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TEACHER/LECTURER
GUIDE
Type and purpose of activity
This experiment can be used to:
- provide evidence
for assessment of Outcome 3. N.B. This is only possible if stomatal
widths are measured using eyepiece graticules. Otherwise, the experiment
is an illustrative practical.
(For advice on marking Outcome 3 report, please contact SAPS Scotland)
- develop good
microscope skills
- develop knowledge
and understanding of how osmosis and the state of turgor in guard
cells affects the opening and closing of stomata
- develop problem
solving skills and in particular Outcome 2 PC:
(d) Experimental procedures are planned, designed and evaluated
appropriately.
-
Commelina
communis provides an excellent epidermal peel. It is invariably
one cell thick while the chloroplasts inside the guard cells are
clearly visible. It should also provide a rare opportunity for students
to see open stomata. The experiment clearly demonstrates that when
guard cells are turgid stomata are open and when guard cells are
flaccid stomata are closed.
-
To
make the results quantitative eyepieces need to be fitted with a
graticule which then allows students to measure stomatal width in
graticule units. (Hence the need for eyepiece graticules if Outcome
3 is to be assessed).
-
Students
need to have a firm grasp of the direction of water movement by
osmosis when the percentage water bathing plant cells is changed.
They also need to be familiar with the terms - turgor, turgid and
flaccid.
-
They
may also be confused by the apparent contradiction that as guard
cells lose water stomata close. This principle is well demonstrated
by pumping up a bicycle tyre tube fully and then folding it in half.
The distance between the two halves will gradually decrease as the
tube is deflated.
Weyers,
J.D.B.(1994) Investigating stomatal physiology with epidermal strips
from Commelina communis L (Dayflower)., Journal of Biological Education,
28(4), 255-259.
The lower epidermal
peel is to be placed in three liquids - distilled water, 5% sucrose
solution and 20% sucrose solution. Each student should be responsible
for the preparation of at least one microscope slide.
To
obtain good results it is necessary to observe the following points:
- the microscopes
must be clean and be capable of x400 magnification
- it is probably
best to demonstrate how to make an epidermal peel
- students may
also require help using the eyepiece graticule
- to measure
stomatal width it will be necessary to move the slide and rotate
the eyepiece so that the graticule scale lies above the stomata
(see diagram in Student Activity Guide)
- cross contamination
between the three liquids must be avoided
- the peel must
be transferred quickly to the bathing liquid. Otherwise, stomata
will close regardless
In
order to satisfy the core skill in problem solving, students will
be required to 'identify and obtain resources' required for themselves.
It is therefore not appropriate to provide all equipment and materials
in e.g. a tray system for each student/group. Normal laboratory apparatus
should not be made available in kits but should generally be available
in the laboratory. Trays could be provided containing one type of
specialist equipment or materials.
-
Bathe
epidermal peels in buffered solutions of different pH.
-
Compare
upper and lower epidermal peels.
-
Compare
responses of epidermal peel in other plants.
-
The
role of potassium ions in stomatal opening e.g. bathe illuminated
epidermal peels with closed stomata in 150 mM KCl, NaCl and LiCl.
-
Bathe
epidermal peels in solutions of abscisic acid - a hormone involved
in the closing mechanism.
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