Download Exercise 3: Plant Form and Function

Exercise 3: Plant Form and Function
Goals
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Identify basic plant body structures.
Recognize that the plant body has a hierarchical organizational structure, like animals.
Identify different organ, tissue system, and cell types macroscopically and microscopically.
Describe the distribution of vascular tissue in Eudicot roots and stems.
Connect the structure of plant cells and tissues to their functions.
Investigate the effects of humidity and light on transpiration.
Use rules of plant organization to identify organs as stems, roots, or leaves.
Part 1. Introduction to the Plant Body
Angiosperms (flowering plants) produce true roots, stems, leaves, flowers and fruit. The leaf blade is attached to the
stem by a petiole (usually) at the point called the node. There is often an axillary bud at the junction of leaf and stem.
The stem between nodes is called the internode. The apical tip of the stem, where growth occurs, is called the shoot
apical meristem. The entire aboveground portion of the plant, including the stem and leaves, is the shoot. Roots often
consist of a taproot and lateral branch roots. The growing tip of the root is called the root apical meristem. With the
help of your TA, label these features on the diagram below.
Figure 1. Basic structure of the plant body. Image from Biodidac
Questions
1. Do humans have apical meristems? Why do plants need apical meristems, while humans do not?
2. How many root apical meristems does the plant in Figure 1 actually have?
Part 2. Structural Organization
Plant bodies contain three separate organs (roots, stems, and leaves) that work in concert to keep food and water
supplied to all parts of the plant. For plants to grow, new cells are added at specific locations (meristems)
throughout the plant. These new cells are initially unspecialized, but then mature into different tissue and cell types.
Each of the plant organs is composed of three different tissue systems (dermal, ground, and vascular). In today's
exercise we will investigate the structural organization of roots, stems and leaves in angiosperms (flowering plants).
As you study the tissues of these different organs you will notice that each tissue is composed of different types of
cells. The three tissue systems (dermal, ground, and vascular) are composed of three different cell types
(parenchyma, collenchyma, and sclerenchyma). However, not all cell types need be present in every tissue in every
plant species. For example, most plants have only parenchyma in their dermal tissues.
Organs
Tissue Systems
Cell Types
Parenchyma
Sclerenchyma
Dermal
Root
Ground
Parenchyma
Collenchyma
Sclerenchyma
Vascular
Parenchyma
Sclerenchyma
Dermal
Stem
Leaf
Parenchyma
Sclerenchyma
Ground
Parenchyma
Collenchyma
Sclerenchyma
Vascular
Parenchyma
Sclerenchyma
Dermal
Parenchyma
Sclerenchyma
Ground
Parenchyma
Collenchyma
Sclerenchyma
Vascular
Parenchyma
Sclerenchyma
Tissue Systems
Dermal Tissue System
The dermal tissue system consists of the outer layer of cells on the stem, leaf, and root. These cells make up the
epidermis, a single layer of cells covering the outside of the plant. Other cells that may be present are guard cells around
stomata (pores) and hairs.
Vascular Tissue System
The vascular tissue system transports water and nutrients throughout the plant body. Water flows by osmosis into the
root hairs and moves through the epidermis and cortex until it reaches the xylem. The xylem provides support for
the plants and moves water and minerals up to the leaves. Phloem transports food and other nutrients from the
leaves to the rest of the plant. The fluid inside the phloem is called sap. An example of sap is maple syrup before it is
boiled down to the concentrate we use. Phloem is typically located near xylem in vascular bundles.
1) Xylem (Figure 2a): The cells of the xylem are elongated and thick and are joined end to end to form tube-like
pipes. Xylem cell walls stain red in prepared slides. Xylem can contain tracheids, vessel elements (both types of
sclerenchyma), and parenchyma (Figure 7.2a). Although vessel elements in secondary woody tissue are typically
dead, those found in elongating portions of plants (primary growth) are alive. Protoxylem vessel elements have
spirals or helices of lignin incorporated into their walls that allow them to stretch during growth. Metaxylem
vessel elements have rigid walls with small pits. Tracheids are relatively narrow vascular cells that have pits in
their cell walls, allowing water to diffuse to adjacent cells. In contrast, the wider vessel elements have
perforations (or openings) in the cell walls that allow the direct flow of water. and thus play a more critical role in
structural support.
2)
Phloem (Figure 2b): and is also made up of long pipe-like cells joined end to end. These are called sieve-tube
members (STMs). STMs are alive at maturity, but do not contain a nucleus. Instead, each STM is partnered
with a companion cell that contains a nucleus and regulates the element. Phloem tissue can also contain rigid
fibers (sclerenchyma) for structural support, and parenchyma cells for storage.
Figure 2. Organization of vascular tissues in plants. A) xylem, B) phloem. Image from
Ground Tissue System
The ground tissue system consists of everything that is not vascular or dermal tissue. It can play many roles, including
support, storage, and photosynthesis (to name a few).
Cell Types
Parenchyma cells are found throughout the plant and are usually less specialized than the other two cells types. The
one exception are the sieve tube members (Figure 2) - specialized cells of the vascular tissue that conduct the plant’s
sap (phloem). In general, parenchyma cells are found in ground tissue and play a role in many of the metabolic
functions of the plant, including photosynthesis and starch storage.
Collenchyma cells are usually found in bundles in ground tissue, providing support to young stems and leaves
(primary growth). They accomplish this through thickened primary cell walls. Good examples of collenchyma cells
are the `strings' found in celery stalks (you will look at these in lab today). When you view these cells under the
microscope, observe that their cell walls are unevenly thickened.
Sclerenchyma cells also provide support to plants through their thickened secondary cell walls that are composed of
either lignin (found in wood) or suberin (found in cork). Sclerenchyma cells are often dead at maturity, and play 2
primary roles:
1) Support: The two main types of sclerenchyma cells that play a role in support are fibers and sclereids. These
sclerenchyma cells comprise the ground tissue of woody stems and roots. Fibers can also be found in phloem.
2) Water transport: In addition to providing support, other sclerenchyma cells are found in the xylem. These are
tracheids and vessel elements.
Tissue System and Cell Identification in Celery
1. Cut a thin cross section of a piece of a celery stalk (actually a leaf petiole) that has been sitting in dye overnight.
Place it in a watchglass and observe it using a dissecting microscope. Draw the cross-section here. Label the dermal,
ground, and vascular tissue systems:
2. Using a razor blade, carefully cut a paper-thin cross section of the celery and make a wet mount. If you cut it too
thick, you will have a difficult time seeing the structures. Using your compound microscope, focus using low, medium
and high power. Identify and focus on the vascular bundles that contain large xylem vessel elements and
parenchyma. Label these on the diagram below. Collenchyma cells can be found toward the edge of the celery stalk.
This collenchyma is part of the gound tissue system, and forms the “strings” that get stuck in your teeth when you eat
celery. Remember that collenchyma provides flexible support to actively-growing primary tissues. The remainder of
the diagram below is filled with unspecialized parenchyma cells of the ground tissue system. Label both the
parenchyma and collenchyma cells.
Figure 3. Cross-section of celery petiole. Image credit: UW-Madison Plant Image Collection
Zoom in on the cross section. Draw a picture of (1) a parenchyma cell, (2) some collenchyma cells with the unevenlythickened primary cell walls and (3) some sclerenchyma (xylem vessel elements) with the evenly-thickened lignaceous
secondary cell walls.
Parenchyma cell:
Collenchyma cells:
Sclerenchyma cells:
Part 3. Stem Structure
Although the celery is useful for studying different cell and tissue types, it is not a true stem. Eudicot stems have a
distinctive ring arrangement of vascular bundles. In the Medicago (alfalfa) stem, ground tissue system is separated into
two areas: 1) the central pith, and 2) the cortex toward the outside, between the epidermis and the vascular bundles.
Figure 4. Tissue distribution in a Eudicot stem. Image credit: Outernet Publishing.
1. Inspect the prepared slide of Medicago stem cross-section using the compound microscope, and label the following
features in the diagram below: pith, cortex, vascular bundle, xylem, phloem, and epidermis. Additionally, locate and label
the red lignified fiber cells outside the phloem.
Figure 5. Medicago stem cross section. Image credit: UW-Madison Plant Image Collection (http://botit.botany.wisc.edu/)
2. Inspect the prepared slide of the Cucurbita (cucumber) stem longitudinal section (sliced lengthwise). First
locate the xylem, which have lignified secondary cell walls that stain red. You should be able to identify both
protoxylem and metaxylem vessel elements, and label them below (Figure 6).
Figure 6. Xylem vessel elements in the Cucurbita stem. Image credit: UW-Madison Plant Image Collection
(http://botit.botany.wisc.edu/)
Next, locate the phloem (Figure 7), which is also composed of elongated cells. Try to find a sieve-tube
member with a porous sieve-plate at one end. The associated companion cells can be difficult to identify,
but try to locate one with a nucleus. Label these in the figures below.
Phloem
Figure 7. Phloem in the Cucurbita stem. Image credit: UW-Madison Plant Image Collection (http://botit.botany.wisc.edu/)
Questions
1. How can protoxylem be supportive if the vessel elements have only periodically-thickened secondary cell walls
(Figure 6)?
2. How does the form of protoxylem help it function in young organs? Why would metaxylem be structurally
inappropriate in young tissues?
3. What are two differences between the two structurally supportive cell types: collenchyma and sclerenchyma?
4. What is an advantage of having collenchyma supportive tissue when sclerenchyma provides more sturdy structural
support?
5. What do companion cells provide to sieve-tube members? Why do sieve-tube members lack a nucleus?
Part 4. Root structure
The large central core of the Eudicot root is the vascular cylinder or stele (Figure 8). The cortex (or
parenchyma) is found outside the stele and the epidermis is on the outer surface of the organ. How does the
arrangement of vascular tissue in the root differ from that in the stem?
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Figure 8. Eudicot root cross section. Image credit: Outernet Publishing
1. Observe and draw the Ranunculus (buttercup) root on medium power. Label the dermal, ground, and vascular
tissue.
Can you see the "cross" shape inside the vascular cylinder? What type of cells compose this cross? Label them
in Figure 8 above. Where do you think the phloem is located? Label this in Figure 8 as well.
The layer of cells between the vascular tissue and the cortex is called the endodermis. Do you notice anything
unique about the color of the cell walls?
The cell walls are impregnated with wax, in a layer called the casparian strip. This layer forces water through
the living protoplast of the endodermal cells.
Questions
1. Why would it be beneficial to the plant to force all solutions entering the vascular cylinder through living
protoplast?
2. If the plant is not photosynthesizing (producing sugar) in its roots, why do the roots contain phloem?
Part 5. Leaf Structure
The last structure you will examine in lab are the leaves (Figure 6). The parenchyma of the leaf is called the
mesophyll. It consists of all the cells containing chlorophyll between the upper and lower epidermis, excluding the
vascular bundles. The palisade mesophyll is located in the upper layer and is tightly packed with cells resembling
"fence posts." The lower layer, or spongy mesophyll, is loosely packed with many air spaces and irregularly shaped
cells.
Figure 9. Eudicot leaf section. Image credit: Outernet Publishing
1. Get a celery leaf from your instructor. Recall that the dye has traveled up the vascular tissue. Do leaves
contain vascular tissue?
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2. View the prepared slide of a Syringa (lilac) leaf cross-section. The leaf should contain at least one vascular bundle.
Locate it using your low power objective and work your way up to the high power objective (400 X magnification).
On Figure 10, label the palisade and spongy mesophyll, the upper and lower epidermis (dermal tissue system), and the
vascular bundle (including xylem and phloem). Also locate and label a stoma with guard cells. Note the air chamber
beneath the stoma.
Figure 10. Syringa leaf blade cross-section. Image credit: UW-Madison Plant Image Collection (http://botit.botany.wisc.edu/)
What are the vascular bundles of a leaf called? __________________________
Questions
1. Explain how the orientation of cells in the palisade and spongy mesophyll help maximize light capture by the entire
leaf.
2. What do you think would be different about the leaf of a plant that is adapted to a dry environment?
Part 6. Transpiration experiment
Water moves through the plant via the plant's cells, but a large amount of it is lost through the leaves during a process
called transpiration. A large tree can move 100 gallons of water per day from its roots, through its trunk and out
through the leaves as water vapor (Figure 11). Water is lost through the leaves via a structure called the stoma (stomata
is plural). The stoma is a small pore or opening in the epidermis of the leaf. It is surrounded by guard cells that regulate
gas exchange and transpiration. It is the loss of water from the leaves that pulls most of the water into the roots and up
the trunk; a very small amount is actually pushed up from the roots to the leaves.
Figure 11. Water transport in plants. Image credit: Outernet publishing
Today we will investigate how different environmental conditions influence the rate of transpiration. Specifically, we
will test whether rates of transpiration differ in the following conditions:
1. Dim light
2. Bright light
3. Bright light with a wind
State a hypothesis. Do you think that amount of light or humidity will have the greatest effect on transpiration? Why?
For this experiment, you will work in groups of four. Each group has a transpiration apparatus (photometer). Your
instructor will show you how to create a continuous column of water in your pipette and insert your plant stem. The
supplies you need to measure transpiration rates are listed below.
Pasteur pipettes
Tygon tubing
petroleum jelly
ring stand with clamps
razor blades
syringes
plastic rulers
fans
plastic bin for water
gooseneck lamps
Ricinus or Geranium plants
Assemble the transpiration apparatus.
1. Attach a Pasteur pipette to the tygon tubing. Make sure the tip of the pipette is not chipped.
2. Place these items in the plastic bin of water and fill them with water. You may need to use your syringe to
get out all of the air.
3. Obtain a long piece of Ricinus or Geranium with many leaves on it from the center bench.
4. Place the plant’s cut end in the water and re-cut it. Try to have the diameter of the stem the same as the
tubing diameter. Insert the stem into the tubing (Figure 12a).
5. Seal the connection with petroleum jelly. The key to making this experiment work is to create a system that
does not have any leaks. If your apparatus leaks, water will not be drawn up from the pipette.
6. Carefully clamp your plant to the ring stand (Figure 12b). Place the pipette end on the bench.
7. Set-up your experimental conditions and let your plant acclimate to these for 5 minutes.
8. Place a plastic ruler next to the pipette so that you can measure the distance that the air bubble travels.
9. After the 5 minute acclimation period is over, reset your air bubble to the 0 mm point (tip of the Pasteur
pipette).
Resetting your pipette
A. Fill your syringe with about 3 mL of water.
B. Place the tip of your pipette into the open end of the syringe.
C. Gently push the air out of the pipette by squeezing the tygon tubing.
D. Release the tygon tubing. Water from the syringe should fill the pipette.
10. Immediately begin timing your experiment. Measure the distance that the air bubble travels over 8 minutes.
Record your results in the table below.
Figure 12. Diagram of method to assemble potometer. Image credit: Outernet publishing
Measure transpiration rates for the two other experimental conditions. Use the same plant and transpiration apparatus
for each experiment. Simply reset your pipette to zero at the start of each experiment. Record your results for each
experiment in 1. Your instructor will summarize the results from each group on the chalk board.
Questions
1. Which variable had the greatest effect on transpiration rates?
2. Did this support your original or alternate hypothesis? Why or why not?
3. Why was it important to use the same plant for each of the treatments?
4. Why was it important to start each experiment at the 0 mm point?
5. What other variables could have affected your transpiration rates?
Part 7. Root, Stem, or Leaf?
Plant growth and development are indeterminate, meaning that there is greater flexibility in plant form than in animal
form. The organs of different plant species have evolved to perform different functions. For example, the potato
stem can form storage tubers, just like the root of a carrot. The diversity in plant form can make it difficult to identify
and categorize plant structure, but there are a few basic rules that you can follow:
1.
2.
3.
Leaves are determinate structures – they have no apical meristems and CANNOT grow new leaves, stems, or
roots. Therefore, you will NOT find any buds on a leaf.
Only stems can grow new roots, stems, and leaves.
Roots can only produce new roots, not stems or leaves.
Inspect the specimens available in lab, and identify the labeled portion as root, stem, or leaf. Check with your TA to
make sure you are correct.
a. Potato:
b. Carrot:
c. Celery:
d. Ruscus (Butcher’s broom):
e. Cactus:
f. Fern:
g. Ginger:
h. Banana plant:
i. Radish:
j. Kohlrabi:
k. Bean plant:
l. Jade plant:
m. Onion: