
Once you have completed this study unit, you should be able to:
•
•
•
•
•
•
•
•
92
Describe the adaptations that enable plants to live on land;
Discuss similarities and differences between algae, mosses and vascular plants;
Explain the alternation of generations in mosses by means of a general lifecycle;
Explain how mosses reproduce asexually and sexually;
Discuss the ecological importance of mosses;
Point out the differences and similarities between the 3 moss phyla, paying particular
attention to the structure of the gametophyte plant, the structure of the sporophyte
plant and stadia in the lifecycles;
Give descriptions and make schematic representations of the lifecycles of
Marchantia and Polytrichum;
Identify, classify and make fully labelled drawings of different structures found in
Hepaticophyta and Bryophyta representatives by studying microscope slides as well
as living material.
Study unit 5
Introduction:
In the previous two study units, you were introduced to the blue-green bacteria and the
algae --- organisms that belong to the kingdoms Bacteria and Protista, respectively. In this
study unit, we shall encounter organisms belonging to the kingdom Plantae for the first
time.
In the previous study unit, you have studied representatives of the class Charophyceae,
and you will remember that Chara has a multicellular, upright-growing thallus, with an axis
that is divided into internodes and nodes. Researchers have come to the conclusion that
the Charophyceae are the organisms that are most closely related to land plants. In the
following paragraphs we shall, amongst other things, discuss reasons for this conclusion.
Terrestrial plants are well adapted to survive on land. Some of the most important
characteristics of land plants include the presence of apical meristems, heteromorphic
alternation of generations, spores that are produced in sporangia, multicellular gametangia
and multicellular dependent embryos.
Individual activity
Study the concept of heteromorphic alternation of generations in the following basic plant
life cycle EXTREMELY well. This life cycle is EXTREMELY IMPORTANT in order to
understand the rest of the work in this module.
Diploid (2n) sporophyte generation
Haploid (n) gametophyte generation
MEIOSIS
Spores
Sporogenous cells
(spore mother cells)
mitosis
Sporophyte
Gametophyte
Antheridium
Archegonium
mitosis
Sperm cell
Embryo
mitosis
Egg cell
Zygote
FERTILIZATION
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Study unit 5
Study section 5.1
An introduction to the Bryophytes (mosses)
Bryophytes (the term used for all mosses) were among the first land plants: The earliest land
plants came about when certain species of green algae were stranded during periods of
seasonal drought. These algae had certain adaptations that helped them to survive the drought
conditions, namely:
•
•
•
vertical stems above ground,
underground stems that were specialised to absorb water and nutrients, and
an ingredient, sporopollenin, in the zygotes of Charophyceae that prevented them from
drying out.
Other characteristics that land plants developed over time to resist desiccation include the
following:
•
•
A fatty cuticle that retards water loss
The gametangia and sporangia became multicellular and surrounded by a sterile cell
jacket.
•
Plant zygotes developed into multicellular embryos
Although the organisms were able to live on land, their sperm cells still needed water to reach
the egg cell. Therefore, bryophytes are mostly limited to damp habitats, or if they grow in dry
habitats, water MUST be present for the process of reproduction. Bryophytes are nonvascular
plants (with no xylem or phloem) and can, therefore, only reach a limited height. Although the
body of bryophytes is often called a "thallus" (as in the case of algae), they do have stem-like
structures, leaf-like structures and root-like structures (rhizoids). However, these are not true
roots, stems and leaves because they do not possess vascular tissue. Although vascular tissue
is absent, there are water-conducting cells (hydroids) and nutrient-conducting cells (leptoids).
Systematists are currently debating the boundaries of the kingdom Plantae. Currently,
organisms are considered to be plants (belonging to the kingdom Plantae) if they produce
embryos. But some botanists propose that the boundaries of the kingdom should be broadened
to include the Charophyceae and that the new kingdom should be called the Streptophyta.
Others propose an even wider definition of plants which includes the Chlorophyceae, and they
suggest that this kingdom be called the Viridiplantae.
In the following paragraphs, the bryophytes ("mosses") will be studied as an intermediary group
between the algae (especially Charophyceae) and vascular plants. For this reason, it will be
crucial to note the similarities and differences between bryophytes and algae on the one hand,
and bryophytes and vascular plants on the other.
Important similarities between green algae and mosses that suggest the green algae as a
precursor of mosses include the following:
94
•
both plants and green algae can grow in water, so this is a characteristic they share
(however, most algae grow exclusively in water, whereas plants grow either in the soil or in
water depending on the variety)
•
•
•
•
•
•
•
Cell walls of most green algae and plants contain 20-25% cellulose
Presence of chlorophyll a and b in both.
Arrangement of thylakoids – in groups of 2-6 with stacked grana.
Starch as storage product.
Alternation of generation present in life cycles.
Water is needed for fertilisation.
Absence of vascular tissue.
Study unit 5
In spite of the similarities between algae (specific the Charophyceae) and mosses listed
above, there are also important differences between algae (in general) and mosses.
These differences are summarised in the table below:
Algae
Mosses
Usually live in water
Usually live on land
Spores distributed by water
Spores distributed by wind
No water-conducting elements
Simple water-conducting cells (hydroids) and
food-conducting cells (leptoids) occur (no
xylem or phloem)
Usually no water and nutrient-absorbing Rhizoids anchor and can absorb water and
tissue
nutrients
Mostly unicellular, colonial or filamentous Only one phase (when present) in some
forms --- only most advanced groups are bryophytes is filamentous (the protonema)
parenchymatous
Gametangia are unicellular or a group of Gametangia are always
cells but are never surrounded by vegetative vegetative cells
cells
surrounded by
Female gametangia are called oogonia
Female gametangia are called archegonia
Gametophyte often a thallus
Gametophyte often an axis with leaves
Sporophyte
and
gametophyte
independently of each other
live Gametophyte lives independently, but
sporophyte is dependent on the gametophyte
Embryo absent
Embryo present
Asexual reproduction by means of spores
Asexual reproduction by means of gemmae
(brood bodies)
Although the Charophyceae are closer related to land plants (mosses and vascular plants)
than the other groups of algae, there are certain key characteristics of land plants that are
absent from the Charophyceae, namely:
•
•
•
•
Apical meristems,
Spores with resistant walls produced in sporangia,
Multicellular gametangia, and
Multicellular embryos.
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Study unit 5
Important information
•
In bryophytes the gametophyte plant (n) is the dominant phase of the lifecycle (in
other words, the phase that can live independently).
•
The gametophyte plants produce gametes (sperm cells and egg cells) in gametangia.
Sperm cells are produced in an antheridium, and egg cells are produced in an
archegonium.
•
A sperm cell is released and fertilises an egg cell in the archegonium, and a diploid
zygote (2n) is formed.
•
The formation of the zygote is the end of the haploid gametophyte generation and the
beginning of the diploid sporophyte generation.
•
The zygote grows out of the archegonium (on top of the gametophyte plant) and
forms a sporophyte plant which is dependent on the gametophyte plant for water and,
to a lesser extent, for nutrients as well.
•
The sporophyte plant (2n) produces spores (n) in the sporangium (2n) through the
process of meiosis.
•
The formation of spores constitutes the beginning of the haploid gametophyte
generation and the end of the diploid sporophyte generation.
•
Spores germinate and produce a protonema, which in turn will give rise to
gametophyte plants.
During the sexual reproduction of bryophytes (described above), there is, therefore, a
clearly defined alternation between the haploid gametophyte generation and the diploid
sporophyte generation.
Some bryophytes can also reproduce asexually through fragmentation or through the
formation of brood bodies (or gemmae).
Individual activity
Study the ecological importance of mosses on the following website, in conjunction with
information given in class:
https://science.jrank.org/pages/1051/Bryophyte-Importance-mosses.html
This can be asked in the exam.
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Study unit 5
Study section 5.2
The classification of mosses
Three phyla of bryophytes can be distinguished, popularly known as the liverworts, the
hornworts and the (leafy) mosses. Of these three groups, the liverworts are most closely
related to green algae, and they are probably the group that first lived on land.
We shall now briefly discuss each of the three phyla.
PHYLUM BRYOPHYTA (MOSSES)
By this time, you would have realised that the term "bryophytes" does not only refer to
representatives of the phylum Bryophyta but to the liverworts, hornworts and the mosses.
Representatives of the phylum Bryophyta differ from liverworts and hornworts in that the
gametophyte plants grow vertically instead of horizontally. They are also more "leafy"
(hence the name "leafy mosses" that was often used previously), with the "leaves" usually
only a single cell layer thick. The gametophyte plants are usually 1 to 15 cm high (although
some may be considerably higher). The sporophyte plants are carried on the gametophyte
plants and are visible with the naked eye (they may be up to 20 cm long). Although they are
green and photosynthesising when young, they turn brown to reddish-brown when they are
ready to release spores.
Photo taken in Scotland by S Janse v Vuuren
Figure 5.1: Leafy gametophyte plants of Phylum Bryophyta with sporophyte
plants on top.
There are three main classes in the Phylum Bryophyta. We shall only discuss
representatives of the most abundant class, the Bryopsida, in more depth. The genera
Mnium or Polytrichum will be used as a representative examples of this class.
Because you have already mastered the basic lifecycle of bryophytes, we shall only
highlight certain aspects here, but it is EXTREMELY important that you must be able to
relate the lifecycle of mosses in the examination.
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Study unit 5
The gametophyte plants of mosses differ from those of liverworts and hornworts in that they
grow vertically and are not flat (Fig 5.9). The gametophyte plant consists of a stem-like
structure around which leaf-like structures are arranged spirally. The gametophyte plant is
anchored in the soil by means of rhizoids.
Like Marchantia, the lifecycle of which you have also studied, Mnium and Polytrichum are
dioecious/unisexual. In other words, there are male and female gametophyte plants.
During sexual reproduction, an antheridial head develops at the top of the male
gametophyte plant. The antheridial head is surrounded and protected by leaf-like
structures. The antheridia with sperm cells develop in the head. Among the antheridia,
multicellular hair-like structures (the paraphyses) can be found. These paraphyses absorb
water to keep the antheridial head damp.
Photos taken by S Janse v Vuuren
Figure 5.2: Longitudinal section through an antheridial head of Mnium.
•
At the top of the female gametophyte plant, an archegonial head develops, which is also
surrounded and protected by leaf-like structures (Fig. 5.11). It consists of a number of
archegonia, each containing an egg cell. Paraphyses occur among the archegonia.
Photos taken by S Janse v Vuuren
Figure 5.3: Longitudinal section through an archegonial head of Mnium.
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Study unit 5
When the antheridia burst, the sperm cells are released and transported to the female
plants with the aid of rain drops. They end up in the archegonial head, and one sperm cell
fertilises an egg cell, which leads to the formation of a diploid zygote (within the
archegonium).
The formation of the zygote marks the beginning of the sporophyte generation and the end
of the gametophyte generation.
The development of the sporophyte is similar to the development of the sporophyte plant in
the liverworts.
The sporophyte also consists of a foot which is embedded in the gametophyte plant and
which absorbs water and nutrients from the gametophyte plant, a long seta and a
sporangium at the tip. Young sporangia are also covered and protected by a calyptra
(remainder of the neck of the archegonium). At a later stage, the calyptra falls off and the
sporangium opens through a small lid (the operculum) which falls off. Under the operculum,
there are teeth-like structures (called the peristome) which aid with the distribution of the
haploid spores (which have been formed through meiosis in the sporangium).
The formation of spores marks the beginning of the gametophyte generation and the end of
the sporophyte generation.
In the case of mosses, these spores each germinate into a protonema (filamentous body;
Fig. 5.12). On the protonema, there are buds that fall off and give rise to new gametophyte
plants.
https://www.flickr.com/photos/stephenbuchan/12059192705/
Stephen Buchen)
(permission
obtained
from
Figure 5.4: Protonema.
Please note:
In none of the mosses you have studied fertilisation can take place without the presence of
water!
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Study unit 5
Practical: Phylum Bryophyta; Genus Mnium or Polytrichum
Individual activity
Study the living material of the gametophyte and sporophyte plants of Polytrichum. Make
sure that you choose a young sporophyte plant where the sporangium is covered by a
calyptra. Make a fully labelled drawing to illustrate the structure of the gametophyte and
sporophyte plants. Supply your drawing with the following labels:
Gametophyte plant (n): Rhizoids, stem-like structure, leaf-like structures
Sporophyte plant (2n): Foot, long seta, sporangium, calyptra (n)
Figure 5.5: Line diagram of a gametophyte and sporophyte plant of Mnium.
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Study unit 5
Again study the living material of Polytrichum, but choose an older sporophyte plant
where the calyptra has been shed and where a red ring is visible on the sporangium. The
red ring of cells is known as the annulus cells and it separates a lid-like structure (the
operculum) from the rest of the sporangium. Make a fully labelled drawing to illustrate the
external structure of the sporangium. Supply your drawing with the following labels:
Seta, sporangium, annulus, operculum, peristome teeth.
Figure 5.6: Line diagram of the external structure of the sporangium of Mnium.
Study the microscope slide of a longitudinal section through the antheridial head and
make a fully labelled drawing of it. Supply your drawing with the following labels:
Antheridial head, leaf-like structures, antheridia, antheridium wall, antheridium stalk,
spermatozoids (or spermatogenous tissue), paraphyses.
Figure 5.7: Line diagram of an antheridial head of Mnium.
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Study unit 5
Study the microscope slide of a longitudinal section through the archegonial head and
make a fully labelled drawing of it. Supply your drawing with the following labels:
Archegonial head, leaf-like structures, archegonium, stalk, venter with egg cell, neck with
neck canal cells, paraphyses.
Figure 5.8: Line diagram of an archegonial head of Mnium.
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Study unit 5
Study the microscope slide of a longitudinal section through the sporangium of
Mnium and compare it with the photograph below. Supply the photograph with the
following labels: Long seta, sporangium, calyptra (if present), operculum, peristome
teeth, spore sac with spores, annulus, columella, apophysis, stomata.
Photo taken by S Janse v Vuuren
Figure 5.9: Line diagram of a longitudinal section through the sporangium of Mnium.
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Study unit 5
PHYLUM HEPATICOPHYTA (LIVERWORTS)
The common name "liverwort", as well as the scientific phylum name "Hepaticophyta" (derived
from the Latin hepaticus, 'liver') comes from the liver-like shape of the thallus of the
gametophyte plant. In the past, the shape of this bryophyte was considered to be a sign that the
plant was possibly suitable for the medical treatment of liver diseases (“doctrines of the
signatures”).
The life cycle of liverworts will be discussed in detail in class. A few key aspects about the
lifecycle are the following:
Note that the gametophyte generation is the dominant (independent) phase in the lifecycle,
while the sporophyte generation is dependent on the gametophyte plant for water and nutrients.
The gametophyte plant is flat, lobed and liver-shaped and is anchored in the soil by means of
unicellular rhizoids which absorb water and nutrients.
During asexual reproduction, cup-shaped hollows (the so-called gemma cups; Fig. 5.1) are
often formed on the upper surface of the thallus. Multicellular gemmae (brood bodies) develop
in these hollows, are released and give rise to new gametophyte plants.
https://www.nps.gov/romo/learn/nature/marchantiaceae.htm
Figure 5.10: Gametophyte plant of Marchantia with gemma cups and gemmae on
the surface.
During sexual reproduction, gametophores (structures that will produce gametes) grow out of
the top of the gametophyte plant. In the case of Marchantia, there are separate male and female
gametophyte plants (in other words, Marchantia is dioecious or unisexual).
From the male gametophyte plant, an antheridiophore (male gametophore) develops (Fig. 5.2).
The antheridiophore consists of a "stalk" and "head". The head is flattened and disk-shaped,
and a whole number of antheridia are formed in its upper layers. The sperm cells develop in the
antheridia. Between the antheridia there are special cells that absorb water to keep the
antheridiophore damp. At the same time, these cells exert pressure on the antheridia so that
they burst open and release the sperm cells, which swim around in the head of the
antheridiophore.
A
B:
C
A: https://en.wikipedia.org/wiki/File:Marchantia_antheridiophores_preserved.jpg
B, C: Photos taken by S Janse v Vuuren
Figure 5.11: A: Antheridiophore on the living gametophyte plant of Marchantia.
B: Longitudinal section through an antheridiophore; C: A single antheridium.
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Study unit 5
Similarly, an archegoniophore (female gametophore; Fig. 5.3) develops from the female
gametophyte plant. The archegoniophore also has a "stalk" and "head". The head of the
archegoniophore is, however, umbrella-shaped. Archegonia are formed under the umbrellashaped head and literally hang from the head. An egg cell is formed in each archegonium.
A
B
A: https://www.nps.gov/romo/learn/nature/marchantiaceae.htm
B: Photo taken by S Janse v Vuuren
Figure 5.12: A: Archegoniophore on the living gametophyte plant of Marchantia.
B: Longitudinal section through an archegoniophore.
When it rains and a drop of water falls onto the antheridiophore, drops of sperm-laden water
splash out, some landing on the soil and on the archegoniophore of a female plant. The sperm
cell swims into the neck of the archegonium and fertilises the egg cell. A diploid zygote is then
formed within the archegonium.
The formation of the zygote marks the beginning of the sporophyte generation and the end of
the gametophyte generation.
The zygote (2n) divides by means of mitosis and gives rise to a sporophyte plant (2n).
Sporophyte plant
A:
B:
A: https://www.flickr.com/photos/gjshepherd/2883149186
B: Photo taken by S Janse v Vuuren
Figure 5.13: A: Archegoniophore with sporophyte plants hanging underneath the
umbrella-shaped head; B: Longitudinal section through 3 sporophyte plants.
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Study unit 5
As the developing sporophyte plant grows and enlarges, the neck of the surrounding
archegonium breaks off, and the neck of the archegonium develops into a calyptra, which
covers and protects the sporophyte plant.
The sporophyte plant (Fig. 5.4) has a foot, which is embedded in the gametophyte plant
and which absorbs water and nutrients from it, a short stalk (the seta) and a sporangium. It
is still found at the underside of the umbrella-like head of the archegoniophore (because it
has developed within an archegonium).
Meiosis takes place in the sporangium, and haploid spores are formed. This marks the
beginning of the gametophyte generation and the end of the sporophyte generation.
Apart from the haploid spores, there are also diploid elaters (elongated cells) inside the
sporangium. Unlike the spores, they are not the product of meiosis (therefore they are not
haploid). The elaters make twisting movements when the sporangium dries out and this
helps with the distribution of the spores.
The spores are released and germinate, again giving rise to gametophyte plants.
Practical: Phylum Hepaticophyta; Genus Marchantia
Individual activity
Study the living material of Marchantia. Make a fully labelled drawing to illustrate the
structure of the gametophyte plant. Supply your drawing with the following labels:
Dichotomously branched thallus, growth point, pores (dorsal), brood bodies with
gemmae (dorsal), scales and rhizoids (ventral).
Figure 5.14: Line drawing of the gametophyte plant of Marchantia.
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Study unit 5
Study the microscope slide of a longitudinal section through the antheridiophore and
make a fully labelled drawing of it. Supply your drawing with the following labels:
Disc-shaped head, stalk, antheridia on dorsal side, antheridium cavity, antheridium
wall, spermatozoids.
Figure 5.15: Line drawing of the antheridiophore of Marchantia.
Study the microscope slide of a longitudinal section through the archegoniophore and
make a fully labelled drawing of it. Supply your drawing with the following labels:
Umbrella-shaped head, stalk, archegonia on ventral side, venter with egg cell, neck with
neck canal cells.
Figure 5.16: Line drawing of the archegoniophore of Marchantia.
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Study unit 5
Study the microscope slide of a longitudinal section through the sporophyte plant and
make a fully labelled drawing of it. Supply your drawing with the following labels: Foot
(2n), short seta (2n), sporangium (2n), calyptra (n), spores (n), elaters (2n).
Figure 5.17: Line drawing of the sporophyte plant of Marchantia.
PHYLUM ANTHOCEROPHYTA (HORNWORTS)
Just like the liverworts, the name "hornworts" or "Anthocerophyta" (from the Greek keras,
'horn') refers to the general appearance of the plant, in this case, the sporophyte.
You will not study the lifecycle of hornworts in detail -- it is very similar to the lifecycle of the
liverworts, which we have discussed above.
https://www.flickr.com/photos/eyeweed/3659135159
Figure 5.18: Hornworts with the horn-like sporophyte plants on top of the
gametophyte plants.
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Study unit 5
Individual activity
Note the similarities and differences between the liverworts, hornworts and mosses and
fill in these differences in the following table:
Gametophyte plant:
Liverworts
Mosses
(eg Marchantia)
(eg Polytrichum)
Pattern of growth
Structure of gametophyte
Structure of "leaves"
Sporophyte plant:
Liverworts
Hornworts
(eg Marchantia)
Mosses
(eg
Polytrichum)
Size and shape/form of
sporangium
How sporangium opens
Spores distributed by
Ability
photosynthesise
to
Stomata in sporangium
Length of seta
Life cycle:
Liverworts
Mosses
(eg Marchantia)
(eg Polytrichum
Protonema
Position of gametangia
(antheridia and
archegonia)
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Study unit 5
Summary of key points
•
•
•
Bryophytes are classified under the kingdom Plantae.
"Bryophytes" is the collective term for all types of "mosses".
Bryophytes are usually found in damp environments because they need water for
reproduction.
Bryophytes are nonvascular plants (they have no vascular tissue), but waterconducting cells and nutrient-conducting cells do occur. Because of the absence of
vascular tissue, they do not have true roots, stems and leaves.
Bryophytes may be viewed as intermediary forms between algae (specifically the
Charophyceae) and vascular plants. They exhibit similarities to both of these groups.
There are, however, also important differences between algae and bryophytes and
between bryophytes and vascular plants.
Alternation of generations occurs in bryophytes. The gametophyte generation is the
dominant stage in the lifecycle. The sporophyte plant is dependent on the
gametophyte plant for water and grows semi-parasitically on it.
The bryophytes can be divided into three phyla: the Hepaticophyta, Anthocerophyta
and Bryophyta.
The structures of the gametophyte and sporophyte plants of the three bryophyte phyla
differ from each other (you must know in which ways), and there are also differences
in their lifecycles. (In the Hepaticophyta, elaters in the sporangium aid with the
distribution of spores; in the Bryophyta, the peristome helps with spore distribution. In
the Hepaticophyta, no protonema is formed, while a protonema is formed in the
Bryophyta. The positions of the antheridia and archegonia differ in the Hepaticophyta
and Bryophyta -- compare Figures 20.6 and 20.13).
Bryophytes can also reproduce asexually by means of fragmentation or brood bodies
(for example, gemmae).
•
•
•
•
•
•
•
Bryophytes play a very important ecological role in nature.
Group activity
Form small groups and make sure you can answer the following questions:
110
1.
"Bryophytes are considered to be intermediary forms between algae and vascular
plants." Discuss this statement, referring to the similarities and differences between
algae, bryophytes and vascular plants.
2.
What is the difference between a sporangium, an archegonium and an antheridium?
3.
Discuss the ecological importance of bryophytes.
4.
Tabulate the similarities and differences between the three phyla of bryophytes that
you have studied. Make use of the following headings:
5.
•
The structure and pattern of growth of the gametophyte plant
•
The structure of the sporophyte
•
Lifecycle
Discuss the adaptations that enable plants to live on land.
6.
Explain how bryophytes reproduce asexually.
Study unit 5
7.
Give brief explanations for the following terms:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
antheridial head
antheridiophore
antheridium
archegonial head
archegoniophore
archegonium
bryophytes
elaters
gametangium
gemmae
alternation of generations
hydroids
calyptra
leptoids
operculum
paraphyses
peristome teeth
protonema
rhizoids
seta
thallus.
Reflection
Make sure that you have fully reached the outcomes set at the beginning of this
study unit.
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Study unit 6
Study unit 6
SEEDLESS VASCULAR PLANTS
Study hours
You will have to spend at least 17 hours to master the contents of this study unit.
Study material
Most of the study material can be found in this study guide. Additional information will be
uploaded on eFundi as part of the PowerPoint presentations.
Study outcomes
On completion of this study unit, you should be able to fulfil the outcomes as listed
under each study section.
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Study unit 6
Study section 6.1
An introduction to the seedless vascular plants
Study outcomes
On completion of this section you should be able to:
•
•
Briefly describe the evolution of seedless vascular land plants with emphasis on
differences between the phyla;
Describe independent gametophytes and sporophytes in the alternation of
generation in seedless vascular plants.
Study material
Study section 1 on the evolution of seedless vascular plants on the following web
page:
https://www2.gwu.edu/~darwin/BiSc151/Plants2/plant2.html
Seedless vascular plants dominated the landscape around 350 million years ago.
Although we do not know much about conditions and organisms on the early earth, it
seems that oxygen in the early atmosphere was enriched by cyanobacterial and algal
photosynthesis to support land-based respiration. Bryophytes and seedless vascular
plants preceded most animals and served as the basis of the first terrestrial food
chains.
Primitive vascular tissues and epidermis possibly evolved in freshwater green algae
(Charophyceae), allowing them to survive dry periods, and eventually seedless
vascular plants evolved approximately 700-450 million years ago.
Seedless vascular plants evolved through adaptive radiation into species that
occupied different land environments. Fertilization of these plants depended upon
water, limiting their habitat and making them vulnerable to drought.
Seedless vascular plants differ from mosses because they developed the skill to
conduct water and nutrients in specialised vascular tissues (xylem and phloem).
Specialised roots are also present to absorb water from the soil. Stems conduct the
water and nutrients to and from the leaves. Mosses do not have vascular tissue, or
true roots, stems or leaves and absorb water directly through the above ground parts.
The gametophyte plants of vascular plants are much smaller and less conspicuous
than those of mosses and they are no longer the dominant generation in the life
cycle, in contrast to mosses. Seedless vascular plants do not produce seeds.
The earliest seedless vascular plants did not have any roots or leaves, but only green
photosynthesizing stems. The first photosynthetic leaf-like structures, without
vascular tissue, termed enations, only later developed in primitive seedless vascular
plants. Still later a single unbranched strand of vascular tissue probably developed in
an enation to form a small leaf called a microphyll. In more advanced seedless
vascular plants a network of branched vascular tissues can be found in the leaves,
called megaphylls.
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Study section 6.2
Types of living seedless vascular plants
Study outcomes
On completion of this section, you should be able to:
•
•
•
Describe the general structure of the sporophyte plants of members of the phyla
Psilotophyta, Lycophyta, Equisetophyta and Polypodiophyta;
compare alternation of generations in the lifecycles of seedless vascular plants
with that of mosses; and
identify permanent microscope slides as well as living material of all the phyla
and make fully labelled drawings of the different stages that you have studied.
Four phyla of seedless vascular plants can be distinguished. Each of these four phyla
will be discussed further on. We will start with the Phylum Polypodiophyta, which is
regarded as the most advanced of all ferns. It is, however, also the most well-known of
the seedless vascular plants and therefore we will start with them. Keep, however, in
mind that vascular tissue is well developed in this phylum in contrast to the more
primitive phyla that we will discuss later.
PHYLUM POLYPODIOPHYTA (ALSO KNOWN AS PTEROPHYTA)
Phylum Polypodiophyta are also known as “true” ferns, and represent the largest group
of seedless vascular plants. Today, this is the most successful and widespread group
of seedless vascular plants. They usually occur in moist terrestrial environments and
are less frequently found in fresh water (advanced characteristic). The phylum contains
vines, epiphytes and trees. Sporophytes have megaphylls – leaves with branched
vascular system (advanced). Megaphylls are also characteristic of all seed plants.
The life cycle of ferns will be discussed in the class. It is important that you know the
lifecycle of homosporic and heterosporic ferns. Pay special attention to vegetative and
reproductive structures. Sporophyte plants produce spores on megaphylls. Leaves are
often compounded and contain leaflets or pinnae that are attached to a rachis.
The sporophyte generation is the dominant (independent) part in the life cycle, while
the gametophyte generation is small and inconspicuous. The gametophyte and
sporophyte plants live independent from each other for the majority of the life cycle.
There is only a short stage where the sporophyte depends on the gametophyte for
water and nutrients.
A few important aspects regarding the life cycle are as follows:
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Study unit 6
The sporophyte plant is large and conspicuous and has true roots, an underground
stem (rhizome) and leaves that can be simple or, more common, compound (Fig. 5.18).
Young leaves are coiled.
Source: Unknown
Figure 6.1: Sporophyte plant of a seedless vascular plant (fern).
A group of stalked sporangia (looks like a dot on the leaf) is called a sorus (Fig. 5.19).
Sometimes a sorus can be covered and protected by an indusium, sometimes no
indusium is present and then the sorus is called naked and sometimes a pseudoindusium is found, where the leaf margin curls over to cover the sorus.
Photo taken by S Janse v Vuuren
Figure 6.2: Longitudinal section through a pinna with sorus.
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Meiosis takes place in the sporangium and haploid spores are produced. When the
sporangium is ready, it breaks open and specialised cells in the wall of the sporangium
(annulus and stomium cells; Fig. 5.20) help with dispersal of spores.
Photo taken by S Janse v Vuuren
Figure 6.3: Sporangium of a fern.
The formation of spores is the start of the gametophyte generation.
The spores germinate and give rise to a gametophyte plant (prothallus; Fig. 5.21).
Prothalli can be monoecious (bisexual) or also sometimes dioecious (unisexual). On
the prothallus antheridia with sperm will develop as well as archegonia with eggs. The
structure of the antheridia and archegonia is similar to that found in mosses.
https://www.flickr.com/photos/17674930@N07/13051724113
Figure 6.4: Gametophyte plant (prothallus) of a fern.
The antheridia will open, the sperm released and they will swim through the water
(Important, water is still needed for fertilisation). The neck canal cells of an archegonium
will dissolve and it will secrete a substance attracting the sperm. One sperm will swim
through the neck of an archegonium and fertilise the egg cell. A diploid zygote will be
formed (in the venter of the archegonium) and this is the beginning of the sporophyte
generation.
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The zygote will grow and develop into an embryo that will develop further to form a young
sporophyte plant (Fig. 5.22). Keep in mind that this development still takes place in the
venter of the archegonium on the female gametophyte plant (prothallus). The young
sporophyte will eventually grow through the prothallus and it is in a young stage dependent
on the prothallus for water and nutrients. As soon as the first leaves are formed, the
sporophyte becomes independent and the gametophyte (prothallus) will die.
https://www.flickr.com/photos/gjshepherd/2939867005
Figure 6.5: Gametophyte plant (prothallus) of a fern with a young
sporophyte plant formed after fertilisation.
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Practical: Phylum Polypodiophyta; Genus Polystichum
•
Study the living material of the adult sporophyte plant provided and compare it with
the drawing below. Add the following labels to the drawing:
Labels: Rhizome with ramentum, adventitious roots, lateral roots, leaves (simple,
once pinnately or twice pinnately compound) - petiole, lamina, rachis, ragilla, pinna,
pinnule, petiolule, young leaves, sporophyll, sori
Figure 6.6: Line drawing of the sporophyte plant of a fern.
Study the sporophylls to illustrate different types of sori
Labels: sporophyll, naked sori, sori with indusia, sori with pseudo-indusia
Naked sorus
Sorus with indusium
Sorus with pseudo-indusium
Figure 6.7: Line drawing of different types of sori found in ferns.
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Study a microscope slide of a cross section through a pinna with sorus.
Labels: Pinna: adaxial (upper) epidermis, abaxial (lower) epidermis, mesophyll,
vascular tissue. Sorus: indusium, indusium stalk, placenta, sporangium, sporangium
stalk.
Figure 6.8: Line diagram of a longitudinal section through the sorus of a fern.
Enlarge one of the sporangia in the previous slide and make a drawing of one
sporangium with specialised cells in the wall.
Labels: annulus,stomium, sporangium stalk, spores (n)
Figure 6.9: Line drawing of a sporangium of a fern.
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Study a microscope slide of a gametophyte plant (make sure whether you are
studying a mono- or dioecious plant) and make a fully labelled drawing of it.
Labels: Heart-shaped prothallus, rhizoids, constriction, assimilation tissue (cushion
tissue), antheridia, archegonia
Male gametophyte plant (prothallus)
Female gametophyte plant (prothallus)
Figure 6.10: Line drawing of a diecious (unisexual) gametophyte plant of a fern.
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FILUM PSILOTOPHYTA
Whisk ferns comprise most of the living members of the phylum Psilotophyta
Living members of this phylum consists of two genera: Psilotum and Tmesipteris. The
vegetative stages have no true roots or leaves. Instead of leaves, the stems have small
scale-like enations. Photosynthetic, whisk-like stems bear yellow, tri-lobed sporangia.
Nutrients are absorbed by rhizoids or rhizomes.
This phylum is homosporous with bisexual gametophytes. After fertilization, sporophytes
grow within the base of the archegonium, eventually detaching from the gametophyte to
become an independent plant. Sporangia, that contain diploid spore mother cells, form on a
photosynthetic branching stem system.
A: https://alchetron.com/Psilotum
B: http://premabotany.blogspot.com/2018/12/psilotum-classification-structure-of.html
A: Photo and permission by M. Clayton - https://search.library.wisc.edu/digital/AHNHAS6YW46AYB8E
B & C: https://www.flickr.com/photos/gjshepherd/3116628921
Figure 6.11: Line diagram and photos of the sporophyte plants of Psilotum.
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Study unit 6
Practical: Phylum Psilotophyta; Genus Psilotum
Individual activity
Study the sporophyte plants of Psilotum sp. in Figure 5.28 and supply the photos with
the following labels: underground rhizome with rhizoids (only visible in living plant),
above-ground stems (dichotomously branched), enations, trophophylls, sporophylls,
tri-lobed synangium.
Study the microscope slide of a cross or longitudinal section through the synangium
of Psilotum and make a fully labelled drawing.
Labels
synangium, three fused sporangiums, synangium wall, isospores
(homospores)
Figure 6.12: Line drawing of a synangium of Psilotum.
PHYLUM LYCOPHYTA
Living members of the phylum Lycophyta include club mosses, spike mosses and
quillworts: Living members of this phylum represent approximately 1,000 species, grouped
into three orders: Lycopodiales, Selaginellales and Isoetales.
Modern Lycopodiales are small herbaceous plants; ancient ancestors were trees. Stems
show dichotomous branching bearing microphylls (small leaves with a single vascular
trace).
There are two main theories of the origin of microphylls: One theory is that they evolved
when vascular tissue extended into existing enations. A competing theory is that
microphylls are short branches that result from different growth rates in the top two twigs or
telomes of a dichotomous branch.
Only the order Selaginellales will be studied in detail. Selaginella is a small, herbaceous
plant with branches that are green and dichotomously branched At the base of the green,
above-ground stems there is a second stem that is white, and non-photosynthetic and it is
known as a rhizophore. This rhizophore carries roots that anchor the plant to the ground.
Small and large microphylls are found on the green, above-ground, dichotomously
branched stems. At the tip of the stems, strobili develop that carry micro- and megaspores
inside micro- and megasporangia respectively. The microsporangia are carried on top of
microsporophylls and the megasporangia on top of megasporophylls. Selaginalla species
are therefore heterosporous, producing two types of spores: microspores and megaspores.
The mature sporophyte carries micro- and megasporophylls.
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Gametophyte development is endosporic. In Selaginella, each male gametophyte grows
within a microspore and after reaching maturity releases the sperm. Each female
gametophyte ruptures the megaspore wall, exposing the archegonia and egg cell.
Following fertilization in the female gametophyte, the young sporophyte is at first attached
to the female gametophyte but eventually becomes an independent plant.
https://uk.m.wikipedia.org/wiki/%D0%A4%D0%B0%D0%B9%D0%BB:Selaginella_denticulata_L
a_Palma01.jpg
Figure 6.13: Photos of the sporophyte plants of Selaginella.
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Study unit 6
Practical: Phylum Lycophyta; Genus Selaginella
Individual activity
Study the living sporophyte plants of Selaginella and compare it with the photo in Fig.
6.13. The gametophyte plants are very small and won’t be studied. Identify the following
parts and label them on Fig. 6.13: Two types of stems: dichotomous branched stems
(carry leaves) and rhizophore (with adventitious roots), stem growth point. Two sizes of
leaves: small and large microphylls, strobili (not always visible)
Study the microscope slide of a longitudinal section through a strobilus of Selaginella.
Compare the strobilus in the slide with that in the photo below and add the following
labels to the photo.
Labels: Strobilus, central axis of strobilus, microsporophyll, microsporangium, wall of
microsporangium, stalk of microsporangium, microspores (n), megasporophyll,
megasporangium, wall of megasporangium, stalk of megasporangium, megaspores (n),
ligule, remnants of tapetum
Photos taken by S. Janse v Vuuren
Figure 6.14: Light microscope photograph of a longitudinal section through a
strobilus of Selaginella.
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PHYLUM EQUISETOPHYTA
Living members of this phylum are known as horsetails. Rhizomes, with adventitious roots,
are found underground. From the rhizome grows above-ground, green, photosynthesising,
hollow stems that are divided into nodes and internodes. The stems feel rough due to
deposits of silica and were commonly used as abrasives and scouring agents in the past.
On the nodes are whorls of side branches, the brachyblasts. Leaves are confined to small
microphylls on the nodes. The bases of neighbouring microphylls are laterally fused to form
a sheath around the stem. Only their tips are free. Strobili develop at the tips of the stems
and are usually 2-4 cm long. A strobilus consists of sporangiophores, each containing 5-10
sporangia, that are projected inwards. Homospores, as well as haploid elaters (they grow
from the spore’s wall), are found within the sporangiums. Elaters help with spore dispersal.
A: http://shop.sweetknowleaquatics.co.uk/Equisetum-fluviatile
B:
C:
B & C: http://www.missouriplants.com/Ferns/Equisetum_hyemale_page.html
Figure 6.15: Sporophyte plants of Equisetum.
A: Stem with strobilus at the tip and whorls of brachyblasts on the nodes; B:
Microphylls fused laterally on the nodes as well as small developing brachyblasts; C:
Strobili at the tips of Equisetum stems.
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Study unit 6
Practical: Phylum Equisetophyta; Genus Equisetum
Individual activity
Study the living material of Equisetum. Make fully labelled drawings of the adult plant
(sporophyte) and identify the following:
Labels: Erect growing branched hollow stem, nodes, internodes, ridges on stem,
microphylls fused to base, short branches (brachyblasts) in whorls on the stem,
underground rhizome, adventitious roots, strobili, sporangiophore.
Figure 6.16: Line drawing of the sporophyte plant of Equisetum.
Group activity
Form groups with your friends and tabulate the differences in the structure of the
sporophytes, gametophytes, sporangia and alternation of generations of the
seedless vascular plants that you have studied. Complete the following table as an
answer to the question. Insert only one word in the gaps.
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Study unit 6
Psilotophyta eg
Psilotum sp.
Lycophyta eg
Selaginella sp.
Equisetophyta eg
Equisetum sp.
Polypodiophyta eg
several true ferns
Sporophyte plants with,
green, dichotomously
branched, erect growing
stems and underground
rhizomes
Sporophyte plants with
above-ground, green,
dichotomously branched,
horizontal growing stems
and above-ground with
adventitious roots
Sporophyte plants with
above-ground, green
stems with nodes and
internodes consisting
of ridges on
underground rhizomes.
Variety of sporophyte
plants, all with aboveground stems or
underground .
Sporophyte plants: no
true roots – ................
present
Sporophyte plants:
rhizophore with
...................
Sporophyte plants:
.................. with
adventitious roots
Sporophyte plants:
rhizomes with
adventitious roots
Sporophyte plants: no
true leaves – ................
(small, scale-like
structure without
vascular tissue) present
Sporophyte plants: leaves
are small and large
..................... (consist of a
single, unbranched main
vein)
Sporophyte plants:
leaves are small
microphylls fused in a
................... at the
nodes
Sporophyte plants:
leaves are ..................
true leaves with
branched veins) – can
be ..................... or
pinnately compound
Sporophyte plants:
Sporangia ...........
carried in the form of
synangia (groups of
three fused
sporangiums)
Sporophyte plants:
Sporangia carried in
........................ (central
axis with sporophylls and
sporangiums) at the end of
dichotomous stems
Sporophyte plants:
Sporangiums carried
underneath
sporangiophores in
strobili
Sporophyte plants:
Sporangiums carried
in the form
of.................. (groups
of unfused
sporangiums) at the
base of leaves known
as ..............., in most
ferns, but exceptions
occur.
Sporophyte plants:
...................... (spores
morphological identical –
isospores or
homospores)
Sporophyte plants:
Heterosporic (spores
morphological different –
small .................. and large
megaspores)
Sporophyte plants:
Homosporic (spores
morphological identical
– .................... or
homospores)
Sporophyte plants:
Homosporic, but also
genera (especially in
water ferns) that are
...........................
No elaters attached to
spores
No elaters attached to
spores
Release of spores
from sporangiums
enabled through...........
attached to spores
No elaters attached to
spores
Isospores give rise to
..........................
(bisexual gametophyte
plants)
Heterospores give rise to
dioecious
(............................)
gametophyte plants –
microspores form
.................. and
megaspores form female
gametophyte plants
Isospore gives rise to
monooecious
(.............................)ga
metophyte plants
.................... give rise
to monooecious
(bisexual)
gametophyte plants, In
genera where
heterospores are
formed, dioecious
(unisexual)
gametophyte plants
are formed
Gametophyte plants
........................... –
develop outside the
spore wall
Gametophyte plants
endosporic (develop..........
spore wall)
Gametophyte plants
exosporic – develop
outside the spore wall
Gametophyte plants
........................ or
exosporic
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Study unit 7
Study unit 7
GYMNOSPERMS
Study hours
You will have to spend at least 17 hours to master the contents of this study unit.
Study material
Most of the study material can be found in this study guide. Additional information will be
uploaded on eFundi as part of the PowerPoint presentations.
Study outcomes
On completion of this study unit, you should be able to fulfil the outcomes as listed
under each study section. You must also be able to discuss the different
gymnosperm phyla individually as well as compare them regarding the following
aspects:
• morphology;
• reproduction;
• lifecycle; and
• demonstrate possible evolutionary developments.
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Study unit 7
Study section 7.1
An introduction to the gymnosperms
Study outcomes
On completion of this study section, you should be able to:
•
•
Describe the selective advantages that seed plants have over seedless plants;
Know how seeds are formed from fertilised ovules.
The name “gymnosperm” is derived from two Greek words: gymnos, meaning “naked”
and sperma, a “seed”. The name refers to the exposed nature of the seeds, which are
produced on the surface of sporophylls and not enclosed within a fruit, as in the
flowering plants. The seed bearing sporophylls are arranged in strobili (seed cones)
that develop at the same time as the smaller pollen bearing strobili (pollen cones).
Seed producing plants (gymnosperms and angiosperms) represent the majority of all
plants. The evolution of the seed enabled seed plants to be much more successful on
land than their seedless ancestral forms.
Gymnosperms form a diverse group of plants with a very dissimilar external
morphology. They are, however, similar in that their seeds are produced naked
(exposed) on modified leaves (sporophylls), rather than being enclosed within fruits like
in the angiosperms, and that they bear cones (strobili), rather than flowers as in the
angiosperms.
Gymnosperms are, however, not as diverse as the bryophytes or the seedless vascular
and, especially, the angiosperms. In earlier classification systems, the gymnosperms
were lumped into a single class (class Gymnospermae) of the seed plants (phylum
Spermatophyta). This classification system was later regarded as artificial because it
was based upon a single characteristic, namely the presence of naked (exposed)
seeds. In more recent classification systems, botanists have regarded the
gymnosperms as sufficiently diverse to separate them into four phyla:
•
•
•
•
Phylum Pinophyta (conifers)
Phylum Cycadophyta (cycads)
Phylum Ginkgophyta (Ginkgo)
Phylum Gnetophyta (gnetophytes)
Please take note: Although the class name Gymnospermae is not used any more, we
still use the general term gymnosperms to refer to all four abovementioned phyla.
Sporophyte plants are well developed in contrast to the gametophyte plant which is
reduced to such an extent that it only consists of a few cells and it cannot be seen with
the bare eye. The female gametophyte develops within an ovule, while the male
gametophyte is on the inside of a pollen grain. The gametophytes are totally dependent
on the sporophyte plants, which usually are large trees or shrubs.
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Study unit 7
Individual activity
Define a seed.
Feedback
VERY IMPORTANT: In mosses, the sporophyte is dependent on the gametophyte. In
seedless vascular plants, the sporophyte and gametophyte live independent for the
majority of the life cycle, except for the early stages where the sporophyte depends on
the gametophyte. In seed plants, the gametophyte is dependent on the sporophyte.
The sporophyte is the diploid, multicellular form in the life cycle, it bears sporangiums
and produces spores through meiosis. The gametophyte is the haploid, multicellular
form in the life cycle, it bears gametangiums and it forms gametes through mitosis. The
sporophyte and gametophyte generations alternate in the life cycle of the plant. The
haploid gametophyte generation begins with meiosis and ends with fertilisation, while
the diploid sporophyte generation starts with fertilisation and ends with meiosis.
Cones consist of three main parts: central axis, sporophylls spirally arranged around
the central axis, and sporangia associated with each sporophyll. On the
megasporophyll megasporangia (ovules) will be formed. When these ovules are
fertilised it will form a seed. The integument of the ovule forms the seed coat (testa) of
the seed.
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Study section 7.2
Types of living gymnosperms
Study outcomes
After working through this study section, you should be able to:
•
•
•
•
Briefly describe the general structure of the sporophyte plants, including the
structure of the strobili of representatives of the phyla Pinophyta, Cycadophyta,
Ginkgophyta and Gnetophyta;
identify representatives of the four gymnosperm phyla in nature and in gardens,
based upon the general characteristics of the specific phylum;
distinguish between representatives of the four phyla based on the structure of
their sporophyte plants and strobili and the development of the micro- and
female gametophytes; and
identify permanent microscope slides as well as living material of all the phyla
and to make fully labelled drawings of the different structures that you have
studied.
Four phyla of gymnosperms can be distinguished (see page 127). Each of these phyla
will be discussed in the following sections. will start with the Phylum Pinophyta as this
phylum includes the large pine trees and conifers, which are familiar to most of you.
PHYLUM PINOPHYTA
Phylum Pinophyta contains conifers, which are the dominant forest trees in cooler
climates: The conifers consist of about 50 genera of trees and include our planet’s
largest trees (Sequoiadendron giganteum - Giant Sequoia or Mammoth Trees), tallest
trees (Sequoia sempervirens – Californian Redwood) and oldest trees (Pinus longaeva
– Bristlecone pines).
Most conifer genera only occur naturally in the Northern Hemisphere. Pinus (pine
trees) is the largest of these genera. Please take note that a variety of pine species and
some other conifers are planted in South Africa and other southern hemisphere
countries for forestry purposes. The only two genera which occur naturally in the
Southern Hemisphere are Araucaria and Podocarpus. A close relative to Araucaria, the
Wollemi Pine (Wollemia nobilis), which was previously thought to be extinct, was
recently discovered in Australia. Podocarpus (yellowwoods) and Widdringtonia (cedars)
are the only genera of the phylum Pinophyta that occur naturally in South Africa. There
are four species of Podocarpus in South Africa. Podocarpus latifolius (real yellowwood)
is the most well-known species as it is one of the country’s most valued timber trees,
and it is also South Africa’s National Tree! All the species of Podocarpus are protected
in South Africa. In the past, they were so sought after as timber trees that, from being
an abundant resource, they became almost extinct in some areas.
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Study unit 7
Individual activity
Additional information from the Internet on the genus Podocarpus
If you want more information on the different Podocarpus species in South Africa and
you have Internet access, the following websites, where the general characteristics,
distribution ecological and economical values are given, are worthwhile to explore:
Podocarpus latifolius (real yellowwood):
http://pza.sanbi.org/podocarpus-latifolius
Podocarpus falcatus (Outeniqua yellowwood):
http://pza.sanbi.org/podocarpus-falcatus
Podocarpus henkelii (Henkel’s yellowwood)
http://pza.sanbi.org/podocarpus-henkelii
Podocarpus elongatus (Breede River yellowwood)
http://pza.sanbi.org/podocarpus-elongatus
There are three Widdringtonia species in South Africa. The most well-know species is
Widdringtonia cedarbergensis (Clanwilliam cedar). The Cederberg Mountains in the
Cape were named after this cedar, which used to be a prominent feature of the
Cederberg, but today it is a rare sight. This tree is critically endangered and on the
brink of extinction after decades of unsustainable harvesting for their long-lasting and
fragrant timber and from too frequent veldt fires.
Individual activity
Additional information from the Internet on the genus Widdringtonia
If you want more information on Widdringtonia cedarbergensis and the two other
Widdringtonia species, W. nodiflora and W. schwarzii and you have Internet access,
use the following website address:
http://pza.sanbi.org/plants/search/advanced?name=Widdringtonia%20cedarbergensis.
Click on the name of the species for more information
The wood of the sporophyte plants of conifers is referred to as softwood due to the
absence of fibres and presence of thinner cell walls than other gymnosperms
The conducting elements of the wood (xylem) consist solely of tracheids (no vessels).
Conifers produce resin, which moves through the tree in resin ducts, thus protecting
the tree from pathogens.
Conifer leaves are different from leaves of many other gymnosperms in that they are
simple (not compound) and are borne singly or in clusters called fascicles.
The microsporangia and megasporangia are borne in microstrobili (pollen cones) and
megastrobili (ovulate cones) (as indicated in section 2.1), but on the same plant
(monoecious).
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The structure of the cones of most of the conifers are similar to those of pine trees
which we will discuss shortly. However, Podocarpus cones differ from that of pine
trees.
The sporophyte plant is the big pine tree that you know. It has roots, an above-ground
stem, side branches and leaves.
Two types of stems are found, namely
• Long branches – all the branches that you can see if you look up into a pine tree.
• Short branches – at the base of a group needle-like leaves underneath the leaf-sheath.
Two types of leaves are also found, namely
•
•
Scale leaves – brown scale leaves on the long branches
Needle-like leaves – green leaves on the short branches. They are covered at the
base with a scale leaf sheath.
https://upload.wikimedia.org/wikipedia/commons/8/8a/Pinus_taeda%2C_a%2C_naaldbundels.jpg
Figure 7.1: Needle-like leaves of Pinus surrounded by a leaf sheath at the
base.
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During the reproductive stage the sporophyte plants form two types of cones (strobili):
microstrobili and megastrobili:
•
Microstrobili (pollen cones; Fig. 7.2 and 7.3) are small and carried in clusters – it
forms in the place of short branches. Each cone consists of a central axis with
microsporophylls (modified leaves) arranged around it.
https://www.flickr.com/photos/jim-sf/3517617531
Figure 7.2:
clusters).
Sporophyte plant of Pinus with microstrobili (carried in
On the lower surface of each microsporophyll (Fig. 7.3) there are two microsporangia
(pollen sacs, 2n) in which the microspores (pollen, n) are formed through meiosis.
Photos taken by S Janse v Vuuren
Figure 7.3: Longitudinal section through a microstrobilus.
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Each microspore (pollen grain; Fig. 7.4) undergoes a couple of mitotic divisions - you are
not required to know all these divisions, only the four-cell stage where two prothallium cells,
a generative cell and a tube cell are formed – this is also the stage during which pollination
will take place. Further development of the male gametophyte will take place after
pollination. Make sure that you know the functions of the generative and tube cells.
The microspores (pollen) are adapted for wind dispersal by means of two air sacs
(Fig. 7.4).
Photos taken by S Janse v Vuuren
Figure 7.4: Microspores (pollen grains) of Pinus.
Megastrobili (ovulate cones; Fig. 7.5) are large and carried single – in the place of long
branches, consist of a central axis with megasporophylls (modified leaves) arranged
around it.
https://www.flickr.com/photos/foxypar4/501043766
Figure 7.5: Sporophyte plant of Pinus with megastrobili (carried single).
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Photos taken by S Janse v Vuuren
Figure 7.6: Longitudinal section through a megastrobilus.
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On the upper surface of each megasporophyll there are two megasporangia (ovules;
Fig. 7.6). The ovule (Fig. 7.7) consists of nucellus cells on the inside and a protective
integument on the outside. In the integument there is an opening, the micropyle.
Photo by S Janse v Vuuren
Figure 7.7: Longitudinal section through the megasporangium/ovule (still
not fully developed).
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One of the megaspore mother cells (2n) in the megasporangium divides through
meiosis and form four megaspores (n). Only one of the megaspores will develop
further. It will undergo multiple mitotic divisions and a female gametophyte will be
formed in which archegonia will develop. In each archegonium (n) there is a central
cell which will divide by mitosis to form an egg cell (n) and a ventral canal cell (n) (Fig.
7.8).
Photo taken by S Janse v Vuuren
Figure 7.8: Longitudinal section through a mature megasporangium/ovule
(fully developed).
When the microspores (pollen) are released they are blown by the wind to a
megastrobilus. The megasporophylls open to expose the megasporangium. The
pollen grain falls on the megasporangium and penetrates it through the micropyle. It is
trapped in a pollen chamber on the nucellus cells and it remains here for a long period
while further development takes place. Two sperm cells are now formed in the pollen
grain and over time a pollen tube will grow into the archegonia and one sperm will
fertilise the egg. Multiple archegonia can be present and can be fertilised, and initially,
more than one embryo develops (polyembryony). Only one embryo will survive. A
diploid zygote (2n) will form after fertilisation and this is the beginning of the sporophyte
generation.
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The fertilised ovule now changes to form a seed (2n) (Fig. 7.9)– the integument
hardens and forms the testa (seed coat) of the seed. The female gametophyte acts as
food for the developing embryo. The seed is adapted by wind dispersal by means of a
wing-like outgrowth.
If the seed germinates it will form a mature sporophyte plant.
https://commons.wikimedia.org/wiki/File:Seed_pinus_sylvestris_1_beentree.jpg
Figure 7.9: Seed of Pinus.
Please note:
The gametophyte generation (male gametophyte inside the microspore and female
gametophyte inside the megasporangium) is reduced to such an extent that it is
surrounded by sporophyte tissue. The gametophyte cannot survive without the
sporophyte.
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Practical: Phylum Pinophyta; Genus Pinus
Individual activity
Study the living and dried material of Pinus supplied. Compare the material with the
drawings of the sporophyte plant in Fig. 7.10 and identify the following by adding
labels to the drawings below:
Two types of stems: long branches and short branches (remove the scale leaf
sheath in order to see the short branches). Two types of leaves: needle-like leaves
(on short branches) and scale-like leaves (on long branches). Scale leaf sheath,
single female cones (megastrobili) in the place of long branches, groups of male
cones (microstrobili) in the place of short branches, seed.
A:
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B:
Source: Unknown
Figure 7.10: A: Part of a long branch of Pinus, B: Pinus sporophyte with
micro and megastrobili.
Study the living material and microscope slides of Pinus supplied to you. Add labels
to Figures 7.1-7.8 as follows:
Figure 7.1:
Needle-like leaves, scale leaf sheath
Figure 7.2:
Microstrobili
Figure 7.3:
Microstrobilus, central axis, microsporophyll, microsporangium (pollen
sac) on abaxial surface of microsporophyll), microsporangium wall,
microspore (pollen grains), vascular tissue, resin canals and
parenchyma tissue.
Figure 7.4:
Microspores, air sacs, nucleus, exine, intine
Figure 7.5:
Megastrobilus, megasporophyll
Figure 7.6:
Central axis, vascular tissue, resin canals, megasporophyll consisting
of two parts: bract scale, ovuliferous scale. megasporangium (ovule)
Figure 7.7:
Megasporangium (ovule) - integument, micropyle, nucellus,
megaspore mothercell, ovuliferous scale, bract scale, microspore
Figure 7.8:
Megasporangium or ovule, integument, micropyle, pollen chamber,
nucellus, sections through pollen grains and pollen tubes in nucellus,
female gametophyte. Archegonium – central cell (divides to form
ventral canal cell and egg cell).
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PHYLUM CYCADOPHYTA
Phylum Cycadophyta contains cycads, which resemble tree ferns or palms: Cycads
are slow-growing gymnosperms of warmer climates. The cycads consist of 11 genera
and 140 species which are a mere remnant of a much wider diversity found during the
Mesozoic era (245-65 million years ago), which is sometimes not only referred to as
the Age of Dinosaurs but also as the Age of Cycads. In South Africa, two cycad genera
occur naturally, namely Encephalartos with about 40 species and Stangeria that is a
monotypic genus (one species in the genus). The name Encephalartos is derived from
the Greek articles “en”, meaning “in, “cephale”. meaning “head”, and “artos”, meaning
“bread”. The generic name refers to the starch obtained from the stems which was
used as food by some indigenous tribes. The name Stangeria honoured Dr William
Stanger who sent the newly discovered plant to experts in Britain.
Individual activity
Additional information from the Internet on cycads:
If you want more information on cycads and have Internet access, the following websites
are worthwhile visiting:
http://en.wikipedia.org/wiki/Cycad
http://www.cycadsociety.org/ is the official website of the Cycad Society of South Africa
– apart from viewing numerous photographs of different cycad species (Click on the
“species flag” at the top), you can also gain some very useful information on aspects
such as the conservation of cycads and threats to their existence in nature. Two primary
threats to the existence of all plants in nature are loss of habitat and competition from the
introduction of non-native species, but for cycads there is an additional factor, namely
human greed. Some cycads are worth a lot of money, mainly because of their rarity.
There is, therefore, a lot of cycad smuggling going on all over the world. Since 1975, an
international treaty known as CITES (Convention on International Trade in Endangered
Species of Wild Fauna and Flora) has tried to protect cycads with mixed success. In the
December 2005 edition of Encephalartos – Journal of the Cycad Society of South Africa
(also available on the website of the society), there is an interesting paper titled “The Cult
of the Cycads” in which the author tried to explain what it is about these strange, squat
plants that give rise to the activities of smugglers and obsessive collectors. If you have a
passion for the conservation of plants in nature and also for detective stories, you will find
this paper extremely interesting.
If you are living in or close to Nelspruit, Mpumalanga, it is important to know that the
Lowveld Botanical Gardens, which are located in Nelspruit, have the most
comprehensive collection of African cycad species in the country. This garden is,
furthermore, ranked as third in the world in terms of the number of cycad species
represented. If you have Internet access, the following website address will give you
more information and photographs of the cycads in this magnificent Botanical
Garden:
The following official website address of SANBI (South African National Biodiversity
Institute) will give you more information on the Lowveld Botanical Garden and other
Botanical Gardens of South Africa. Click on the garden of your choice:
https://www.sanbi.org/gardens/
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The sporophyte plants have a single trunk and large pinnately compound leaves
(resembling palm leaves or fern fronds) and because of their structure they are often
mistaken for ferns or flowering plants.
Reproduction in cycads is similar to that of pines, except that their sperms are motile by
means of flagella and that their megastrobili (ovulate cones) can be massive
(sometimes more than a metre in length).
All cycads are dioecious (separate pollen and seed-producing plants).
Many cycad species are pollinated by beetles, an example of insect pollination that is
more common in angiosperms (flowering plants).
Each microsporophyll bears many microsporangia on the lower surface.
Practical: Phylum Cycadophyta; Genus Encephalartos/Cycas
Individual activity
Study the living and preserved or dried material of Encephalartos or Cycas. Identify
the following on the photos supplied below:
A: Stem, pinnately compound leaves
B: Microstrobilus, microsporophylls
C: Megastrobilus, seed.
A:
https://cdn.shopify.com/s/files/1/0200/5036/products/01005_1800x1800.jpeg?v=1421152659
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B: https://commons.wikimedia.org/wiki/File:Cycas_revoluta_male.jpg
C: https://www.needpix.com/photo/1357116/plant-fern-cycad-female-seeds-orange
Figure 7.11 A: Sporophyte plant of Cycas, B: Microstrobilus of Cycas, C:
Megastrobilus of Cycas.
PHYLUM GINKGOPHYTA
Phylum Ginkgophyta contains only one living species namely Ginkgo biloba. The
maidenhair tree (Ginkgo biloba) is the only living member of the Phylum Ginkgophyta.
Living plants of this species seem unchanged from fossils that are 150 million years
old; therefore, the term “living fossils” is sometimes used to describe this plant and the
cycads. Ginkgo would probably have become extinct if they had not been grown in
temple gardens in China and Japan for thousands of years. The name Ginkgo is well
known to users of phytomedicines (herbals). In 2000, it was the best-selling
phytomedicine on the European market, and sales were in excess of 280 US dollars
per year. Ginkgo is widely used as a treatment for Alzheimer’s disease, vertigo, tinnitus
(ringing in the ear), heart diseases and eye diseases and as an enhancer of short-term
memory.
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Individual activity
Additional information from the Internet on Ginkgo biloba:
If you want more information on this plant, especially its general medicinal purposes and
you have Internet access, the following website addresses would be interesting to you,
but take note that there are literally hundreds of other websites about this interesting
plant. Before you visit these websites and might be carried away with all the positive
effects of Ginkgo, always remember that herbal plants are not always safe. Ginkgo can
interfere with prescription drugs or can cause unwanted side effects of its own, as with all
plants with a medicinal effect.
PLEASE CHECK WITH A DOCTOR FIRST BEFORE EMBARKING ON A NEW
TREATMENT, AS WITH ALL HERBAL REMEDIES.
http://www.stevenfoster.com/education/monograph/ginkgo.html
http://www.aafp.org/afp/20030901/923.html
The sporophyte plant is a deciduous tree with simple, fan-shaped leaves which are
deeply lobed (hence the name of the specific epithet biloba) on the short branches,
while the leaves on the long branches are not lobed. Long branches have unlimited
growth in length, while short branches have limited growth in length.
Like the cycads, Ginkgo has pollen and seed-producing strobili on separate trees
(dioecious of unisexual). Ginkgo is also similar to cycads in that the sperms are motile
and that the ovule contains a fleshy outer layer (integument). Ginkgo differs from
cycads in that the cones are small and that the ovulate cones usually bear only two
ovules.
Take note that the fleshy seeds (fertilised ovules) of Ginkgo biloba have a nauseating
odour. To avoid the stinky and messy seeds, most nurseries propagate these trees by
means of cuttings of the pollen producing trees and not of the seed producing trees.
Much of the genetic diversity of the genus has, therefore, probably been lost during
cultivation.
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PRACTICAL: PHYLUM GINKGOPHYTA; Species Ginkgo biloba
Individual activity
Study the living and preserved or dried material of Ginkgo biloba. Distinguish the
following on the given photographs in Fig. 7.12:
A: Long branches, Short branches, fan shaped, bilobed leaves, petiole, lamina
B: Bilobed leaves, dichotomous venation, lamina
C: Microstrobili with microsporophylls
D: Seed (fertilised ovules), megasporophyll, short branches, scars of leaf abscission
A: https://www.flickr.com/photos/tim-waters/5628801491
B: https://www.flickr.com/photos/monteregina/30643567781
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C: https://commons.wikimedia.org/wiki/File:Ginkgo_biloba_-_male_flower.JPG
D: https://commons.wikimedia.org/wiki/File:Ginkgo_biloba_007.jpg
Figure 7.12: A: Leaves carried on short branches on the sporophyte plant, B:
Structure of a leaf of Ginkgo biloba, C: Microstrobili of Ginkgo biloba, D: Seed
(formed after fertilisation from megasporangia or ovules).
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PHYLUM GNETOPHYTA
The gnetophytes (phylum Gnetophyta) consist of 70 living species, divided into three
genera, namely Ephedra, Gnetum and Welwitschia. All 3 these genera are located in
tropical forests or in deserts. Although the three genera differ distinctly in their external
morphology, they do have a number of similar characteristics regarding molecular
evidence and certain angiosperm-like features. One of these features is the occurrence
of a type of double fertilisation: After one sperm fertilises the egg cell, the other sperm
fuses with another cell in the female gametophyte, but no endosperm is formed as in
the angiosperms. The process of double fertilisation in gnetophytes yields extra
embryos.
In this module, emphasis will be placed on Welwitschia mirabilis which is the sole living
representative of its genus. It is a peculiar looking plant growing in the coastal desert
regions of Namibia and Angola. Most of the moisture for this slow-growing plant is
derived from fog that rolls in from the ocean at night. In some textbooks, this plant is
described as looking more like something out of a science fiction movie than a real
plant. The specific epithet mirabilis, meaning miraculous, describes this fact. The genus
name Welwitschia honoured Dr Friedrich Welwitsch, an Austrian explorer who
“discovered” the plant although it was noticed by several previous travellers.
Individual activity
Additional information from the Internet on Welwitschia mirabilis:
If you want to know more about this weird and wonderful plant, including looking at
photographs and you have Internet access, the following website addresses would
be worthwhile visiting:
http://pza.sanbi.org/welwitschia-mirabilis
http://www.mobot.org/gradstudents/olson/welwitschia.html
The stem of the sporophyte plant is woody and concave in shape and may be as much
as 1.5 m in diameter and grow up to 3 m into the soil where it is connected to a large
taproot. Mature plants have two large strap-like, leathery leaves of up to 6 m long
which persist throughout the life of the plant, growing continuously along the ground.
These two permanent leaves are unique in the plant kingdom as they are the original
leaves from when the plant was a seedling, and they just continue to grow and are
never shed. Each leaf has a meristem at its base (it is called an intercalary meristem),
which constantly replaces tissue that is lost at its drier, ageing tip. Leaf tips gradually
split into many segments as a result of the leaves being whipped by the wind, which
gives the plant a very untidy and unhealthy appearance even if it is quite healthy.
Welwitschia is dioecious, like many other gymnosperms.
Gnetophytes have many angiosperm-like features, which include flowerlike compound
strobili, vessels in the secondary xylem, loss of archegonia, loss of prothallium cells in
the pollen and double fertilisation.
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Practical: Phylum Gnetophyta, Species Welwitschia mirabilis
Individual activity
Study the dry material and photographs of Welwitschia mirabilis.
between the following in Fig. 7.13:
Distinguish
A: Welwitschia mirabilis plant with long leaves of which the tips seem to be dead
B: Concave stem, leathery leaves, microstrobili (male flowers)
C: Megastrobili (female flowers)
A: https://upload.wikimedia.org/wikipedia/commons/f/f4/Welwitschia_mirabilis%281%29.jpg
B: https://storage.needpix.com/rsynced_images/nature-3293059_1280.jpg
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C: https://upload.wikimedia.org/wikipedia/commons/0/05/Welwitchia_Mirabilis_%28female%29.JPG
Figure 7.13: A: Welwitschia mirabilis plant with long leaves of which the tips seem
to be dead, B: Stem and microstrobili of a male Welwitschia mirabilis, C:
Megastrobili of a female Welwitschia mirabilis.
Group activity
Complete the table on the next page – only one word must be used in the open
spaces. Make sure that you understand these differences.
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Pinophyta eg Pinus
spp.
Cycadophyta eg
Encephalartos spp.
Ginkgophyta eg
Ginkgo biloba
Gnetophyta eg
Welwitschia
mirabilis
Sporophyte plants:
evergreen or ……….
trees
Sporophyte plants:
single trunk with
crown of leaves,
evergreen
Sporophyte plants:
deciduous trees with
long branches and
short branches
Sporophyte plants:
evergreen, with
conical stem and two
leathery strap-like
leaves
Leaves simple,
narrow, needle-like
and sometimes
situated in …………
Leaves …………
compound
Leaves simple,
……………. on long
branches
Two simple leaves
through entire life of
plant
Resin is produced by
some species to
protect the tree from
pathogens
No resin is produced
No resin is produced
No resin is produced
Xylem contains only
……………
Xylem contains only
……………
Xylem contains only
……………
Xylem contains
……………. and
……………..
Monoecious (pollen
and ovulate cones on
same plant)
………… (pollen and
ovulate cones on
separate plants)
………… (pollen and
ovulate cones on
separate plants)
………… (pollen and
ovulate cones on
separate plants)
Pollen cones simple
and smaller; ovulate
cones complex and
larger with some
except ions, eg
Podocarpus
Pollen cones and
ovulate cones simple
and massive
Pollen and ovulate
cones simple and
small (ovulate cones
usually with only two
ovules)
Flowerlike compound
cones
Sperms not motile –
are taken by pollen
tube to egg cell
Sperms ………… by
means of flagella
Sperms ………… by
means of flagella
Sperms ………… by
means of flagella
Archegonia (with egg
cell) present in
female gametophyte
(female)
Archegonia (with egg
cell) present in
female gametophyte
(female)
Archegonia (with egg
cell) present in
female gametophyte
(female)
Archegonia (with egg
cell) ………. in
female gametophyte
(female)
Integuments of
ovules papery; only
fleshy in some, eg
Podocarpus
Integuments of
ovules form a
………….. layer
Integuments of
ovules form a
………….. layer
Integuments of
ovules papery, not
fleshy
No double
fertilisation
No double
fertilisation
No double
fertilisation
Type of double
fertilisation, but no
……………… is
formed
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Individual activity
These questions will help you to determine if you have mastered all the outcomes.
1.
Explain how the sporophyte plants of cycads differ structurally from those of
conifers.
(10)
2.
Explain how the leaf structure of Ginkgo biloba differs from those of the conifers
and the cycads.
(6)
3.
Explain why Welwitschia is ecologically highly specialised and is adapted to
grow under extremely arid conditions.
(7)
4.
Describe the angiosperm-like features of the gnetophytes and explain why
double fertilisation in gnetophytes is not the same as in angiosperms.
(6)
5.
Tabulate the major structural differences between the four gymnosperm phyla.
(8)
6.
Give definitions of the following concepts:
Dioecious/unisexual
Monoecious/bisexual
Short branch
7.
Define the following concepts:
•
•
•
•
•
•
•
152
(6)
Sporophyll
Sporophyte
Megastrobilus
Microstrobilus
Pollination
Fertilization
Micropyle
(14)
Study unit 8
Study unit 8
ANGIOSPERMS
Study hours
You will have to spend at least 6 hours to master the contents of this study unit.
Study material
Most of the study material can be found in this study guide. Additional information will be
uploaded on eFundi as part of the PowerPoint presentations.
Study outcomes
On completion of this study unit, you should be able to fulfil the outcomes as listed
under each study section. You must also be able to demonstrate an understanding
of Angiosperms regarding their:
•
•
•
•
morphology;
reproduction;
lifecycle; and
demonstrate possible evolutionary development of flowers
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Study section 8.1
Introduction to the Angiosperms
Study outcomes
On completion of this study section, you should be able to:
•
•
•
•
•
•
•
•
Describe the structure of bisexual and unisexual flowers;
Explain how and where the male and female gametophyte plants develop in
angiosperms;
Explain what happens from pollination through fertilisation until seeds and fruits
are formed;
Explain the structure of male and female gametophytes in Angiosperms;
Explain how an ovule (megasporangium) develops into a seed;
Discuss the further reduction in size and cell number of the male gametophyte
and female gametophyte of angiosperms, in comparison with gymnosperms;
Discuss the importance of double fertilisation in angiosperms and explain how it
differs from double fertilisation in gnetophytes;
Compare the alternation of generations of the mosses, seedless vascular
plants, gymnosperms and angiosperms with each other.
Angiosperms are also known as flowering plants. They are the most successful of all
plant groups in terms of their diversity - Nearly 90% of all living plants are angiosperms.
They are the last major group of plants to have evolved, appearing about 130–145
million years ago. In terms of structure and environmental adaptation, angiosperms,
with over 250,000 species and at least 12,000 genera, represent an astonishing
diversity.
As you know by now, heteromorphic alternation of generations in the sexual lifecycle is
a general characteristic of all plants (bryophytes, seedless vascular plants,
gymnosperms and angiosperms). Like the gymnosperms, flowering plants are
heterosporous, and the gametophytes develop endosporically within megaspores and
microspores. Unlike gymnosperms in which ovules are exposed on the sporophylls, the
ovules (megasporangia with integuments) of angiosperms are located inside ovaries
that will develop into fruits after fertilisation.
Angiosperms, like gymnosperms, have a dominant sporophyte and dependent
gametophyte. The gametophytes (male and female) are reduced to such an extent
that it consists of only a few cells (much less than the gymnosperms).
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Study unit 8
Individual activity
Revise the structure of flowers that you have done in the first semester.
Give a definition of a flower and name the main parts of a typical bisexual flower.
A flower (Fig. 8.1) is a modified stem with modified leaves. A flower is formed at the tip of a
stem called a pedicel. A pedicel has a swollen tip, the receptacle which bears the different
parts of the flower (modified leaves). The modified leaves in a flower can be sterile or
fertile:
•
Sterile leaves include sepals (which collectively form the calyx) and petals (that
collectively form the corona). Petals are sometimes brightly coloured to attract
insects for pollination. Sepals and petals are found in dicot plants and the term
perianth is used when you can distinguish between them. In the case of monocot
plants, you cannot distinguish between sepals and petals and the leaves are called
tepals. When you cannot distinguish between sepals and petals (as in monocots) we
use the term perigone instead of perianth.
•
Fertile leaves include the leaves that produce pollen (part of male gametophyte) or
ovules (part of female gametophyte).


Microsporophylls are now modified (smaller) and called filaments.
Microsporangia, borne in anthers on microsporophylls (filaments), are also
called pollen sacs. Inside the microsporangia, microspores (pollen grains)
are formed. These “stamens” (filaments and anthers with microsporangia)
are collectively called the “androecium” of the flower and it represents the
male part of the flower.
Megasporophylls are also called carpels and one carpel may fold to form a
closed structure – the ovary, or alternatively more than one carpel can fuse
sideways with one another to form an ovary. The ovules (megasporangia)
are located on the inside of the ovary (in contrast to the ovules of
gymnosperms that are carried naked). On top of the ovary there is a style
with a sticky stigma to which the pollen will adhere. The stigma, style and
ovary are called the pistil and collectively the pistil(s) of a flower is called
the “gynoecium” and it represents the female part of the flower.
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https://simple.wikipedia.org/wiki/Flowering_plant_sexuality#/media/File:Mature_flower_diagram.svg
Figure 8.1: Structure of a bisexual flower.
Most flowers are bisexual having both androecia and gynoecia, while others are
unisexual and can either be male or female.
Individual activity
Make sure that you know where in the flower the male and female gametophytes will
develop. Read the following paragraphs as a revision if you cannot remember:
1.
2.
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Endosporic development of haploid male gametophyte: Each anther in the
stamens consists of four microsporangia (pollen sacs) in which microspore mother
cells formed microspores through meiosis. Each microspore forms a tube cell (form
pollen tube later) and a generative cell (form two sperms later). Make sure that you
can describe the different wall layers of a pollen grain. Also, take note that further
development of the male gametophyte will only takes place after pollination.
Endosporic development of haploid female gametophyte: A megaspore mother
cell in a megasporangium (ovule), enclosed in an ovary, will undergo meiosis to
form four megaspores. Only one of the megaspores develops into a female
gametophyte (embryo sac). No archegonia will form inside the female gametophyte.
Study unit 8
After pollination, a pollen grain will land on the stigma and a pollen tube will grow down
the style and into the ovule through the micropyle. The tip of the pollen tube will break
open en 2 sperm cells (part of the male gametophyte on the inside of the pollen grain)
will enter the female gametophyte (embryo sac). The female gametophyte (embryo
sac) consists of ONLY 7 cells, but 8 haploid nuclei. Two of the nuclei, found within a
large central cell, are called polar nuclei. The egg cell (with 1 nucleus) is near the
micropyle and is surrounded by two cells, the synergids. Three other cells (antipodal
cells) are situated on the opposite end of the embryo sac.
https://commons.wikimedia.org/wiki/File:Embryosac-en.svg
Figure 8.2: Structure of the ovule surrounded by integuments and female
gametophyte (embryo sac) on the inside.
One sperm cell will fuse with the egg cell (fertilisation) to form a diploid zygote, while
the other sperm cell will fuse with the 2 polar nuclei to form a triploid (3n) tissue, called
the endosperm. This process is called DOUBLE FERTILISATION and differs from the
fertilisation found in gymnosperms. In most gymnosperms only 1 sperm will fuse with
the egg cell while in the most advanced gymnosperms (eg Gnetophyte) a primitive type
of double fertilisation will take place, but endosperm is never formed (additional
embryos may be formed when the second sperm fuses with another egg cell located in
more than one archegonium). Note that NO ARCHEGONIA are present in the
Angiosperms (see Fig. 8.2) and there is only ONE EGG CELL in contrast to
gymnosperms where several archegonia with egg cells may be present.
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After fertilisation, the zygote (inside the fertilised ovule) will develop into a multicellular
embryo and the surrounding endosperm will act as food for the growing embryo. The
integuments will harden to form a thick, protective coating around the embryo and
endosperm. These changes result in the formation of a seed (fertilised ovule). The
seed (Fig. 8.3) is protected with the seed coat (testa = hardened integuments) and on
the inside, the endosperm (feeding tissue) and embryo (young new plant) are formed.
The ovary (around the ovule) also expands after fertilisation and it will form the fruit
(with the seeds on the inside).
Photo taken by Arthurita Venter
Figure 8.3: Structure of a seed.
Group activity
Complete the following table. Only one word must be inserted in the open spaces.
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Development of
male gametophyte
Development of
female
gametophyte
Gymnosperms
Angiosperms
Immature pollen grain with four
cells (two prothallial cells, tube
cell, generative cell) when
pollination occurs
Immature pollen grain with two cells
(tube cell and ………………) when
pollination occurs
Mature pollen grain has six cells
(two prothallial cells, two sperms,
sterile stalk cell, tube cell)
Mature pollen grain has three cells
(two ……… and tube cell)
Female gametophyte consists of
many cells (100-1,000+)
Female gametophyte consists of
eight nuclei and seven cells (three
……………. cells, two ……….. cells,
one …… cell, a cell with two polar
nuclei)
Archegonia form part of female
gametophyte, except for
gnetophytes
……………. absent
Individual activity
Selfevaluering:
1.
Explain the lifecycle of a flowering plant in your own words under the following
headings:
•
Development of gametophytes
•
Pollination
•
Fertilisation
•
Development of seeds
2.
Tabulate the differences in the development of the male gametophyte and
female gametophyte between angiosperms and gymnosperms.
Describe the unique process of double fertilisation in angiosperms and focus on
the products of double fertilization and the differences between double
fertilization in angiosperms and gnetophytes.
Briefly discuss the differences in male gametophyte development between
angiosperms and gymnosperms.
3.
4.
5.
Briefly discuss the differences in female gametophyte development between
angiosperms and gymnosperms.
6.
Define the following concepts:
•
Exine / intine
•
embryo sac
•
endosperm
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Study unit 9
Study unit 9
FUNGI
Study hours
You will have to spend at least 11 hours to master the contents of this study unit.
Study material
Most of the learning content of this study unit is contained in this study guide.
Study outcomes
On completion of this study unit, you should be able to fulfil the outcomes as listed
under each study section.
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Study unit 9
Study section 9.1
An introduction to Fungi
Study outcomes
On completion of this study section, you should be able to:
•
•
•
•
describe the morphological structure of a fungus;
briefly describe sexual and asexual reproduction in fungi;
name the five major groups of fungi;
understand the partnership between the individual components of a lichen.
The study of fungi (singular, fungus) is called mycology.
Fungi are very important for life on earth as they, together with bacteria, are
responsible for decomposition and the recycling of nutrients. Some can also be used
as a food source, for the manufacturing of antibiotics, the brewing of beer and baking of
bread. There are also fungi that can cause diseases of plants and animals (pathogens).
Fungi are diverse with more than 100 000 species currently described. Estimates are,
however, that there are more than 1.5 million species on earth.
Fungi are heterotrophic and grow in aquatic and terrestrial environments where they
absorb their nutrients from the substrate. They cannot photosynthesize, like algae and
plants.
Fungi can be unicellular (the yeasts are examples) or multicellular (most other types of
fungi), in which case they are filamentous. The filaments of fungi are called hyphae
(singular, hypha). A group of interwoven hyphae forms a mycelium. Hyphae are
surrounded by cell walls of chitin – similar to that found in arthropods (e.g. insects or
spiders). Cells in hyphae can be separated with cross walls or septa, in which case the
hyphae are known as septated hyphae. If there are no cross walls (and therefore cells)
in hyphae, the hyphae are termed coenocytic hyphae (see Figure 9.1). Coenocytic
hyphae are therefore multinuclear, while septated hyphae have a nucleus inside each
cell.
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A
B
Drawing: S Janse van Vuuren
Figure 9.1: A) Septated hyphae with cross walls and uninuclear cells; B)
Coenocytic hyphae without cross walls, thus multinucleate.
Fungi reproduce by spores only and spores are dispersed by wind or water. The life
cycle of fungi differs from that found in plants because spores can be formed during
asexual and sexual reproduction. Some fungi can only reproduce asexually, while
others can reproduce both asexually and sexually.
Asexual reproduction in yeasts either occurs by ordinary cell division, or small buds are
pinched off from a parent cell. In most filamentous fungi the mycelium will form
sporangia with haploid spores on the inside. The spores will be distributed and when
they land in a moist place with enough nutrients they will germinate to form a new
mycelium.
During sexual reproduction hyphae from two different mycelia release pheromones that
attract each other. Two hyphae will meet and their cytoplasms will fuse (plasmogamy).
The nuclei do not fuse immediately, resulting in a dikaryotic stage (2 nuclei in the cell).
A long time may pass before the nuclei fuse (karyogamy). After fusion of the nuclei, a
diploid zygote is formed. The zygote will undergo meiosis leading to the formation of
“sexual” spores which will germinate to form new mycelia.
Interesting from an evolutionary point of view is that fungi and animals are more closely
related to each other than they are to plants or other eukaryotes, hence the placement
of fungi at the end in this module.
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Currently, 5 major groups of fungi are recognised:
•
•
•
•
•
Chytrids – very primitive fungi.
Zygomycetes – common fungi, often growing on bread and fruit.
Glomeromycetes – they mostly live in mutualistic relationships e.g. mycorrhizae
(fungus in/on plant roots).
Ascomycetes – a huge group of fungi with almost 65 000 species - also
sometimes called “sac fungi”. Their fruiting bodies, formed during reproduction,
are called ascocarps. Ascomycetes also live in symbiosis with green algae or
cyanobacteria to form lichens (see below), or some can also form mycorrhizae
with plants.
Basidiomycetes – fungi with dikaryotic stages in the life cycle. The fruiting
bodies of these fungi are called basidiocarps (mushrooms and puff-balls are
examples of basidiomycetes).
Lichens are symbiotic partnerships of two separate organisms, namely:
•
•
•
a fungus and a green alga, or
a fungus and a cyanobacterium, or
a fungus, a cyanobacterium and a green alga.
The algae and cyanobacteria photosynthesise and provide the lichen with food. The
lichen provides the algae or cyanobacteria with water and mineral nutrients and
protects it from desiccation. The fungus is usually the dominant partner, and it gives the
lichen the majority of its characteristics. Lichens are classified by the fungal component
- lichen species are given the same scientific name (binomial name) as the fungus
species in the lichen.
Only examples of the Zygomycetes and Basidiomycetes and lichens will be studied
during the module.
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Study unit 9
Study section 9.2
Zygomycetes
Study outcomes
On completion of this study section, you should be able to:
•
•
describe the general structure of Rhizopus as an example of the Zygomycetes;
describe sexual and asexual reproduction in zygomycetes with reference to
Rhizopus.
Introduction
There are approximately 1 000 known species of zygomycetes. This phylum includes
species of moulds growing on bread and several fruit types. The life cycle of Rhizopus
stolonifer (black bread mould) is typical and therefore the genus Rhizopus will be used
as an example of the zygomycetes.
PHYLUM ZYGOMYCOTA, Genus: Rhizopus
Rhizopus has coenocytic hyphae (no septa or cross walls present, except where
reproductive structures are formed), and hyphae are thus multinucleate (Fig. 9.1)
Hyphae that enter the substrate are branched for firm anchorage – these hyphae are
called rhizoids.
During asexual reproduction the tips of hyphae enlarge – the swollen tip is termed a
columella. A sporangium, with spores on the inside, develops on top of the columella.
Hyphae that carry sporangia are called sporangiophores. When the sporangium is
mature, the sporangium wall will break open and the spores will be released. When
they fall in/on a suitable substrate they will germinate to form a new mycelium.
If environmental conditions are unfavourable Rhizopus will reproduce sexually through
the process of conjugation. Identical hyphae of different mating strains (called plus and
minus hyphae) will send outgrowths towards each other. These big swollen structures,
the progametangia, will eventually touch each other. Progametangia do not have any
cross walls. In the tips of the progametangia there is an accumulation of nuclei
surrounded by cytoplasm. In each progametangium a cross wall will develop, forming a
gametangium with gametes on the inside. The basal part is now called the suspensor.
The wall between two adjacent gametangia will dissolve and plasmogamy will take
place. This will form a multinucleate, heterokaryotic zygosporangium in which
karyogamy will occur. The zygosporangium is resistant and can survive unfavourable
conditions. When conditions improve, the diploid nuclei in the zygosporangium will
undergo meiosis to form haploid spores on the inside. These spores are released and
they will germinate to form a new mycelium.
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PRACTICAL: PHYLUM ZYGOMYCOTA; Genus Rhizopus
Individual activity
Study the general structure, as well as asexual reproduction, of Rhizopus in the
slides labelled Rhizopus sporangia or Rhizopus asexual reproduction. Make a fully
labelled drawing indicating the structure of the mycelium and stages in asexual
reproduction.
Labels: mycelium, hyphae, rhizoids, sporangiophore, sporangium, columella, spores
Figure 9.2: General structure and asexual reproduction of Rhizopus.
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Study the different stages in sexual reproduction of Rhizopus in the slides labelled
Rhizopus conjugation. Compare your slides with the photographs given below and
add labels to the photos.
PROGAMETANGIA: Labels: hypha, progametangia, zygosporangium.
GAMETANGIA: Labels: cross wall (septum), gametangia with gametes, suspensor
IMMATURE ZYGOTE: Labels: suspensor, young zygote
ZYGOSPORANGIUM: Labels: suspensor, zygospore (or zygosporangium)
Figure 9.3: Sexual reproduction of Rhizopus.
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Study section 9.3
Basidiomycetes
Study outcomes
On completion of this study section, you should be able to:
•
•
describe the general structure of the basidiomycetes;
describe sexual reproduction in basidiomycetes.
Introduction
Approximately 30 000 fungi, including mushrooms, puff-balls and shelf fungi, are called
basidiomycetes and are classified in the phylum Basidiomycota. The name of the
phylum is derived from a structure, the basidium that forms sexual spores. During
sexual reproduction, a structure called the basidiocarp (fruiting body) is also formed.
The mushrooms that we usually eat are in fact the basidiocarps. Asexual reproduction
is not common in basidiomycetes. Basidiomycetes are important decomposers of wood
and plant material.
PHYLUM BASIDIOMYCOTA, Genera Agaricus or Coprinus
In basidiomycetes, the mycelium forms below the soil’s surface and it can be extremely
huge in size, covering many square kilometres. During sexual reproduction, a fruiting
body, named the basidiocarp, is formed from a very dense cluster of hyphae and the
basidiocarp appears above the soil’s surface – this is the common “mushroom” that we
often eat.
The basidiocarp consists of a stalk, called the stipe. On top of the stipe, there is a
structure that resembles a hat, the pileus. Underneath the pileus there are gills and the
spores (basidiospores) will be released from the gills. In a young stage, the gills are
protected and covered by a thin membrane that attaches the edge of the pileus to the
stipe – this membrane is called an annulus. As the spores mature and must be
released the annulus will rupture and break away and only small remnants of it
sometimes remain around the stipe or the edge of the pileus.
The gills consist of three different layers of hyphae. The inner layer consists of loosely
packed, elongated hyphae and this layer is termed the central trama. Just on the
outside of the central trama is a single layer of rounded hyphae, called the subhymenium. The outermost layer of each gill is called the hymenium and this is the
fertile layer where sexual spores will be formed. The hymenium consists of sterile hairlike hyphae, the paraphyses and fertile basidia. Each basidium will form 4
basidiospores through the process of meiosis. The basidiospores are carried on top of
short stalks (the sterigmas) on the basidia. The basidiospores will be pinched off and
released between the gills. When they fall onto a suitable substrate they will germinate
to form a new mycelium.
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PRACTICAL: PHYLUM BASIDIOMYCOTA; Genus Agaricus
Individual activity
Study the living material of the basidiocarp of Agaricus. Make a fully labelled drawing of
it.
Labels: Stipe, pileus, gills, remains of the annulus.
Figure 9.4: Structure of the basidiocarp of Agaricus.
PRACTICAL: PHYLUM BASIDIOMYCOTA; Genus Coprinus
Individual activity
Study a cross section of the basidiocarp of Coprinus, mounted on a permanent
microscope slide under low magnification. Compare your slide with the photo below and
add the following labels to the photo.
Labels: Stipe, pileus, gills
Photo taken by S Janse van Vuuren
Figure 9.5: Structure of the basidiocarp of Coprinus as seen under low
magnification.
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Study unit 9
Study a cross section of the basidiocarp of Coprinus, mounted on a permanent
microscope slide. Make a fully labelled drawing of the entire basidiocarp under high
magnification. Enlarge anyone of the gills on the same slide. Identify the three layers
in a gill. Focus on the hymenium and identify the paraphysis and the basidia with the
basidiospores. Add labels to the photo below:
Labels: Gill, central trama, sub-hymenium, hymenium, basidium, paraphyses,
sterigma, basidiospores.
Photo taken by S Janse van Vuuren
Figure 9.6: Structure of a gill of Coprinus.
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Study unit 9
Study section 9.4
Lichens
Study outcomes
On completion of this study section, you should be able to:
•
•
•
•
•
describe the vegetative body of a lichen;
describe the different types of lichens based on morphology;
describe asexual reproduction in lichens;
describe the biological, ecological and economic importance of lichens;
understand the partnership between the individual components of a lichen.
Introduction
Lichens are small and slow-growing organisms that can survive in environments that
experience extreme temperature or moisture. Scientists estimate that there are about
14 500 lichen species. Lichens are distributed from the Arctic to the Antarctic and are
often dominant in areas of high altitude or in polar environments where the low
temperature limits the growth of vascular plants. Lichens grow on a variety of
substrates such as rocks, tree trunks and leaves, soil, dead organic matter and some
are even aquatic species. They also grow on artificial structures such as glass,
concrete, metal, plastic and even cloth.
The vegetative body of a lichen is called the thallus and consists of a fungal
component, the mycobiont, and an algal (usually from the Chlorophyta) or
cyanobacterium component, called the photo- or phycobiont. The thallus consists of
three or four layers (Fig. 9.7). The upper layer consists of hyphae that are so tightly
compressed that they resemble parenchyma cells and this layer is therefore called the
upper cortex. This is followed by a layer consisting of algal or cyanobacterial cells
scattered among strands of hyphae. Below the algal layer is the medulla, consisting of
loosely packed hyphae. Some, but not all, lichens have a fourth layer of tightly packed
hyphae that resembles the upper cortex. This layer is, therefore, called the lower cortex
and contains rhizoids (short twisted strands of hyphae) that anchor the lichen to the
substrate. It should also be mentioned that some lichens do not form a definite algal
layer and that the algae are then uniformly distributed throughout the thallus.
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Upper cortex:
Tightly woven
fungal layer
Algal layer:
green algae or
cyanobacteria
Medulla:Loosly
woven fungal
hypae
Lower cortex:
Tightly woven
fungal layer
Rhizoid
Modified from https://courses.lumenlearning.com/microbiology/chapter/lichens/
Figure 9.7: Structure of a lichen showing the four layers, and a rhizoid.
Three distinct types of lichens can be distinguished, based on the morphology of the
thallus, namely crustose, foliose and fruticose type. The crustose type (Fig. 9.8A)
appears as coloured crusts or spot on rocks, trees or leaves. The thallus is small and
flattened dorsiventrally and attached to the substrate over its entire lower surface. In
the foliose type (Fig. 9.8B), the thallus is leafy and flattened and attached to the
substrate by the rhizoids. The thallus in the fruticose type (Fig. 9.8C) is cylindrical,
deeply divided or have a lacy appearance. It is common epiphytes on trees and shrubs,
especially in rain forests.
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Study unit 9
A: Crustose lichens
B: Foliose lichens
A: https://commons.wikimedia.org/wiki/File:Caloplaca_marina.JPG
B: https://www.flickr.com/photos/juliek1967/19474820359
C: https://www.flickr.com/photos/easyfordkid/7170616513
C: Fruticose lichens
Figure 9.8: Three types of lichens based on the morphology of the thallus.
Lichens reproduce asexually by means of fragmentation or soredia (singular, soredium)
(Fig. 9.9). Soredia are small clusters of algae surrounded by hyphae that become
detached from the main thallus and can grow into a new lichen. Soredia are dispersed
by agents such as rain, wind, running water or even animals and, in this way they can
colonize new areas. The algae reproduce through mitosis and cell division. The thallus
as a whole cannot reproduce sexually, only the fungal component through the
formation of sexual spores. The germinating hyphae must, however, come into contact
with algal cells if they are to form new lichens.
Source: Rushforth SR. (1976). The plant kingdom. Evolution and Form. Prentice-Hall, New
Jersey.
Figure 9.9: Drawing of a soredium showing the algal cells surrounded by
fungal hyphae.
As mentioned in the introductory paragraph, lichens can survive in extreme environments.
This is possible because lichens have the ability to become dormant when environmental
conditions are unfavourable and then resume their metabolic activity once conditions
become favourable again. This is aided by a gelatinous substance found in the upper
cortex which becomes opaque when the lichen dry, blocking out light and in doing so stop
photosynthesis.
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BIOLOGICAL, ECOLOGICAL AND ECONOMIC IMPORTANCE OF
LICHENS
•
Lichens can be used as bioindicators.
Lichens are extremely sensitive to pollution, especially by sulphur dioxide, and can be
used to assess air quality. Lichens can even be used to assess pollution by radioactive
substances following the crash of satellites, during uranium mining or nuclear fallout
from weapon testing.
•
Lichens are colonisers and play an important role in the ecosystem.
Lichens are often the first life forms to colonize newly exposed rocks and over time
they initiate soil formation. This is done by organic acids which the lichen excretes that
chemically weather the rocks they grow on. Dead and decaying lichens contribute
organic substances to the newly formed soil. Lichens with nitrogen fixing cyanobacteria
can fix atmospheric nitrogen. Lichens are also used as nest material by birds and
mammals.
•
Lichens are a source of food.
Lichens can serve as a food source for animals. It is consumed by reindeer and
caribou in the Arctic regions during the winter. The nutritional value is due to lichenin (a
carbohydrate that is allied to starch). Lichens are not readily consumed by humans as
the acids in the lichen make it unpalatable and it can have a laxative effect. It is
sometimes used as a food supplement (e.g. in soups) in Europe and Cetraria islandica
is consumed in India and Japan.
•
Lichens have medicinal applications.
Lichens are reported to have antibiotic properties and have been used in the treatment
of tuberculosis and cancer.
•
Lichens can be used in the perfume industry.
Essential oils obtained from lichens can be used for manufacturing perfume and soaps.
•
Lichens can be used for making dyes.
Many lichens were formally used to obtain dye in shades of yellow, brown, red and
purple to dye wool and silk fabrics. It is still used today to dye Scottish tweeds and East
Indian cotton fabrics. Lichens are also used to make Litmus paper, an acid-alkaline
indicator widely used in chemistry laboratories.
•
Lichens can be used for the tanning of leather and in the preparation of alcohol.
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PRACTICAL: Lichens
Individual activity
Study the live material of lichens provided or living material in the botanical garden
and make a fully labelled drawing thereof.
Labels: Substrate, crustose or foliose lichen
Figure 9.10: Living material of lichens.
Study the permanent slide of a longitudinal section through a lichen thallus and
compare it with the line diagram in Fig. 9.7. Make sure that you can distinguish the
following labels on the microscope slide.
Labels: Upper cortex, algal layer, medulla, lower cortex, rhizoid, thallus, hyphae,
algal cells
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Summary of key points:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
The study of fungi is called mycology.
Fungi are more closely related to animals than to plants and other eukaryotes.
Five major groups of fungi are recognized, namely, chytrids, zygomycetes,
glomeromycetes, ascomycetes and zygomycetes.
Fungi are heterotrophic and absorb their nutrients from a substrate.
Fungi consist of filaments called hyphae and a group of interwoven hyphae forms
a mycelium.
Cells in hyphae can be separated with cross walls or septa (septated hyphae) or
there can be no cross walls (coenocytic hyphae).
Fungi reproduce asexually and sexually by the formation of spores.
Zygomycetes includes species of moulds growing on bread and several fruit
types.
During asexual reproduction, hyphae in Rhizopus form sporangia with spores.
Sexual reproduction takes place through conjugation.
The basidiocarp of the basidiomycetes consists of a stipe and pileus. Beneath the
pileus are gills consisting of three different layers, namely, central trama,
subhymenium and hymenium. The hymenium consists of paraphyses and basidia.
Each basidium forms 4 basidiospores.
Lichens are small, slow-growing organisms that can survive in environments that
experience extreme temperature or moisture.
The vegetative body of a lichen (thallus) consists of a fungal component
(mycobiont) and an algal or cyanobacterium component (photo- or phycobiont.)
The thallus consists of three (or four layers), namely, upper cortex, algal layer,
medulla (and lower cortex).
Three distinct types of lichens can be distinguished based on the morphology of
the thallus, namely crustose, foliose and fruticose type.
Lichens reproduce asexually by means of fragmentation or the formation of
soredia (small clusters of algae surrounded by hyphae).
Lichens have various ecological, economical and biological importance.
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Study unit 9
Individual activity
Self-evaluation:
1.
Name and briefly describe the 5 major groups of fungi.
2.
How do fungi obtain their nutrition?
3.
Describe the sexual reproduction in Rhizopus.
4.
Make a fully labelled drawing to show the structure of a gill of Copricus.
5.
What is the relationship between an alga and the fungus in a lichen?
6.
Describe the basic structure of a typical lichen.
7.
Why can lichens survive in extreme environments?
8.
Describe how reproduction in lichens takes place through the formation of
soredia.
9.
Briefly explain what you understand under the following terms:
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
mycelium
coenocytic hyphae
plasmogamy
karyogamy
columella
sporangiophore
suspensor
zygosporangium
basidiocarp
pileus
annulus
basidium
thallus
mycobiont
photobiont / phycobiont
medulla
soredium
Reflection
Make sure that you have fully reached the outcomes set at the beginning of this
study unit.
***THE END!***
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