Lab 2
Flower Morphology and Plant Structure
Overview
In this lab you will have the opportunity to observe images of the structures (including flowers)
for a reference strain of Arabidopsis thaliana plant and learn about the different functions for
these structures. You will then compare these structures with those of a mutant Arabidopsis
plant and make predictions about how these differences might impact how the mutant plant
functions. Finally, you will receive published information on your mutant strain to determine if
your predictions are correct.
Objectives
1. Become familiar with plant growth and meristems, as well vegetative organs.
2. Become familiar with terms associated with plant structure, through lab exercises, as
well as readings in textbook, lecture notes, and lab manual.
Required Preparations
1. Read Lab 2 carefully before coming to your lab.
2. Read Chapter 27 and Chapter 29 in your textbook (Morris et al. 2019) and review
relevant lecture notes.
Lab 2 Exercises
1. Summary of plant organ function and observation of reference strain of Arabidopsis
thaliana
2. Observation of mutant strain of Arabidopsis thaliana
3. Using internet resources for information on mutant strain of Arabidopsis thaliana
4. Complete Lab #2 Assignment on Quercus. This must be completed within 24 hours
after the end of your scheduled lab period.
Background Information: Plant Structure
In this lab, you will have an opportunity to become familiar with some of the basic features of
plant structure. Most of what you will learn relates primarily to vascular plants (primarily
angiosperms), since these are the most prominent land plants currently found on the planet.
The knowledge of plant structure is critical to understanding how plants function. To provide
adequate context for this laboratory, the background information is divided into four subject
areas:
1.
2.
3.
4.
Growth and meristems
Tissue systems
Plant vegetative organs (leaf, stem, root)
Plant reproductive organs (angiosperms only - flowers)
1. Growth and meristems
For a plant to grow and develop, cell division, followed by cell enlargement, and finally cell
differentiation are required. Unlike animals, plants may continue to grow throughout their
lives due to the continued presence of regions of dividing cells. These regions are called
meristems. There are two basic groups of meristems: (a) primary and (b) secondary
meristems (Figure 2.1)
(a) There are two primary meristems. Once is located at the tip or apex of the shoot
and is called the shoot apical meristem. The second is at the tip or apex of the root
and is called the root apical meristem. These two apical meristems give rise to all
simple and complex tissues and tissue systems in the shoots and roots of a plant.
(b) The secondary meristems have site-specific roles. See Figure 2.1 for names and
locations of secondary meristems.
Growth from the primary meristem is known as primary growth, generally resulting in a
lengthening of root and shoot and in the addition of new plant parts.
Figure 2.1. Primary and basic secondary meristems. Primary meristems consist of shoot apical
meristem and root apical meristem, whereas secondary meristems include lateral
shoot and root meristems, axial meristems, adventitious meristems, vascular
cambium (e.g. in leaves; not indicated in the figure) or cork cambium (in the
periderm; not indicated in the figure).
From: http://vannocke.hrt.msu.edu/plb865/Meristem%20dynamics/meristems.html
The leaves, stems, and roots of all monocotyledons and some eudicots have only primary
growth. However, in all gymnosperms and some eudicots, a process known as secondary
growth also occurs in stems and roots. This secondary growth is due to the appearance of
two new lateral meristems, the vascular cambium and the cork cambium. Secondary
growth from these regions results in an increase in the diameter of shoots and roots. Some
plant parts can continue to grow throughout a plant’s lifetime. For example, the growth of
roots and shoots over long periods of time shows such indeterminate growth.
Determinate growth is growth that stops at a particular point. This is characteristic of most
animals and of some plant parts such as leaves, flowers, and fruit.
2. Plant tissues
In multicellular organisms, such as plants, cells often exist in communities. A biological
tissue is a community of cells that work together to perform a specific function. In plants,
‘simple tissues’ are composed of just one cell type and include parenchyma, collenchyma,
and sclerenchyma. Whereas ‘complex tissues’ are composed of more than one cell type
and include the epidermis, periderm, xylem, and phloem.
Plant tissues are organized into three basic tissue systems (Figure 2.2), based on their
function:
a. Dermal tissue system – This tissue system consists of the epidermis and periderm
tissues. It provides an outer covering for the plant.
b. Vascular tissue system – This tissue system is specialized for the transport of
different substances from one part of the plant to another and consists of both
xylem and phloem tissue. Among plants, only vascular plants (pteridophytes,
gymnosperms, and angiosperms) contain the vascular tissue system. Non-vascular
plants (bryophytes) lack this tissue system.
c. Ground tissue system – This tissue system serves largely as a storage organ or as a
filler of space between the dermal and vascular tissue systems.
Reproductive
organs
Dermal tissue
Shoot
Leaf
Stem
Ground
tissue
Vascular
tissue
Root
s
Figure 2.2. The tissue systems and vegetative organs of plants. From Morris (2019).
3. Plant vegetative organs
In vascular plants, the three tissue systems of a plant are organized into four major organ
types: leaves, reproductive organs, stems, and roots (Figure 2.4). The leaf, stem, and
reproductive organs form the shoot of the plant. Reproductive organs are covered in more
detail later in this document.
a. Leaves (Figure 2.3)
The primary function of a leaf is to perform photosynthesis. There are two zones of
photosynthetic cells, or cells that contain a large number of chloroplasts, within the
ground tissue of a leaf: the palisade mesophyll and the spongy mesophyll. The
palisade mesophyll consists of elongate cells found below the upper epidermis. The
spongy mesophyll consists of irregular shaped cells found below the palisade
mesophyll. Compared with the palisade mesophyll, spongy mesophyll cells are very
loosely packed creating a large volume of intercellular airspace within this part of
the leaf. Gas exchange for photosynthesis is dependent on this cell arrangement, as
well as on the presence of pores called on the upper and more so on the lower
surface of the leaf. These pores are called stomata (singular = stoma). The pair of
cells that surround these openings, the guard cells, will change shape in response to
environmental stimuli and cause the stomata to open and close. The dermal tissue
system or epidermal cells are found on both the upper and lower surfaces of the
leaf. To help mitigate water loss, the epidermal cells are covered by a waxy layer
called the cuticle. Leaves may also possess tiny hairs called trichomes. These
trichomes can be found on other parts of the shoot including the stem and sepals.
The function of these tiny hairs to protect the plant from harmful UV light and
herbivores.
The vascular tissue system (xylem and phloem) is found in the veins of the leaf. The
veins are enclosed by a layer of a cells known as the bundle sheath cells. The
pattern of veins (leaf venation) facilitates close contact between the vascular tissue
and the photosynthetic mesophyll cells. This arrangement facilitates efficient
movement of water into the leaf from the roots and movement of sucrose out of the
leaf to other parts of the plant.
Palisade mesophyll cells
near the upper surface of
the leaf have a columnar
arrangement that
maximizes light
interception.
Cuticle
Upper epidermis
Mesophyll cells
~40,000 ppm
Cuticle
Guard cell
Stoma
Vein
H2O
CO2 gradient
Lower epidermis
Figure 2.3. Typical leaf structure. Adapted from Morris (2019).
Water vapor gradient
CO2
Spongy mesophyll cells
close to the stomata
have a honeycomb-like
arrangement to allow
CO2 to spread easily
throughout the leaf.
b. Stems (Figure 2.4)
The function of the stem is to elevate and position both leaves and reproductive
organs. Similar to the leaf, the dermal tissue system surrounds the stem and
provides protection from pathogens and against water loss. The vascular tissue
system transports substances between the leaves and roots of the plant. Unlike
roots, the vascular tissue of the stem is usually arranged into discrete strands called
vascular bundles, which are oriented such that the xylem is closest to the center of
the stem and the phloem is closest to the epidermis. The arrangement of vascular
bundles may differ in monocots and eudicots. For example, the sunflower, a
eudicot, has vascular bundles that are arranged in a single ring, while the vascular
bundles of a monocot are scattered throughout the stem. The ground tissue of
stems is made up of undifferentiated parenchyma cells, or pith.
Xylem
Phloem
Epidermis
Thin-walled
parenchyma cells
Figure 2.4. Typical leaf structure. Adapted from Morris (2019).
Sclerenchyma
c. Roots (Figures 2.5 and 2.6)
Roots enable vascular plants to take up water and nutrients from the soil, as well as
provide an anchor for the plant, especially for those that can grow to great heights.
The root apical meristem will form a root cap at the tip of the root through mitosis.
Cells of the root cap mature into parenchyma cells toward the tip. The root cap
protects the root apical meristem as it pushes its way through the soil and is unique
to root apical meristems. The shoot apical meristem does not have an equivalent
structure. There are three zones of root development behind the root apical
meristem in this order: (1) the zone of cell division, (2) the zone of cell elongation,
and (3) the zone of cell differentiation.
(1) Zone of Cell Division: The root apical meristem and the cells immediately behind
it are actively dividing through mitosis. This cell division will give rise to the
three tissue systems in the roots.
(2) Zone of Cell Elongation: As the name implies, following cell division the cells of a
root undergo a period of rapid cell lengthening.
(3) Zone of Cell Differentiation: In this zone, the now fully expanded cells
differentiate into particular cell types. In the order of outer cells to inner cells:
I. Epidermis: This is the outer layer of cells that constitutes the dermal tissue
system of the root. Many epidermal cells grow long extensions called root
hairs. Root hairs increase the absorptive surface of the root, allowing
efficient uptake of nutrients and water from the soil.
II. Cortex: This is a region usually made up of thin-walled parenchyma cells
involved in the storage of food within the root.
III. Endodermis: This is a single layer of cells. The cell walls of these cells contain
a band of suberin called the Casparian strip. Suberin provides a barrier that
prevents any further movement of water or solute through the cell walls
(apoplastic water transport) and instead forces these substances to move
through the cell membranes (symplastic water transport). Although water
has no problem moving through osmosis across cell membranes, the
Casparian strip allows plants to be selective about which solutes are taken up
from the soil.
IV. Stele: This central region of the root contains the vascular tissue system. The
three tissue types that make up the stele are the pericycle, xylem, and
phloem. The pericycle is the outermost layers of cells in the stele. Some of
these cells may become meristematic and form root meristems, which give
rise to branch roots. The arrangement of xylem (tracheids and vessel
elements) and phloem (sieve tube elements and companion cells) in the stele
differs in monocots and eudicots. In monocots, the central region of the
stele contains additional ground tissue called the pith, which consists of thinwalled parenchyma cells.
Figure 2.6. Typical root structure (part 1). From Morris (2019).
Xylem
Phloem
Root hairs
Zone of cell
differentiation
Epidermis
Cortex
Pericycle
Endodermis
Root apical
meristem
Root cap
Root apical
meristem
Root cap
Figure 2.5. Typical root structure (part 1). From Morris (2019)
Zone of cell
elongation
Zone of cell
division
3
At the endodermis, the Casparian
strip prevents ions and water from
moving in the walls, forcing them to
pass through cell membranes.
1
Ions that enter the
cytoplasm of a root cell can
move to the xylem through
plasmodesmata.
Endodermis
Casparian strip
Phloem
Epidermis
Xylem
Cortex
Plasmodesmata
Ion
Root hair
2
Or they can move in the water-filled
spaces of the cell walls.
Figure 2.6. Typical root structure (part 2 – water movement through endodermal
cells). From Morris (2019)
4. Plant/Angiosperm reproductive organs
A flower is the reproductive structure found in flowering plants (angiosperms). The biological function of
a flower is to accomplish reproduction through the union of pollen with eggs. Angiosperms show
remarkable variation in flower form and gender function. Because a single flower can contain both male
AND female organs or be unisexual (male OR female sexual organs), and because a single plant
individual can produce one to many flowers (with varying genders), angiosperms are characterized by a
very wide range of reproductive systems. Depending on the gender distribution within or among
flowers, flowers may facilitate outcrossing (fusion of pollen and egg from different individual plants of
the same species) or allow selfing (fusion of pollen and egg from the same individual plant). This varied
gender distribution and function in flowering plants is in stark contrast to reproduction in animals, in
which the majority of animals are unisexual. Following pollination, the flower transforms into a seed
containing fruit. Fruit both protect immature seeds from being consumed by animals and enhance seed
dispersal, once seeds are mature.
Flower morphology: Introduction of the basic flower morphology
The basic organs (Box 2-1) in a flower are arranged in four whorls (Fig. 2.7):
(1) Sepals in the outer-most whorl. The typical role of sepals is the protection of a developing
flower. They come in a variety of shapes and sizes. Some sepals are long and thin while others
are short and thick. Some sepals are individualized or free while others are fused together to
make a cup formation around all other whorls of the flower (Fig. 2.8).
(2) Petals in the second whorl. The typical role of petals in animal-pollinated flowers is to attract
pollinators, hence petals often are large and colorful. Similarly, as in sepals, petals can be either
free or fused (Fig. 2.8).
(3) Male sexual organs in the third whorl. Male sexual organs are called stamens and are made up
of filaments and anthers (with two pollen sacks) (Fig. 2.7).
(4) Female sexual organ in the fourth and inner-most whorl. The female sexual organ is called the
pistil. It is composed of the ovary (containing the ovule and the egg), style, and the stigma (Fig.
2.7).
Angiosperms, or flowering plants, include approximately 275,000 species from a large number of
taxonomic groups, of which the eudicots and monocots are the largest and most important.
Representative plant species of these two groups differ in their basic flower characteristics (Fig. 2.9).
Flower organs of eudicots (approximately 200,000 species) often come in multiples of 4s or 5s, whereas
flowers of monocots (approximately 60,000 species) come in multiples of 3s. In monocots, sepals and
petals are often indistinguishable in size, shape, and color and, in such cases, are referred to as tepals
(Fig. 2.10). Note that often the numbers of pistils, and, less so, stamens, do not follow these basic
eudicot and monocot organ multiples.
Both eudicot and monocot flower organs are arranged in two basic symmetries: in radial
(actinomorphic) or bilateral (zygomorphic) symmetry (Fig. 2.11; Box 2-1). Floral symmetry describes
whether, and how, a flower can be divided into two or more identical or mirror-image parts. Radially
symmetric flowers can be divided into three or more identical sectors, whereas bilaterally symmetric
flowers can be divided by only a single plane into two mirror-image halves, much like a person's face.
TERMS OF FLOWER MOPHOLOGY; BOX 2-1
Sepal
The outer parts of the flower inserted in the first whorl. Sepals are often
green and leaf-like and protect the developing bud.
Petal
Flower parts inserted in the second whorl. In animal-pollinated flowers,
petals are often conspicuously colored and used to attract the pollinators.
Tepal
This term is applied when the sepals and petals are of similar shape and
color, or undifferentiated. This is often the case in monocots.
Stamen
The pollen-producing or male part of a flower, usually with a slender
filament supporting the anther. Stamens are inserted in the third whorl.
Anther
The structure at the top of a stamen where pollen is produced. It typically
consists of two pollen sacks.
Filament
The part of a stamen that supports the anther; the stalk of a stamen.
Pistil
The ovule-producing or female part of a flower, inserted in the fourth or
inner-most whorl of the flower. The pistil consists of a stigma, style, the
ovary, ovule and egg(s).
Stigma
The part of the pistil where pollen is supposed to land and germinate. The
stigma is thus the landing platform for the pollen and located on top of the
style.
Style
Pillar-like stalk through which pollen tubes grow to reach the ovary.
Ovary
The enlarged basal portion or chamber of the pistil in which ovules are
produced. The ovary bears ovules (unfertilized and immature seeds) and
ripens into a fruit.
Ovule
The plant part that contains the embryo sac and hence the female germ
cell (egg), which after fertilization and maturation develops into a seed.
Radial
Flower of this symmetry can be divided into three or more identical
(actinomorphic) sectors which are related to each other by rotation about the center of
symmetry
the flower. Typically, each sector might contain one petal and one sepal
or one tepal.
Bilateral
(zygomorphic)
symmetry
Flower of this symmetry can be divided by only a single plane into two
mirror-image halves, much like a person's face. Zygomorphic flowers
generally have petals of two or more different shapes, sizes, and colors.
Pedicel (the stalk that supports the flower)
Fig. 2.7. Schematic diagram of a flower and an example of an apple flower.
Fig. 2.8. Free (individualized) or fused sepals and petals.
Fig. 2.9. Typical eudicot and monocot flowers. Typically, the number of sepals and petals is indicative
for whether a flower is a eudicot or a monocot, whereas the numbers of stamens and pistils are more
likely to diverge from the basic numbers of 4s/5s and 3s.
Fig. 2.10. If sepals and petals are indistinguishable (often the case in monocots), they are referred to as
tepals. Left: schematic diagram of tepals. Mid, right: actual monocot flowers with indistinguishable
tepals.
Fig. 2.11. Radial or actinomorphic symmetry with more than one possible way to divide the flower in
mirror-image parts (left) and bilateral or zygomorphic symmetry, in which a flower can only be
divided by a single plane in two mirror-image halves (right).
Exercise 1: Summary of plant organ function and observation of reference strain of
Arabidopsis thaliana
As you learned in Lab 1, Arabidopsis thaliana (Arabidopsis) was the first plant to have its
genome completely sequenced. Although technically a weed, this plant has transformed into an
important model system for plant research.
Research experiments using Arabidopsis as a study species will often include a wild-type or
reference strain. These strains have no or minimal mutations and therefore produce a wildtype or non-mutant phenotype. The images of the reference strain that you will be looking at
today are of the Landsberg erecta (Ler-0) strain of Arabidopsis. This laboratory strain contains
an X-ray induced mutation in the ERECTA gene, which causes the plants to have more upright
growth. This mutation makes laboratory experiments easier for the researchers and does not
affect any other part of the plant’s phenotype. For that reason Ler-0 is widely used to generate
mutants, and serves as the reference strain for these mutants.
1. With your groups members in your breakout room, use the information in this lab, your
textbook, and your lecture notes, fill in the column labelled ‘Function’ in the Lab 2
worksheet for all plant organs. Be as succinct as possible when filling out this table.
2. Carefully observe the images of the Landsberg erecta strain of Arabidopsis. With your
groups members in your breakout room, fill in the column labelled ‘Observations of
Reference Strain: Landsberg erecta’. Include as much detail as you can.
Exercise 2: Observation of mutant strain of Arabidopsis thaliana
Using various methods including X-ray, chemical mutagens, T-DNA inserts, as well as others,
scientists have engineered a large number of mutant strains of Arabidopsis thaliana. These
mutant strains often display a mutant phenotype due to a loss of function in one or more of the
genes in their DNA. These mutant strains have allowed plant researchers to learn a great deal
about plants and how they function.
1. Enter the name of the mutant strain in the heading of the column
2. Carefully observe the images of the mutant strain of Arabidopsis. With your groups
members in your breakout room, fill in the column labelled ‘Observations of Mutant
Strain’ in the Lab 2 worksheet. Include as much detail as you can.
3. After observing the mutant strain, predict how the phenotypic differences you observed
would impact the function of the relevant organ in mutant. Complete the “Predictions
as to how this would affect function of organ in mutant ” column in the worksheet.
Exercise 3: Using internet resources for information on mutant strain of Arabidopsis thaliana
1. Follow the Arabidopsis Biological Resource Center link that is provided by your TA for
the mutant strain you observed in Exercise 2. Paraphrase the phenotypic description
given on this site for your mutant in complete sentences. Be sure to include an in-text
citation and reference in CSE: Name Year format. For more information on this format
please see this very comprehensive U of Guelph link:
https://guides.lib.uoguelph.ca/CSENameYear/Basics
The phenotypic information would be considered information from an
‘Organization/Group as Author’.
2. Does this phenotypic description match with your observations of your mutant strain?
Describe how it does and does not as applicable.
Exercise 4: Complete Lab #2 Assignment on Quercus
After you have completed your online synchronous Lab 2, please complete Lab Assignment #2
on Quercus. This assignment will require you to interpret parts of the worksheet you
completed. You will also be required to add to the paragraph you wrote in Exercise 3.
You will have one three-hour attempt to complete this assignment. It must be completed 24
hours after the end of your scheduled Lab 2. No late assignments will be accepted