Plant roots are responsible for bringing in water, oxygen and ions to the cells, to be used for
photosynthesis and other metabolic processes. Water is taken in continually; plants achieve this
by using active transport to concentrate ions in the root cells, thereby creating concentration
gradients that encourage osmosis.
The amount of water that is taken in daily far exceeds the amount required for photosynthesis
and other metabolic reactions in the tissues; most of the water simply passes through the plant
and is evaporated off at the leaves in the process of transpiration. In transpiration differences in
water concentration cause osmosis, thereby moving water from the soil to the roots and into the
xylem. A continuous column of water molecules is created, and is kept intact by cohesion and
adhesion. As water evaporates at pores on the leaf, called stomata, the entire column is pulled
up. If the column of water is broken, the plant will wilt.
Evaporation through the stomata is a major route of water loss for a plant. However, the stomata
must be open to allow for the entry of CO2 for photosynthesis. A balance must be obtained
between these two conflicting needs. Many environmental conditions, such as temperature and
humidity, influence the opening and closing of the stomata and so affect the rate of transpiration.
I. MICROSCOPE STUDY
Use the compound (monocular) microscope.
Diagram, label, and annotate the field of view. Record magnification!
A. LEAF CROSS SECTION:
Identify the following leaf structures. The epidermis is the top layer of cells, often
covered with a thin layer of wax, called the cuticle. Below these cells are the green
palisade cells, where most of the photosynthesis occurs. Further down into the leaf is the
mesophyll, described as spongy since it has air pockets which allow for the movement of
gases. Finally, at the underside of the leaf are guard cells surrounding stomata, where
gases and water enter/exit the leaf.
B. STOMATA :
• Lilium epidermis prepared whole mount (w.m.)
The outermost layer of a leaf is largely composed of colorless epidermal cells. Scattered
throughout the epidermal cells are stomata (singular: stoma) surrounded by pairs of
guard cells. Here, all the stomata are closed. As guard cells take on and lose water in
osmosis their shape changes. In a hypotonic environment, the guard cells become bowed
thus opening the stoma. In a hypertonic environment the guard cells lose water, and the
change in shape effectively closes the stoma. This way the plant is able to respond as
internal water, oxygen and carbon dioxide needs dictate.
• Daphne odora epidermis wet mount
Tear a leaf at an angle to expose a thin, almost transparent section of epidermis. Lay the
epidermis on a clean slide and add a few drops of tap water to make a wet mount. View it
with the compound microscope at 1000x magnification. Look for the dark, curved guard
cells surrounding small stomata.
Make a second wet mount slide, this time with a salt solution. Again find guard cells.
After a few minutes, compare these cells with those of the first slide noting any change in
the size of the stomata.
Remove a leaf from a sprig of Anachris and prepare a wet mount slide. At 100x or 400x
magnification, focus on cells which are full of large, green chloroplasts. Look for the
circulation called cytoplasmic streaming.
D. ROOTS :
Make a wet mount slide of a section of a fibrous root. Look for microscopic root hairs. Use
the compound microscope to see the tiny root hairs that serve to increase the surface area
for water absorption.
II. COMPREHENSION CHECK
1. What needed molecules enter the plant via the root? Why so many root hairs?
2. What enters the plant at the stomata? What exits?
3. When stomata are open and water escapes, where does it go? What’s this called? Why
does it happen?
4. What happens to the water content (and therefore turgidity, or fullness) of the guard cells
as they are subjected to the salt water? Why? Describe the tonicity of the 5% salt solution
compared to the guard cells.
5. As the guard cells dehydrate, how do the stomata change? Why?
6. The spongy mesophyll has space for the movement of gases. What gases are plants
exchanging at the leaves?
III. EXPERIMENTAL LAB
What is the Effect of Leaf Mass on the Amount of Water Transpired?
• Pre-Lab : Discuss and record a lab objective, summarize the methods, and hypothesize
the relationship between the amount of leaf mass (from which we can infer surface) on
quantity of water transpired.
(Optional extension: hypothesize the effect of humidity on transpiration.)
• Day 1
A. Label a test tube with your name and period. Using a graduated cylinder, almost fill your
test tube with a measured amount of water. Record initial water amounts (in mL) in Table I.
Tightly cover with parafilm. Use a dissecting needle to poke a hole in the parafilm. Set
B. Gently loosen the soil from the roots of a plant in a tray of water. Keping the plant in the
water, cut the roots off at an angle. Quickly and gently slide your plant stem through the hole
in the parafilm and into the water of the test tube. Set under the light bank until next class.
(Extension: Do the same set up again, but seal it all in a plastic bag.)
• Day 2
C. Remove the plant from the test tube, shaking any water drops clinging to the stem back
into the test tube. Measure the water volume remaining in the test tube. Record.
D. Calculate the change in water volume to determine the amount of water transpired by the
E. Carefully pick off all leaf matter from the plant and determine the total leaf mass. Record.
Graph the effect of leaf mass on amount transpired.
Summarize the information from the microscope work
Analyse the experimental lab results as per lab notebook handout.
1. What brings in and transport the water to the stem? What determines if water will move
into the roots from the soil?
2. How does the nature of water allow it to move it up the xylem?
3. What is the role of evaporation in the transpiration process?
4. Plants need water for what metabolic process? Compare this amount to the total amount
transpired by the plant.
5. Why cut off the roots?
Follow Up Questions
1. How do the root cells ensure that water will diffuse in and not out?
2. Why do plants continually transpire water to the atmosphere?
3. Infer why the upper epidermis of the leaves of many dry-climate plant species lack
4. What is the impact on a plant if the stomata are always open?
5. What is the impact of the stomata being closed?
Table I: Results
Start water Final water Difference Total Leaf
amount amount in water mass (g)
(mL) (mL) level (mL)
Table IV: Class results
water leaf mass (g)
Student transpired (ml)