SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL107): Instructor: Elmar Schmid, Ph.D.
Chapter 17: Plants, plant evolution & Plant life cycles
- Part II -
Evolution of plants and prominent members of the plant kingdom

Fossil records, biochemical and molecular biological evidence tell us that all today
existing “modern” plants descended from an enigmatic multi-cellular green algal
ancestor

Among the different green algal groups discussed, especially the Charophytes (see
Figure below) and the green algae Coleochaete (see Image below) , are believed to
represent an ancestral algal type

based on these evidence scientists assume that algae once dominated the ancient
oceans of the Precambrian time over 700 million years ago

between 500 and 400 million years ago, some green algae made the “revolutionary”
transition to land and eventually evolved into “green plants” by developing a series of
critical adaptations which enabled them to successfully make the transition onto
land

in the past 400 millions years, green plants successfully inhabited and conquered
almost every region on Earth

today we classify these “modern” plants in five major groups
E
Evvoolluuttiioonn ooff ppllaannttss iinnttoo 44 m
maajjoorr ggrroouuppss
- in this image, a Charophyte (far left) is proposed as a green algal plant ancestor
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
I. Algae (Chlorophyta, Rhodophyta, Phaeophyta)

today, three major groups of algae thrive in almost all aqueous environments on planet
Earth
1. Green algae
2. Red algae
3. Brown algae
(= Chlorophyta)
(= Rhodophyta)
(= Phaeophyta)

all three groups perform photosynthesis by using the photosynthesis pigment
chlorophyll a; but they differ in their use of extra accessory pigments, such as
phycoerythrin ( red algae) or fucoxanthin ( brown algae)

algae differ from green plants since they do not have a rigid support and vascular
tissue

in algae the entire body has access to water and they therefore obtain their CO2 for
photosynthesis from the surrounding water and not from the air

algea are the aquatic ancestors of all modern plants; Scientists have evidence, that
modern plants have evolved very early in Earth’s history from a simple “archaic” green
algae, which existed already some 500 million years ago (!!)
- the ancestral green algae theory is supported by series of experimental findings,
which are:
1. both groups use chlorophyll a for photosynthesis
2. both life forms possess chloroplasts
3. the cell walls of both life forms are made up from cellulose
4. both groups store their sugars in form of starch
5. the most primitive, earth historically oldest plants, i.e. bryophytes, reproduce
with the help of flagellated sperm

the “modern”, multi-cellular green algae C
Coolleeoocchhaaeettee probably resembles these early
“archae-plants” in many ways (see Image below)
1. it lives on regularly flooded habitats, e.g. fringes of lakes
2. it possesses already pocketed zygotes

it develops a haploid, multicellular, vegetative thallus, which produces egg cells toward
its periphery
 once the eggs are fertilized, the “pocketed zygotes” are retained in this position on
the parent thallus and supplied with the necessary nutrients

some scientists think, that Coleochaete resembles our algal ancestor of land plants;
they consider it as a modern representative of the algal group that gave rise to our land
plants
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Image of the haploid vegetative thallus of the green algae Coleochaete
Multi-cellular
thallus
Cell
Pocketed zygote
( protected oocyte
 “modern feature”)

other scientists favor the green algae family of C
Chhaarroopphhyyttaa as the closest algal
relatives of plants
 both probably evolved from an early common, but unidentified, algal ancestor
Tracing evolutionary history of life & Molecular biology
 recently, phylogenetic scientists, which used modern DNA analysis techniques (rDNA
sequencing), found evidence, that a scaly, unicellular biflagellate, called Mesostigma
is the earliest common ancestor of the so-called Streptophyta lineage
 this lineage includes the Charophytes, the mosses (= bryophytes),
ferns, and all other multicellular land plants (= Embryophyta)

the earliest terrestrial organisms known were simple, so-called Rhynia-type plants,
often referred to as Rhyniophytes (see Images below)

the oldest fossil of these terrestrial plant ancestors was found in more than 400 million
year old (Late Silurian) rock formations; this Rhynia-type plant was called Cooksonia;

Cooksonia was a primitive, leaf-less plant, with an already developed primitive vascular
tissue; it carried spore-producing sporangia (= spore carrier) on top of its simple
formed stems

the plant ancestor Rhynia, which fossils were found in Rhynie, Scotland, consisted of
an underground stem (rhizome) with simple, erect dichotomous branches bearing
terminal, ovoid sporangia
 the stems must have been photosynthetic, since the plants lack leaves
 the rhizome had simple rhizoids (that may have helped anchor the
plants), but no true roots
 due to the sporangia, the life cycle might have been fern-like
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
R
Reeccoonnssttrruucctteedd iim
maaggeess ooff tteerrrreessttrriiaall ggrreeeenn ppllaanntt aanncceessttoorrss
R
Rhhyynniiaa
C
Cooookkssoonniiaa
the first spore-carrying plant parts, called sporangia




the first primitive roots, called “rhizoids”
 the first evolved anchoring devices for land plants
The characteristics of early land plants






the earliest plants, which fossils were found in the Early Devonian sediments of
Australia, were small; the majority with less than a few centimeters height
they were leaf-less plants, most likely equipped with primitive conducting vessels
similar to the xylem and phloem found in modern land plants
they possessed underground stems called rhizoids, but showed no roots
they had spore-carrying sporangia on the tips of the stems, no cones, no flowers
they had simple branched (= dichotomous) stems
their habitats were the huge, wet marshy land areas of the Devon

the Rhynia-type plants of the earliest Devonian period evolved very rapidly and
radiated (see:  radiation of biological species: has nothing to do with radioactivity!)
into the many ecological niches in the new terrestrial habitat

the terrestrial plants tremendously increased in size and developed a astonishing
complexity of branching
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

by the end of the mid-Devonian, several plant lineages, first of all the so-called
llyyccooppooddss, had developed leaves
 the primitive lycopods evolved leaves by vascularization of
simple outgrowths (= enations) and flattening of the terminal branch systems

plant evolution went fast and by the end of the Devonian period (= 375 million years
ago), most major lineages of spore-bearing plants, the Pro-gymnosperms (=
ancestors of naked seed plants) and the first Gymnosperms (= naked seed plants)
had appeared
numerous land plants had evolved, which already showed well developed leaves
and roots
Reconstruction of the Mid-Devonian landscape
from eastern North America
(Knight, Field Museum of Natural History)
B
Calamophyton
(= early horse tail)
A
C
Protolepidodendron
Eospermatopteris
(= primitive fern tree)
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
G
GR
RE
EE
EN
NP
PLLA
AN
NTTS
S

evolution on our planet created 4 major groups or divisions of terrestrial green plants

members of all 4 divisions of the plant kingdom still exist today
1. Division: Bryophytes (Mosses, Liverworts & Hornworts)

the first plants with features characteristic for the plant group called bryophytes
evolved about 480 million years ago; the term “Bryophytes” (bryo = sprout or burst
forth; phyta = plant) refers to a group a closely related “primitive” plants with shared
characteristics (see sections below), which includes the:
1. Mosses
2. Liverworts
3. Hornworts
- even though liverworts and hornworts differ significantly from each other regarding
morphology (= outer appearance), they have similar life cycles (see section
below)
- if scientists refer to an organism or its characteristics as “primitive”, they mean
that they earth-historically evolved earlier and they appear in the fossil records
earlier than the so called “advanced” organisms or characteristics

today, all still existing bryophytes have a waxy leaf coating called cuticule

the gametes (= sperm and egg cells) as well as the embryos of bryophytes develop
within protected plant structures, the so-called gametangia
- a gametangium is a plant structure which is responsible for the generation of the
haploid gametes (= sex cells)
- as you will see in more detail below, the male gametangium – which produces the
haploid sperm – is called antheridium, while the female egg cell-producing
gametangium is referred to as archegonium

bryophytes are avascular plants, which means that they have no vascular tissue (=
phloem or xylem) and therefore lack the internal stem support which (in higher,
vascularized plants or tracheophytes) is provided by the rigid, lignified cell walls of the
xylem and phloem cells

bryophytes produce flagellated sperm (similar to green algae), which swim from the
male gametangium (= antheridium) over to the egg cell (= oocyte) usually located at
the bottom of a cup-like female gametangium (= archegonium)
- therefore, fertilization in these plants, is a strongly water-dependent process
- without the presence of water, the flagellated bryophyte sperm have little chance
to reach the oocyte embedded into the archegonium;

the embryo grows out into a new, differently looking plant part, the so-called
sporangium which produces spores
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

all existing bryophyte plants thrive and are mostly found in wet and humid habitats on
planet Earth (most species are found in temperate climate zones)
Images of Bryophytes
Liverwort (Aneura pinguis)
Moss (Bartramia stricta)
 with typical, antennaelike sporangia
Moss (Hypnum cupressiforme)
2. Division Pteridophyta: Seedless vascular plants (Ferns, Lycopods,
Horse tails)

Ferns, which together with the horsetails and psilophytes, make up the division
Pteridophyta (pter = wing; phyta = plant) show major advancements in crucial plant
structures, such as leaves and stems, while retaining a primitive life cycle (see section
“plant life cycles” below)

The common ancestor of today’s ferns and horse tails (= Equisetophytes) evolved
some 410 million years ago; they were once the dominant plants on Earth and build the
huge Paleozoic forests
 about 12,000 species of ferns still exist today
 the numerous paleozoic sphenophytes grew up to thirty meters tall
 today the mostly tiny sphenophytes consist of only one genus,
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Equisetum, or commonly called horse tails

pteridophytes were the first vascular plants (= tracheophytes) on Earth and they have
a real vascular tissue (= xylem and phloem) in their upright and well-developed
stems; they generally show strong stems and tall upright growth; ferns have so-called
megaphyllous leaves or fronds.

The dominant sporophytes of pteridophytes, e.g. the mature fern plant, also show well
developed roots instead of the more primitive rhizoids or rhizomes

After fertilization of an oocyte with mobile sperm, the embryo develops in the female
gametangium (plural: gametangia) located on an inconspicuously small fern structure
called the prothallium
the prothallium has only a size of about 1-2mm and is ‘hiding away” in the moist
soil
this step of the reproductive life cycle of a fern is strictly water-dependent (see
sections below)

they were the first land plants, which did not require a permanent water layer or aquatic
environment for successful fertilization

instead of mobile sperm they produce so-called spores
 a spore is a hardy, nonflagellous plant vehicle which bears the
information for the new haploid gametophyte (see section below)

the spores develop in sporangia, called sori (singular: sorus) which in most ferns are
positioned on the underside of the frond-like leaves of the sporophyte; upon
maturation, the tiny spores are carried away by mostly wind
Image of sporangia (= sori) on the underside of a fern leaf
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

after landing on fertile, moist ground, the spores sporulate and develop a gametophyte,
which shows both, female and male, gamete-producing structures, called gametangia;
in ferns this plant stage is referred to as a prothallium
Images of seed-less plants
3 genera of ferns (Family: Gleicheniaceae)
Horse tail (Equisetum schaffneri)
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
3. Gymnosperms or seed plants (e.g. Conifers, Cycads, Ginkgos)

About 250 million years ago, the once dominating Pteridophyta, i.e. ferns and horse
tails, were successively replaced by the so-called gymnosperm plants (gymno = naked;
sperm = seed) as evidenced in the global fossil records

The “currently” 700 living species of gymnosperms on Earth are placed into four
taxonomic divisions, which include:
1. Division Cycadophyta
- also called cycads or “sago palms”
- even though they have palm-like leaves, they are not true palms (which are
flowering plants)
- members of this ancient, once thriving and very successful, species-rich form
of plant life - had its hay days during the Mesozoic period
- cycad fossils date back more than 250 million years into the Mesozoic, a
time where the dinosaurs roamed on planet Earth!
- they reproduce with the help of cone-like structures, which produce the seeds;
- today they are mostly found in warm, tropical zones on planet Earth
- they show a very slow growth, have a limited range and require special
habitats to survive; therefore they are extremely vulnerable to extinction!
Read more about this fascinating plant life forms under:
http://planetnet.rbgsyd.gov.au/PlantNet/cycad/
2. Division Coniferophyta
- also called conifers;
- most are evergreen plants which possess needle- or scale-like leaves;
- only the larches and bald cypresses are conifers that loose their leaves
every year;
- includes the pine trees, cedars, cypresses, firs, hemlocks, junipers, larches,
pines, redwoods, sequoias and yews;
- conifers have 2 types of cones, male and female cones, that are the
reproductive structures that produce the seeds;
- as one can easily see from the huge diversity of this group, it is an enormously
successful life form, and the coniferophyta are the dominating gymnosperms
on planet Earth today;
- they are one of the oldest groups of woody plants and thrive especially well
in the northern hemispheres and in higher elevations;
- conifer fossils have been unearthed in rocks that are about 300 million years
old!
- conifers form about 30% of the world’s forests today!
- this truly fascinating group includes with the redwood trees the tallest, with the
sequoias the largest (see Image below), and with the bristle cone tree (see
Image below) the oldest life forms on planet Earth;
- all three of these confers are indigenous to California, U.S.A.!
- the redwoods may reach heights of more than 360 feet (110 meters)!
- the “General Sherman” sequoia tree in Sequoia National Park in California
is about 275 feet tall and has a circumference of 103 feet (31.4 meters) at
the base of its massive trunk;
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
- the age of one of the famous bristle cone trees in the White Mountain range
of Southern California has been dated to be about 4,600 years old!
- Conifers are of great commercial and ecological value for humans; in the
U.S. and other parts of the world, conifer wood is used as timber and
construction material to build houses and other buildings;
- especially redwood, Douglas fir and loblolly pine are commonly cut for
that commercial purpose;
- Conifers also provide much material to make the wood pulp in industrial
production of paper and cardboard;
- the rooting and leave system of conifer forests have a tremendously
important ecological role for natural filtration of rain water and for enrichment of the air with oxygen;
Cones and branches of a bristle cone
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Ancient bristle cone tree
(White Mountain range, California)
3. Division Ginkgophyta
- also called Ginkgos or maidenhair trees;
- they have fan-like leaves that are shed during winter time (= deciduous leaves)
- products and extracts of Ginkgo biloba – the only surviving species of a once
thriving group of plants – are sold as herbal remedies in many stores;
4. Division Gnetophyta
- includes a plant called mormon tea (Ephedra)
- this primitive looking plant has green upright stems and lacks well-developed
leaves
- parts and extracts, i.e. ephedrine, of mormon tea are sold as herbal remedies
or are part of over-the-counter supplements
12
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

Gymnosperms (or naked seed plants) were the earliest seed plants appearing on
Earth
they have cones as reproductive structures, build seeds but show no flowers nor
build any fruit
the earliest Gymnosperms developed during the Paleozoic Era and began to
flourish (or radiate as biologists say) during the Mesozoic Era, some 360 – 280
million years ago (see reconstructed Images of Mesozoic plants below)
Reconstructed images of seed plant ancestors and early seed plants
(based on fossils finds of the Carboniferous period: 360 – 286 million years ago)
- other seed plants were: Neuropteris, Pecopteris, Alethopteris, Sphenopteris, Odontopteris and the scale trees Lepidodendron and Sigillaria
30m
Medullosa
(seed fern tree)
Lepidodendron
(scale tree)

Today, the conifers (= trees with seed-bearing cones) build the largest group of
gymnosperms, which includes the pines, spruce, fir, bald cypress and Norfolk Island
Pine (Araucaria)

Gymnosperms share the following unique combinations of characteristics:
1.
They are true tracheophytes, i.e. they have vascular tissues
2.
Their sporophyte is dominant
3.
They do not require water at any stage of their life cycle for successful
fertilization
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
4.
They produce embryo-protecting seeds
5.
The seeds are either unprotected or develop within a modified bud called
cone

As mentioned already in the section above, one of the hallmark characteristics of
gymnosperms is the observation that the embryo is packed into so-called seeds
- a seed is a protective shell or covering packed with food supply in form of the
endosperm layer for the (later) germinating embryo
- the seeds are not contained in a thick layered fruit
Auracaria sp.
Examples of typical “modern” gymnosperms
Sequoia sempervirens
 with large female cones
 this species forms the largest trees on Earth
Sequoia s. leaf and cone
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
4. Angiosperms (= Flowering Plants)

The common ancestor of all angiosperm (= covered seed) plants or flowering plants
evolved approx. 130 million years ago during the Cretaceous period
the early angiosperm plants had structurally very primitive, cone-like flower heads
(see Image below), called strobili
Reconstructed images of early ancestors of flowering plants (angiosperm) living
during the Cretaceous period (144 – 65 million years ago)
• Lily-like pollen found in Upper Triassic rock formations (France)
• Magnoliaceae, the most primitive angiosperms, thrive which had
cone-like flowers and developed fruits ( Magnolia or Liriodendron)
Strobili
- cone-like, primitive
flower heads
Archaestrobilus
cupulanthus
Sanmigueli
lewisii
Both plant reconstructions show Gnetophytes
 Plant intermediates between conifers and angiosperms
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

at the beginning of the Cenozoic (around 60 million years ago), angiosperm life forms
began to flourish dramatically (“angiosperm radiation”) and eventually evolved into
the many different species of flowering plants we know today

the latest, great “success story” of angiosperm evolution is the rise of plants called
grasses, which are characterized by inconspicuously small flower heads and rapid
growth
- grasses form one of the largest and most varied families within the plant kingdom
- their evolutionary success is reflected in the fact that grasses can be found on almost
all land surfaces on earth
- grasslands began to spread in many areas of the world during the late Cenozoic,
about 10 to 15 million years ago and co-incites with the radiation of ruminant animals
- grasses are C4 plants, i.e. they use the C4 pathway to fix atmospheric carbon dioxide
- the reproductive floral organs of grasses, which are grouped together in flower
clusters, includes the male stamens, the pistil (the female floral part) and 2 or 3
delicate little scales called lodicules
- the two-ranked grass leaves surround the culm (stem) and are made up of a sheath,
blade, ligule and auricle
- grasses are of high ecological and economical value and includes important
groups, such as:
1. Grazing and forage grasses
- the principal source of food for grazing animals, e.g. cows, buffalos, sheep
- e.g. Blue grama (Great plains of the U.S.), Buffalo grass, Crested wheatgrass,
Sudan grass, Kentucky bluegrass, Bermuda grass, Timothy,
2. Turfgrasses
- used to cover lawns, golf courses, athletic fields and play grounds
- e.g. Colonial bent, Kentucky bluegrass, Bermuda grass,
3. Ornamental grasses
- used in flower gardens, parks and other landscaped gardens
- e.g. Chinese silvergrass, pampas grass, uva grass
4. Cereals
- one of the worlds most important food crops to assure nutrition of the world’s
human population
- the seeds of the cereals provide grain that is ground into flour
- e.g. wheat, barley, rice, oats, corn, sorghums, Emmer, millets,
5. Sugar cane
- grass which provides more than half of the world’s sugar (sucrose) supply!
- also produces a fiber called bagasse, that is used in making wallboard,
plastics, and fuel to heat homes
- in recent years sugar cane is also more and more used to make gasoline
(Bio-ethanol) to fuel cars, mostly in Brazil
6. Woody grasses
- grasses with strong, wood-like stems that are used for building houses, rafts,
bridges, and furniture
- e.g. Bamboo, Reed is used in some parts of the world to build roof covers
16
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

today, the modern angiosperm plants build the great majority of the known 240,000
species of terrestrial plants
Nearly 80% of all plants existing on planet Earth today are angiosperms

while conifers are the dominating seed plants which cover most of the parts of the
northern hemisphere of our planet, the angiosperms or flowering plants are the seed
plants which successfully conquered most other land areas on Earth

whereas gymnosperms (= e.g. conifers) currently supply most of our raw material for
lumber and paper production (= cellulose!), it is the angiosperm plants of which we
receive the most diverse form of materials
1. Food (carbohydrates, proteins)
 carbohydrates from cereal grains e.g. wheat, corn, oats, rice, barley, sugar
cane
 proteins from soy bean, beans, lentils, etc.
 vitamins from fruits, citrus, vegetables, etc.
2. Fibers for textiles
 e.g. cotton, hemp, coconut, pineapple
3. Medicinal, life style and illegal drugs from medicinal plants
 angiosperm plants are known to synthesize a plethora of molecules, the socalled secondary plant metabolites, for their special (mostly unknown)
purposes (see secondary plant metabolites special section below)
 e.g. the anti-cancer drugs taxol and vinblastine, the plant supplements
Echinacea, St. John’s word, Ephedra
 e.g. the life style drugs caffeine (coffee), nicotine (cigarettes), theophylline (tea)
 e.g. the illegal drugs morphine, heroine, cocaine, etc.
4. Fine hard wood for furniture
 e.g. oak, cherry, walnut, teak, etc.

Evolutionary biologists assume that angiosperms evolved and developed further from a
gymnosperm ancestor; they evolved novel and unique plant parts:
1. they developed a so-called closed sporophyll which to supply further protection
for the seed (see Image below)
2. they have highly developed vascular tissues with thicker and stronger cell walls
than gymnosperms
 especially the larger vessel elements of their phylem enabled more efficient
transport of nutrients than the “evolutionary older” tracheids
17
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Comparison of a closed sporophyll as seen in angiosperms with the open
sporophyll characteristic for gymnosperm plants
Gymnosperm
(“Naked seed” plant)
Open
sporophyll
Angiosperm
(“Covered seed” plant)
Closed
Sporophyll
 Ovary
Ovule/
 later seed
3. flowering plants evolved a unique reproductive part called flower which mostly
accounts for the unparalleled success of this family in plant evolution
the usually very colorful and variant forms of flowers solely serve the reproduction
of the plant; flowers also dramatically vary in size amongst different flowering
plants; a flower head – in the case of grasses – can be inconspicuously small –
with only millimeter size or – in the case of the gigantic Rafflesia flower head –
reach up to 1 meter in diameter ! (see Image below)
the flower exposes the plants reproductive organs, which are:
a. the male stamen (= consists of anthers and filament)
 usually surrounding the female carpel
b. the female carpel or pistil (consists from top to bottom of: stigma, style and
ovule-containing ovary)
(for parts of a flower see image below)
- the flower is the site of (usually animal) pollination and fertilization
- the flower is also the place where the later fruit (see 4. below) develops
- the fruit harbors the seeds with the angiosperms embryo packed into it
18
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Important structural parts of a typical flower of an angiosperm plant
19
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Flower of Rafflesia tuan-mudae (Euphorbiaceae)
- The world’s largest flower head -
Flower diameter: ~ 1 meter: Weight: 7 kg
Raffleciacea species, such as the shown Rafflesia tuan-mudae, produce
the world’s largest flower heads which can bloom up to 1 meter in diameter.
Rafflesia is a leaf-less, stem- and rootless, non-photosynthetic plant (= holo-parasitic
angiosperm), which lives as a parasite embedded into its obligate host plant
Tetrastigma (Vitaceae)
4. a thickening of their ovule wall lead to the evolution of a fruit; over time
angiosperms developed different forms of fruits which are dependent on the
architecture of the flowers and its reproductive organs
a. Simple fruits
 develop from a flower with a single carpel and ovary
 e.g. apple, pea pods, cherry, peach
b. Aggregate fruits
 develop from a flower with many carpals
 e.g. blackberry, raspberry
 each small fleshy part of the blackberry developed from a single ovary
20
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
c. Multiple fruits
 develop from a group of separate flowers which are tightly clustered
together
 the walls of the separate ovaries fuse together and build one fruit
 e.g. pineapple

the angiosperms develop seeds in complex reproductive structures, called flowers,
within protective chambers called ovules

angiosperms are classified into 2 groups based on:
1. the structural features of the first appearing leaf, called seed leaf or cotyledon
2. on the morphology of the embryo within the seed
11.. cchhaarraacctteerriissttiiccss ooff m
moonnooccoottyyllee aannggiioossppeerrm
mss







the embryo within the seed has only one seed leaf (= monocotyl)
approx. 65,000 species are known today
the developing leaves of the plant have usually parallel veins
the vascular bundles in the stem are in complex, irregular arrangements
the floral parts, e.g. petals, are usually in multiples of three
monocotyles have a fibrous root system
prominent examples of monocotyle angiosperms are orchids, palms, lilies,
grass, corn
2. characteristics of dicotyle angiosperms









the embryo embedded into the seeds has two seed leaves (= dicotyl)
approx. 170,000 species are known
the main leaf veins of dicotyle angiosperms are usually branched
the vascular bundles (xylem/phloem) are arranged in rings
the floral parts appear in multiples of 4 or 5
dicotyls have a characteristic tap root, which stores food in form of starch
some dicotyl species develop so-called rhizomes (e.g. irises)
 these horizontal stems have storage function
 some rhizomes can spread out to form new plants
some dicotyl plants, e.g. white potato, have rhizomes that end in enlarged
structures called tubers
 tubers store enormous amounts of sugar in form of starch
prominent dicotyl plants are shrubs, trees (not conifers!), carrots, sugar beets
and food crops (not corn!), sunflower and other ornamental plants
21
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
S
Seeccoonnddaarryy m
meettaabboolliitteess aanndd ttooxxiinnss pprroodduucceedd bbyy aannggiioossppeerrm
mss

since plants do not have the option of leaving the area when danger, e.g. caused by
herbivorous animals or insects, approaches, plants have developed many strategies to
protect themselves

one of the highly efficient strategies especially used by angiosperm plants is the
production of plant chemicals to guard themselves against specific threats, such as
hungry insects or an invading bacterium or a fungus

angiosperm produce an amazing variety of organic chemical compounds over and
above their everyday components and metabolites, which are called primary
compounds or primary metabolites

the other branch of chemical compounds, which plants produce from the primary
metabolites and use for defense and protection, are called secondary metabolites
 these chemical compounds are not central to metabolism and
often it can be difficult to identify their function

more than 20,000 natural compounds, or secondary metabolites, have been identified
and isolated from angiosperm plants

the 3 most important types of secondary plant metabolites are the:
1. Terpenoids

many spices and fragrances contain terpenoids
e.g. Geraniol
Menthol
Pinene
Carotenoids
in geranium
in mint
in pine resin
accessory pigment in all green leaves
2. Phenolics

Phenolics are a huge and diverse group of so-called aromatic compounds (containing
benzene rings) usually with hydroxyl groups

they play an important role in plant - herbivore interactions and in disease resistance of
plants.
e.g. Coumarin
Anthocyanins
Tannins
Lignin
scent of new-moan gras
pigment in red, blue and purple flowers
are extracted from tree bark and
occur in grape peel;
they reduce the digestibility in insects
polymer of phenyl propanoid molecules,
it strengthens plant cell walls,
particularly in the xylem of woody plants;
22
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
it is interwoven with, and chemically
bonded to cellulose
3. Alkaloids

another important (and diverse) type of secondary metabolites are the so-called
aallkkaallooiiddss, which are nitrogen containing cyclic compounds
 this class includes many plant poisons, medicinal and illegal drugs:
Caffeine
Nicotine
Atropine
Quinine
Cocaine
Mescaline
Ephedrine
Lupinine
Colchicine
main alkaloid of the coffee and tea plant
main alkaloid of the tobacco plant
alkaloid of the deadly nightshade Atropa belladonna
anti-malarial alkaloid in the bark of the Cinchona tree
alkaloid in the leaves of the Coca plant
alkaloid found in many cacti plants
alkaloid of the Ephedra plant
bitter alkaloid of lupines
alkaloid of Colchicium autumnale

many of the angiosperm’s secondary metabolites have pharmacological effects:
anticoagulant, anti-inflammatory, diuretic, antibacterial, anticancer, etc.

almost all plant alkaloids are poisonous to humans; when ingested in concentrated
levels or consumed in large quantities, they can cause illness and in some cases lead
to death

Examples include:
1. Rhubarb
2. Potatoes
3. Lima beans
4. Squash/cucumbers
5. Legumes
6. Tomatoes
7. Broccoli
contains oxalates
produce chaconine and solanine
contain cyanide compounds
contain cucurbiticin
contain high levels of lectins
contain tomatine and quercetin glycosides
contains nitrates and glucosinolates
23
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
TThhee ppllaanntt cchhaarraacctteerriissttiiccss tthhaatt eennaabblleedd tthhee ssuucccceessssffuull m
moovvee ooff ppllaannttss oonnttoo llaanndd

in order to adapt and to meet the challenges of life on land, e.g. to be exposed to air,
intense sunlight and high gravity, green plants developed 7 unique structures
1. Cuticule


A well-developed cuticule, which is a wax-like, water-proof layer made of special fats
which covers and protects the epidermal cells of the leaves from dehydration and
damage
the cuticule prevents the water loss of plants due to evaporation during day times
 therefore desert plants or succulents, e.g. ice plant, have an
extremely thick cuticule
2. Stomata


these pore-like openings, mostly underneath the leaf, enable the regulated gas (CO 2,
O2) and water exchange with the surrounding air
they are formed by two bean-shaped cells, called guard cells, which can regulate their
cell shape
3. Stems

plants evolved stems and rigid vascular tissues within the stems to meet the challenge
of higher gravity on land
 stems enable the plant to rise above the ground and to hold itself
upright
4. Roots


with the help of evolved roots, plants were able to successfully compensate for the
increased water loss they were facing on land
they provide water and mineral supply from the soil for the upper parts of the plant as
well as anchorage
5. Vascular tissues (phloem/xylem)

vascular tissues are found in all modern plants

2 types of vascular tissues are known in green plants
1. Xylem
 passive transport of water and minerals
 microscopic tubes made of dead cells
2. Phloem
 active transport of synthesized sugars and other
plant metabolites by live cells
24
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
6. Seeds

it is the seed plants which dominate among the plant kingdom today on Earth
7. Flowers

flowers are a hallmark of angiosperms which build the largest group among the seed
plants today

the flower of angiosperms consists of 4 different kinds of modified leaves:
1. sepals
 enclose the flower before it opens
2. petals
 the most colorful and variant part of the angiosperms
 has important function in attracting animals (e.g. bees,
birds, flies, bats, etc.) to transport the pollen from the
plants
3. stamen
 consists of stalk and anther = the saculus-like male
organ which develops the pollen
4. carpel
 consists of 2 major parts
1. the stalk with the ovary at the base; the ovary is the protective chamber
which contains one or more ovules
2. the stigma = it is the sticky tip which traps the male pollen
25
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
PLANT LIFE CYCLES

green algea and the different higher plants do not only differ morphologically but
also regarding their sexual reproduction and life cycles
Definition : Life Cycle
a life cycle is the closed cycle starting with the fertilization of two sex or gender cell (=
gametes), formation of an embryo, development into an new plant and ends with the
production of new gametes

in green algae the female gametes leave into the water where they are fertilized
by the free-swimming male sperm cells, while in higher plants fertilization and the
development of the embryo occurs in a protected gamete-containing plant
structure, the so-called female gametangium
 the gametangium of green plants provides an ideal moist surrounding where
the fertilization by the wind- or animal-carried pollen takes place
 the only common feature between algae and higher plants is, that in both the
gametes are produced in gametangia (singular: gametangium)
 a gametangium is a plant structure consisting of protective layers of cells
which surround and protect the gamete-producing cells

plants have in contrast to higher animals so-called alternating generations

that means diploid (2n) plant types called sporophytes and haploid (1n)
gametophytes generate each other during a typical plant life cycle (see Figure
below)
Alternating generations during a plant life cycle
26
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.

-
the life time of the haploid generation in plants is much longer than in animals
the haploid stages in animals are only in form of sperm and egg cells in the gonads

in the general life cycle of a plant there is an alternating appearance of a haploid
gametophyte (1n) and a diploid sporophyte (2n) generation each with typical
perfectly adapted features

the features depend on the different life conditions and therefore the life cycles
amongst the different types of plants vary

during their more than 400 million years of existence on our planet, green plants
evolved unique and amazing reproductive strategies to enable the creation of new
offspring
E
Exxaam
mpplleess ooff ppllaanntt lliiffee ccyycclleess

now let’s have a closer look at the life cycles of different, prominent members of the
plant kingdom
A. The life cycle of algae

fossil records of algae date back to approximately 3 billion years into the
Precambrian era

algae exhibit a wide range of reproductive strategies, from simple, asexual cell
division to complex forms of sexual reproduction
usually algae multiply vegetative; one cell buds from another by
simple mitotic (not meiotic) cell division to form genetically and morphologically
identical daughter cells/organisms
the daughter organism is - in molecular biological terms – a clone of the parent
among the larger kinds of algae, however, methods of reproduction vary greatly

some algae reproduce by breaking off small sections of their main body, which
grows into another complete new plant; this form of reproduction is called
fragmentation

some algae produce spores, called gametes; two gametes of opposite sex must
unite to produce the cell that will develop into a new alga
 spores and gametes are simpler than the later evolved seeds

in green algae the female gametes leave into the water; there they are fertilized by
the free-swimming male sperm cells
 this is a risky maneuver, since the unprotected gametes as well as
the fertilized zygotes can become easy prey to all sorts of aqueous
predators
 this risk is calculated and over-run by the mass production and
release of thousands of gametes
27
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
 in higher plants fertilization and the development of the embryo
occurs in a protected chamber, the female gametangium
 the gametangium of green plants provides an ideal moist
surrounding, where the fertilization by the wind- or animal-carried
spores or pollen takes place

the only common feature between algae and higher plants is, that in both, the
gametes are produced in gametangia (singular: gametangium)
 a gametangium is a plant structure consisting of protective layers of
cells which surround and protect the gamete-producing cells
B. The life cycle of Bryophytes (= mosses, hornworts, liverworts)

the group of bryophytes includes the mosses, liverworts and hornworts

they are small, nonvascular plants that first evolved approximately 400 million years
ago
 the earliest land plants were most likely bryophytes

bryophytes lack a water-conducting vascular tissue, which limits their size
 mosses are usually under 5 inches high, which makes the study of
their life cycles intricate

mosses, hornworts and liverworts evolved a ggaam
meettoopphhyyttee that is larger than the
ssppoorroopphhyyttee

more interestingly, the sporophyte develops on (!!) the gametophyte
 the green cushiony moss pillow (which we usually see) is a haploid
plant (!!), the gametophyte of the moss
The stages of the moss life cycle
1. ggaam
meetteess develop in the haploid (1n) ggaam
meettaannggiiaa on the gametophyte by
mitosis; the flagellated sperm swim to the egg on a haploid gametophyte
2. after fertilization in the cup-like ggaam
meettaannggiiuum
m (1n), the diploid zygote (2n)
stays there
3. the zygote divides by mitosis and develops into the ddiippllooiidd ssppoorroopphhyyttee (2n)
4. meiosis occurs in the so-called ssppoorraannggiiaa at the tips of the sporophyte stalks
and haploid ssppoorreess are released
5. the transported and settled spores (under ideal growth conditions) undergo
mitosis and develop into the hhaappllooiidd ggaam
meettoopphhyyttee (1n) again
28
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
6. the cycle is closed and a new life cycle with new players and generations can
begin
Stages of the moss life cycle
29
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
C. Life cycle of ferns

ferns have (like almost all modern plants) a dominant, diploid ssppoorroopphhyyttee
generation
 it is the sporophyte stage of the ferns which we usually see
as the typical large, fan-like fern leaves

the haploid ggaam
meettoopphhyyttee is a tiny heart-shaped plant tissue
 this coin-sized and thallus-shaped gametophyte usually
thrives hidden in the moist soil of the fern habitat
The stages of the fern life cycle
1. the small ggaam
meettoopphhyyttee (1n) produces haploid ssppeerrm
m (1n) by mitosis; the
flagellar sperm swim to the haploid eegggg on the same plant structure
2. after fertilization the diploid zzyyggoottee (2n) remains on the gametophyte
3. after multiple mitotic cell divisions the diploid ssppoorroopphhyyttee grows out of the
gametophyte
4. specilized (germ) cells in the ssppoorraannggiiaa of the mature ssppoorroopphhyyttee undergo
meiosis and produce haploid ssppoorreess (1n)
5. the spores are released, carried away (usually by wind) and develop into the
haploid ggaam
meettoopphhyyttee again after multiple rounds of mitosis

ferns together with other seedless plants, e.g. horse tails, once build the vast socalled coal forests during the tropical-humid Carboniferous period on our planet,
about 285-360 million years ago
 they disappeared at the end of the Carboniferous period because
of a dramatic and global change of the Earth’s climate
 the dryer and colder Earth climate made the vast swamps and
tropical forests almost completely disappear
 the remains of the crumbling fern forests were fossilized into a
black sedimentary rock, called coal

the massive extinction of the Carboniferous fern, lycopods, horse tails and other
seedless plant forests cleared the way for the evolution and dominance of the socalled seed plants
30
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Stages of the fern life cycle
r
e
d
( red = haploid gametophyte; bblluuee = diploid sporophyte)
31
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Earth periods & the occurance and distribution of plants groups
 the coming & going of different plant groups over long periods of time

the seed plants ( conifers!) were much better adapted to the long harsh winters
and the much colder climate which dominated on Earth at the end of the Carbon
period

another major advantage over the seed-less plants was, that they were the first “real
terrestrial plants”; they were the first plants with the capability to complete their
whole life cycle on land
 their protected and sturdy pollen are able to reach and fertilize eggs
without being immersed in water

after the swamps had disappeared, the so-called gymnosperms (= “naked seed”
plants) were among the first plants which appeared on the scene; they successfully
“filled the gaps” and populated the ecological niches, once dominated by the ferns
and horse tails

especially the naked-seeded conifers turned out to be best adapted to the changed,
dry-cold climate of the northern hemispheres on our planet
 today, conifers, such as fir, pine, are the dominating plant types of the
vast fir forests of Canada, Scandinavia, Russia and Northern China

here are 700 living species of gymnosperms, placed into four divisions: conifers,
cycads, ginkgos, and gnetales
32
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
D. The life cycle of gymnosperms (e.g. conifers or ginkgos)

conifers have huge sporophytes which we see as the typical pine trees;

the diploid (2n) ssppoorroopphhyyttee dominates the plants appearance while the haploid
(1n) ggaam
meettoopphhyyttee generation consists only of microscopically small stages that
grow inside the tree’s cones

the cones harbor all of the conifer’s reproductive structures

a pine tree bears 2 types of cones
1. the hardy, woody ffeem
maallee ccoonneess
 consists of many scales each bearing one pair of
ovules
2. the soft and smaller m
maallee ccoonneess
 each scale produces many sporangia, which make
the numerous haploid spores by meiosis
 they make up the typical yellowish conifer pollen clouds
we seasonally see in the landscapes
The stages of the life cycle of pine trees
1. Within the ovules (= macrosporangium) of the female cone scales, diploid
macrospore mother cells (2n) undergo meiosis to generate haploid (1n)
spore cells which develop into the ffeem
maallee ggaam
meettoopphhyyttee (1n); after months, a
fertile egg (oocyte) appears in the female gametophyte after mitosis of special
cells;
2. Within the pollen sacs of the scales of the male cones, diploid microspore
mother cells (2n) undergo meiosis and generate winged haploid pollen grains
(1n) which are dispersed by wind into the air; they eventually reach the sticky
surface of a neighboring female cone where they became trapped; they attach to
the ssppoorraannggiiuum
m (2n) in a process called pollination;
3. the m
maallee ggaam
meettoopphhyyttee, which is the pollinated pollen grain on the female cone,
forms a pollen tube which works its way towards the egg cell within the embryo
sac; the nuclei of the generative cell fuses with the nucleus of the oocyte to form
the diploid zygote (2n)  fertilization event;
4. after fertilization of the egg (which occurs more than a year after pollination!) in
the ffeem
maallee ggaam
meettoopphhyyttee (1n), a diploid zygote (2n) develops and an embryo
forms by consecutive mitotic cell divisions
33
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
5. the ovules integument (= surrounding cell layer) develop into a sturdy, highly
lignified seed coat, which surrounds and protects the diploid embryo; the ovule
has turned into a seed
6. approx. two years after pollination, matured, winged seeds are carried away by
wind, gently fall to the ground and (if conditions are favorable) start to germinate
(see seed germination); the embryo grows into a seedling and further into the
matured ddiippllooiidd ssppoorroopphhyyttee again, which is the pine tree;
7. this tree produces again male and female cones and the conifer life cycle closes
Stages of the conifer life cycle
bblluuee = diploid sporophyte
rreedd = haploid gametophyte
Male and female gametophyte generation
34
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
D
Diippllooiidd ssppoorroopphhyyttee ggeenneerraattiioonn
Summary of the major factors which account for the successful development of
conifers on land
1. all reproductive stages are housed in protected cones on the same sporophyte
2. the evolution of an ovule
 creates a shared place for the crucial reproductive stages
= pollination, fertilization and embryonic development occur at one
spot
3. development of a seed
 carries and protects the embryo
E. The life cycle of angiosperms (= flowering plants)

Angiosperms, or flowering plants, were the last of the seed plant groups to evolve
on Earth; they appeared over 140 million years ago at the beginning of the
Cretaceous Earth era

while conifers are the dominating seed plants, which cover most of the parts of the
northern hemisphere of our planet, the angiosperms became the seed plants, which
successfully conquered most other land areas on Earth
 today nearly 80% of all plants on Earth are angiosperms
35
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
The stages of the life cycle of an angiosperm plant
1. ddiippllooiidd m
miiccrroossppoorree m
mootthheerr cceellllss (2n) within the antheres of the stamen
undergo meiosis and each cell builds 44 hhaappllooiidd m
miiccrroossppoorreess;
2. the microspores undergo mitosis and form the pollen grains; inside the pollen
are two (or, at most, three) spore cells that comprise the immature m
maallee
ggaam
meettoopphhyyttee (1n);
3. the hhaappllooiidd ssppoorreess cceellllss divide mitotically to form 2 cells, i.e. the tube cell and
the generative cell; a thick wall, the exine, starts to surround both cell types and
forms the final hhaappllooiidd ppoolllleenn ggrraaiinn
 the haploid sperm in angiosperms is produced by mitosis
4. the ddiippllooiidd oovvaarryy within the carpel contains several ovules (macrosporangia)
within the ovule is a central cell surrounded by protective cover cells; the central
cell enlarges and undergoes meiosis to produce 44 hhaappllooiidd ssppoorreess;
5. only one of the so-called macro-spores survives and builds the ffeem
maallee
ggaam
e
t
o
p
h
y
t
e
metophyte (1n) or also called embryo sac after mitosis; the embryo sac
contains a large central cell (with two haploid nuclei!) and the hhaappllooiidd eegggg;
 the haploid egg in angiosperms is obtained by mitosis
6. after pollination, the pollen grain (carried over by wind, an insect or another
animal) germinates on the stigma; the tube cell starts to form the pollen tube,
while the generative cell divides only once to build the hhaappllooiidd ssppeerrm
m; the pollen
tube grows out from the pollen grain and reaches down to the egg in the ovule
7. in angiosperms it comes to a so-called double-fertilization in the ovule; only one
sperm (1n) fuses with the egg (1n) to form the diploid zzyyggoottee (2n);
8. the other sperm (1n) contributes its haploid nucleus to the diploid central cell (2n)
of the embryo sac and builds a triploid nucleus (3n); the triploid cell divides and
forms the so-called endosperm, which is a rich, multicellular mass;
 the endosperm has the function to nourish the embryo until it
becomes a self-supporting seedling after germination
9. the zygote develops into the eem
mbbrryyoo which pushes itself into the endosperm; the
ovule coat starts to lose water and shrinks in order to turn into a highly lignified,
desiccation-resistant seed coat; a mature seed with an embedded embryo
developed; the embryo stops dividing and falls into so-called seed dormancy;
 it stays in this stage until optimum growth conditions trigger the
re-growth of the embryo again in process called germination
10. while the seed develops in the ovule, the ovary wall in the meantime thickens
and develops into a fleshy plant structure called fruit; the fruit covers and
protects the seed
11. ripe fruits are carried away and dispersed mostly by animals which eat them;
36
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
 most of the seeds survive the digestive tract of animals and
re-enter the natural cycle unharmed after defecation
12. under favorable conditions (humidity, light, temperature) the seed germinates
and the embryo grows into a m
maattuurree ssppoorroopphhyyttee (2n) which completes the life
cycle
Stages of the life cycle of an angiosperm plant
anthers
 many microspore mother
cells in microsporangium
Meiosis
Macrospore
mother cell
1 pollen tube cell (1n)
1 generative cell (1n)
Embryo
 embryonic dormancy
 seed formation
Pollination
 by animals
Meiosis
Pollen germination
1 egg cell (1n)
2 polar cells (2 x 1n)
 pollen tube forms

on stigma
Fertilization
 Formation of embryo (2n)
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SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
Why did angiosperms become the most successful plants on our planet?

Several reasons account for the tremendous success of angiosperm plants on our
planet
1. Packaging of the embryo (within the seeds) into a complex tissue called
fruit

a fruit is the ripened ovary of a fertilized flower

a fruit is an “attractively designed bioshuttle”, with which help the angiosperms
disperse and transfer their genetic material in form of an embryo over to other
places

many forms and shapes of fruits evolved
e.g. the kite-like fruit of the dandelion
e.g. cockleburs, melons or berries

the ‘invention’ of the angiosperm fruit triggered many mutually dependent
relationships of angiosperm plants with many animals (which are more reliable
seed carriers than climate factors, e.g. wind)

many fruits are eatable for humans (e.g. tomato, strawberry, etc.); but some fruits
contain substances which are toxic for us, i.e. they negatively affect our human
body functions and health

fruit toxins played and still play an important role in the selection of the seed
carrying species; only these animals and species, which are “immune” against
the fruit toxins are able to function as a potential carrier

angiosperm fruit toxins had a major impact on the development of very intricate
interaction patterns and mutually beneficial relationships between plants and
animals, in a process called co-evolution (see also UNIT 14)
2. A rapid fertilization

fertilization in angiosperm plants usually occurs 12 hours after pollination

ripe seeds are produced in only a few days or weeks; much faster than in
gymnosperms which take years to produce a ripe seed

it enables the angiosperms to rapidly adapt to changing and variable
environments
38
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
GERMINATION

germination is the process by which the embryo ends the seed dormancy and
starts to divide and grows into a plant
germination is not the beginning of new life it is only the continuation of a
already pre-existing one
seed dormancy can last many month and in some plant species the seeds can
stay in the dormant stage for years
e.g. coconut seeds under unfavorable conditions can float for many years in the
ocean until they reach land

the embryo in a seed is already a miniature plant consisting of the embryonic root
and shoot

after hydration (= uptake of water) of the seed in the soil, enzymes in the endosperm
or cotyledons become activated and begin to digest the stored nutrients

the embryo starts with mitotic cell division again and begins to grow
 first the embryonic root which grows into the soil, then the embryonic shoot
which reaches for the light

after the shoot left the soil, the so-called foliage leave ( monocotyl) or two leaves
( dicotyls) unfold and expand from the shoot tip

photosynthesis starts for the first time in the seedling
ASEXUAL OR VEGETATIVE REPRODUCTION OF PLANTS

plants not only reproduce by sexual reproduction (= fertilization and formation of a
diploid zygote) but are also capable to reproduce by so-called asexual or vegetative
reproduction

during vegetative reproduction, plant parts separate from the parent plant which start
to regenerate into a whole plants
 e.g. sprouts from the roots of trees or the so-called runners of strawberry and
many grass varieties (dune grass, Bermuda grass)

the major advantage of asexual reproduction is that a plant that perfectly adapted to
its environment can quickly copy itself; these plant copies are also called clones

vegetative reproduction of plants bears a huge potential and provides various
opportunities for modern agriculture

large numbers of plants with desirable traits e.g. large fruit, appealing flower color,
etc., can be easily propagated and grown
39
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology Lecture (BIOL 107): Instructor: Elmar Schmid, Ph.D.
 most of our fruit trees, ornamental garden plants and shrubs are asexually
propagated from stem or leaf of a parent plant

but today plants can also be multiplied (= cloned) by various laboratory methods and
techniques
1. test tube cloning
 multiple, genetically identical plants are regrown from cultured meristem cells
 e.g. pine trees, orchids
2. protoplast cultures
 protoplasts are isolated embryonic cells from the apical so-called growth cone
of a plant, of which the cell wall has been enzymatically removed
 these pluripotent cells can re-grow into a full plant again when brought
into a nutrient culture medium
 this technique enables the introduction of foreign genes (= on plasmids or with
the help of plant-specific viruses) into a plant cell; genes which encode for,
e.g. a resistence factor against a certain pathogen or insect
e.g. a bacterial gene has been introduced into strawberry which produces
a compound that protects the strawberry from getting frost damage
(frost bites)
3. artificial seed germinating
 the embryo of a plant, e.g. alfalfa, is packaged into a polysaccharide
capsule which provides the nutrients

all three techniques enable the breeding of genetically uniform plants each with the
same desirable trait

but modern agriculture despite its tremendous progress and its successful methods
in breeding faces some serious problems

scientists warn of a dramatic decrease in the genetic variability in our contemporary
crop plants
 this is primarily caused by the intensive use of many inbred and/or genetically
cloned species in huge agricultural so-called monocultures
 a monoculture is the agriculturing of a single plant variety (with a typical genetic
trait) on vast areas of land
 it is the belief of many scientists that low genetic variability of our
‘mono plants’ may make them highly susceptible to many plant diseases;
a small number of pathogens (viruses, fungi) could devastate large crop areas

today many agricultural laboratories and plant breeders make efforts to ‘refresh’ the
so-called gene pool of plants by breeding the mono cultures with varieties deposited
world-wide in so-called gene banks
 e.g. farmers and plant breeders in Europe started to agriculture and breed the
hardy and very robust so-called Dinkel crop, which has been widely used in the
medieval ages throughout Europe but later been replaced by our modern crops
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