Stomata: Microscopic Openings
That Let Plants “Breathe”
SUMMARY / ABSTRACT:
Plants move in ways that may not seem obvious. The opening and closing
of stomata
is one example of this movement. This lab was designed to allow students
to observe and measure this daily activity.
The following
experiment can be utilized during units involving plant anatomy and physiology,
photosynthesis, or to illustrate ecological concepts such as photoperiods,
pollution, and the greenhouse effect. This is an inquiry-based lab exercise;
it may be modified to fit a wide variety of student questions and hypotheses.
For example, students may be curious about the effects the following on
stomata:
-
Carbon dioxide concentrations
-
Temperature
-
Humidity
-
Type of plant
-
Water availability
-
Wind
-
Photoperiod
This lab exercise focuses on the question of photoperiod. Additionally,
the technique demonstrated can be used at many levels of instruction. It
allows for qualitative and quantitative analysis including, but not limited
to, the questions above.
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National
Science Education Standards addressed:
-
Science as Inquiry, (NSES
Content Standard A);
Abilities related to scientific inquiry
Understanding about scientific inquiry
-
Developing Student Understanding, (NSES
Content Standard C),
Behavior of organisms
Diversity and adaptations of organisms
Interdependence of organisms
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OBJECTIVES:
Upon completion of this lab, the students will be able to:
-
correctly locate and identify stomata on a leaf.
-
explain the role of stomata in the daily functioning of a plant.
-
evaluate the effect of daily light cycles on stomata opening / closing.
-
utilize proper lab procedures to make an imprint slide.
-
formulate and test student-generated hypotheses.
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TARGET AUDIENCE:
Biology / Life Science 7 - 12 (varying degrees of student
inquiry are encouraged to meet learner needs)
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NOTES TO TEACHER:
Vocabulary list:
stomata
guard cell
epidermis
turgor
osmosis
diffusion
concentration gradient
Plant use of turgor pressure to open and close stomata is often difficult
for students to understand. This concept can easily be demonstrated by
using two long balloons and some rubber cement. First, the
balloons should be inflated about 50% full and taped in the manner illustrated
below. Next, the inside ends of the balloons are glued together with
rubber cement. This represents the relaxed state of the guard cells
resulting in closed stomata. If the balloons are more fully inflated,
each balloon will expand outward, resulting in the creation of a space
in the middle. This is representative of the stomatal pore when open.
Both ends of balloon glued, inflated 50%. |
Further inflated, showing stomatal pore. |
(copied with permission from Plant Physiology, 4th Ed., F.B. Salisbury,
C.W. Ross, Wadsworth Pub. Belmont, CA, 1992)
It is suggested that the teacher try various brands of superglue before
starting this project. Gel types often remain tacky for too long,
and won't take the imprint. The type used for this lab was simply
a generic storebrand - and the cheapest. Also, use a new box of slides;
any oils from fingers or previous use tend to prevent the glue from sticking.
For a class management idea, don't allow the students to allocate the glue;
the teacher can control the amount (and the tube!) of glue.
Depending on the type of leaf used, different amounts glue and time
are needed to produce an imprint. Again, the teacher should test
this before the students. Various types of leaves were examined in
trials for the project; leaves with a thick, hairy layer on the bottom
were very difficult to imprint. Waxy leaves, such as holly or wandering
jew, are easy to imprint and peel away nicely from the glue. Other
possibilities include: pea plants, beans, grass, or any other plant
students may be growing for class. Dicots seem to work the best,
but the sky is the limit!
The following sample imprints were taken from a holly bush at different
times of the day, in Princeton, New Jersey on July 15 using the methods
described above. Results may vary depending on season, location,
temperature and humidity.
6:30 a.m. |
10:00 a.m. |
11:00 a.m. |
1:00 p.m. |
4:30 p.m. |
8:15 p.m. |
(Photos taken by web development group, using a Flexcam
and Snappy video capture device)
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MATERIALS:
-
Plant with many leaves
-
glass slides
-
Superglue
-
microscopes
-
permanent markers
-
graph paper
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BACKGROUND:
Since their colonization of land, plants have been faced with a constant
problem. Plants must exchange gasses through their leaves
in order to conduct photosynthesis and respiration; they also must permit
evaporation / transpiration in order to assist in the movement of water
from the ground to leaves, where it is needed to build carbohydrates.
Yet, if transpiration is uncontrolled, a plant may become dessicated (dehydrated)
and die.
The stoma is a pore formed by a pair of guard cells. The guard
cells are located in both epidermal layers of the leaf, with a higher concentration
on the underside (demonstrating another strategy to reduce water loss).
Research has indicated that plants contain a range of 10,000 - 100,000
stomata per cm2. Stomatal opening is regulated by turgor
in the guard cells. The inner walls of the stomata are fused at both
ends, and, as the cells fill with water, the fused walls remain the same
length, while the outer walls stretch. As demonstrated by the taped
balloons, this stretching is limited by the orientation and strength of
lignin fibers surrounding the cells.
(copied with permission from Biology: The Science of Life; R.A.
Wallace, G.P Sanders, and R.J. Ferl, Harper-Collins, New York, 1991)
Water enters and leaves guard cells in response to changes in solute
concentrations. Potassium is actively pumped into the guard cells
from surrounding cells. This in turn increases solute concentration;
as a result, water potential decreases and water enters the guard cell
against a pressure gradient.
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EXPERIMENTAL PROCEDURE:
Slide Preparation Technique:
1. On a clean glass slide, dispense a small amount of superglue,
about one quarter the size of a pea.
2. Let the drop sit for a few seconds. (if more than 30 seconds,
the glue adheres to the glass in a circle, and won't imprint)
3. Obtain a large leaf and press the underside into the drop
of glue.
4. Apply firm pressure to the leaf for approximately 30 seconds.
(The thinner the leaf, the shorter the time; pea leaves should be held
about 20 seconds, holly leaves about 60 seconds) Students may need
to place a paper towel over the leaf so their fingers don't stick to the
slide.
5. Remove pressure, and let the leaf remain on the glue for another
15 - 20 seconds.
6. Carefully peel the leaf off of the glue. (Some of the leaf
may stick in the glue, try to get most off)
7. The imprint will be in the glue; let the glue dry before viewing.
Suggested Photoperiod Procedure:
Three sample slides were made every hour, over a 24-hour period, using
the same tree. Each sample was taken from three different areas of
the tree. Humidity, light intensity, and temperature were recorded
with each sample.
To analyze the data, images were captured using a flex-cam attached
to a compound microscope (400x). A "Snappy-cam" (video capturing
device) was used to photograph the imprints on the slide. These images
were then transferred to "SCIONIMAGE"
for total area measurement of stomatal pore openings (at 400x, the entire
stomata has a length of 20microns). Scionimage can be downloaded
free from the internet!
This procedure can be altered to fit any student-generated hypotheses
mentioned in the summary
section above.
If you do not have access to a flex-cam, snappy-cam and scionimage,
an alternative way to assess the area of the open pores is to compare them
to the following images, photographed using a compound microscope (400x).
100% open 
|
75% open 
|
50% open
|
25% open 
|
15% open 
|
0-5% open
|
For sample results of this photoperiod lab, click here - RESULTS.
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EVALUATION / ASSESSMENT:
Students demonstrate mastery of science concepts by completing the imprinting,
data collection, graphing, and evaluation process described above. They
may complete an oral or written lab report investigating a hypothesis or
question generated by the basic procedures. Teachers may thus encourage
divergent inquiry, utilize "real-life" evaluation, and maintain a portfolio-type
record of the achievements of individual students or groups.
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EXTENSION:
Upon learning the slide preparation method, students can generate inquiry-based
questions, form hypotheses and continue a lab of interest.
Student lab repoorts should include: hypothesis, materials, procedure,
data tables / graphs, analysis and conclusion.
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REFERENCES:
Plant Physiology, 4th Ed. (1992), F.B. Salisbury, C.W.
Ross, Wadsworth Pub. Belmont, CA
(a comprehensive, undergraduate-level text in plant physiology, explaining
the interrelationships between structure, function, and biochemical processes
in plants)
Biology: The Science of Life (1991), R.A.
Wallace, G.P Sanders, and R.J. Ferl, Harper-Collins, New York
(a standard: comprehensive undergraduate text in general biology; also
suitable for advanced placement or gifted secondary classes)
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ADDITIONAL LINKS:
Stomata
information and pictures
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Last updated July 15,1999
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