SJB Create 10/2011 Revised 7/31/2017
Botany
Page 1 of 6
Lasting Impressions
Counting Stomata
[Stomata]. (n.d.). Retrieved October 23, 2011, from http://www.biologycorner.com
Adapted from: Flinn Scientific Inc. Bio Fax Lasting Impressions – Counting Stomata
http://www.cjhs.org/teacherssites/taylor/accbio/PLANTS/STOMATE%20LAB.pdf
SJB Create 10/2011 Revised 7/31/2017
Botany
Page 2 of 6
Background
Plant tissue, just like animal tissue, is composed of specialized cells to perform specific
functions. Plants have an epidermis layer, an outer skin-like layer, just like animals. Animal skin
contains specialized “holes” or pores for specific body regulatory functions. The plant epidermis
likewise has “pores.” A single pore in the plant epidermis is called a stoma; multiple pores are
called stomata.
Stomata play an important part in water movement within plants through the process of
transpiration. This relies on the adhesion and cohesion properties of water molecules and the
process of evaporation of water molecules through the stomata. As with all processes in both
animal and plant organisms, regulation to stay within normal values is very important for the
well-being and survival of the organism. Therefore, the process of transpiration must be
regulated. This regulation of the size of the stoma is accomplished by guard cells. Each stoma is
bordered by two sausage-shaped cells that are usually
smaller than surrounding epidermal cells. These small
cells are called guard cells and, unlike other cells in the
Epidermal cell
epidermis, contain chloroplasts. See Figure 1
Guard Cell
The photosynthesis that takes place in the guard cells
aids in the functioning of these cells i.e., the opening and
closing of the stomata. This regulated opening and
closing of the pores permits gas exchange between the
interior of the leaf and the outside atmosphere. The
opening and closing of the stomata also helps regulate
the water balance inside the plant as water can more
easily escape when the stomata are open. See Figure 2.
Thick inner
wall of guard
cell
Stoma
Chloroplast
Figure 1.
It is the unique structure of the guard cells that allows the opening and closing to occur. Internal
microfibrils and thicker inner walls of the guard cells cause these guard cells to “bulge” when
osmotic pressure builds up inside them. When the water content of the guard cells is high the
stoma is open and when the water content is low the stoma is closed. (With fresh epidermal
tissue this open and closing can be viewed under the microscope by applying different water
concentrations.)
The location and density of these
numerous pores is interesting and
relates to plant genetics and niche
adaptations. Stomata are most
numerous on the leaves of plants.
They occur on both the upper and
lower epidermis of the leaves in some
species (alfalfa, corn), exclusively on
the upper epidermis in some plants
Figure 2.
(water lily), and are absent altogether
on submerged leaves of aquatic plants. Stomata are very numerous, ranging from about 1,000 to
more than 1.2 million per cm2. An average-sized sunflower leaf has about 2 million stomata on
its lower epidermis.
Adapted from: Flinn Scientific Inc. Bio Fax Lasting Impressions – Counting Stomata
http://www.cjhs.org/teacherssites/taylor/accbio/PLANTS/STOMATE%20LAB.pdf
SJB Create 10/2011 Revised 7/31/2017
Botany
Page 3 of 6
Pre-Lab Questions:
1. How many guard cells surround each stoma?
2. How are guard cells different from the surrounding epidermal cells?
3. What process takes place within the guard cells that aids in their opening and closing
function?
4. What causes the guard cells to open?
5. Are stomata only located on the bottom of leaves?
6. What are some examples of plants that have stomata on both the upper and lower
epidermis?
7. What is an example of a plant that has stomata exclusively on the top epidermis of the
leaves?
8. Why would the plant (answer to #7) only have stomata on the upper epidermal surface of
the leaves?
9. Do aquatic plants have stomata? Explain why.
10. What is the range of the amount of stomata per square centimeter? Explain why there
would be such a large variation in amount.
Adapted from: Flinn Scientific Inc. Bio Fax Lasting Impressions – Counting Stomata
http://www.cjhs.org/teacherssites/taylor/accbio/PLANTS/STOMATE%20LAB.pdf
SJB Create 10/2011 Revised 7/31/2017
Botany
Page 4 of 6
Lasting Impressions - Counting Stomata
Materials:
Plant leaves
Clear fingernail polish
Clear cellophane tape (clear package sealing tape)
Microscope
Microscope slides
Scissors
Procedure:
1. Obtain a sturdy leaf or other plant tissue.
2. Paint a thick patch of clear nail polish on the leaf surface being studied (top side or bottom
side). Make a patch at least one square centimeter (about the size of a penny).
3. Allow the nail polish to dry completely.
4. Fold the very end of a piece of tape under on itself to create a handle for easier grasping and
pulling without leaf damage before adhering tape to nail polish on leaf.
5. Tape a piece of clear cellophane tape to the dried nail polish patch. (The tape must be clear.
Do not use any other opaque tape. Clear carton-sealing tape works well).
6. Gently peel the nail polish patch from the leaf by pulling on folded-under handle end of the
tape and “peeling” the fingernail polish off the leaf. This is the leaf impression you will examine.
(Only make one leaf impression on each side of the leaf, especially if the leaf is going to be left
on a live plant.)
7. Tape your peeled impression to a very clean microscope slide. Use scissors to trim away any
excess tape. Label the slide “top” or “bottom” as appropriate for the specimen being examined.
8. Examine the leaf impression under a light microscope. Use the scanning (red 4x) objective
for initial focus and progress up to 400X. (Use the 40x objective [Blue]).
9. Search for areas on the slide where there are numerous stomata. Avoid areas where there are
thumb prints, dirt or smudges, damaged areas, or large leaf veins.
10. Count all the stomata in one microscopic field. Record the number in the data table.
11. Repeat counts for at least two other distinct microscopic fields. Record all the counts in the
table. Determine an average number per microscopic field.
12. From the average number/400X microscopic field calculate the stomata per square
millimeter by multiplying by 8.
13. Trade slides with classmates so you examine three different leaves (top and bottom) under
the microscope. Repeat steps 8–11.
Adapted from: Flinn Scientific Inc. Bio Fax Lasting Impressions – Counting Stomata
http://www.cjhs.org/teacherssites/taylor/accbio/PLANTS/STOMATE%20LAB.pdf
SJB Create 10/2011 Revised 7/31/2017
Botany
Page 5 of 6
Lasting Impressions – Counting Stomata
Data Table 1.
Leaf 1
Leaf 2
Leaf 3
Leaf sketch 400x
(example)
Top
Bottom
Top
Bottom
Top
Bottom
# Stomata in field 1
# Stomata in field 2
# Stomata in field 3
Average # stomata
per field
Stomata / mm2
(multiply average
stomata per field by 8)
Post Lab Questions:
1. On the leaves examined, was the greatest concentration of stomata located on the top or
bottom of the leaf?
2. Explain why you think the concentration of stomata was greatest in this location. (Hint: Think
about how this location would help regulate transpiration.)
Adapted from: Flinn Scientific Inc. Bio Fax Lasting Impressions – Counting Stomata
http://www.cjhs.org/teacherssites/taylor/accbio/PLANTS/STOMATE%20LAB.pdf
SJB Create 10/2011 Revised 7/31/2017
Botany
Page 6 of 6
3. What time of day/night would stomata most likely to be open? Explain your reasoning.
4. What could you do as an experiment to investigate whether your prediction was accurate?
5. How do you think weather and environmental factors affect the number of stomata a species
produces on its leaves? Fully explain your reasoning.
Adapted from: Flinn Scientific Inc. Bio Fax Lasting Impressions – Counting Stomata
http://www.cjhs.org/teacherssites/taylor/accbio/PLANTS/STOMATE%20LAB.pdf