Structure and Function of Seeds
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Ben Stark Ill. Inst. of Technology - Division of Biology
Dept. of Chemical and Biological Sciences
IIT Center - Chicago IL 60616
Seeds of flowering plants have two main parts, the "cotyledon(s)" (one or
two, depending on the seed type) and the "embryo" (a tiny "baby" plant in a
state of suspended animation). When the seed is planted and begins to
germinate, the embryo will "reawaken" and begin to grow and develop. Initially
it uses resources stored in the cotyledon(s) for this purpose; when the growing
plant gets a bit bigger, it is able to (and must) obtain minerals and water from
the soil and carbon from the air on its own. In these exercises the structure
of a seed (particularly the embryo) is observed and two experiments are
performed to show the function of the cotyledons in providing nourishment to the
plant during the early times after germination.
The following list is intended for a class of 25 students.
1. one bag of lima beans or raw peanuts (note: roasted peanuts will not
2. 150 paper cold drink cups (10 ounce or larger size)
3. 10 pounds of potting soil
4. one big bag of vermiculite
5. one box of miracle grow (dry) plant food
6. three one gallon plastic milk containers
7. hand lenses
8. razor blades or suitable substitute (choice adjusted for appropriate grade
1. Hand out one or two seeds to each child. For this part of the exercise
peanuts work a bit better than lima beans, but both are suitable and work the
same way. Separate the two cotyledons from each other by carefully inserting a
razor blade or other sharp edge between the cotyledons and gently prying them
apart (you will see a line in the seed which denotes the point of separation
between the cotyledons). At the bottom of one of the two halves you will find
the embryo. Close inspection of the embryo, particularly with the hand lens,
will show that the seed does, in fact, carry a tiny version of the plant,
including leaves and root. Also note the size and location of the cotyledons in
relation to the embryo.
2. Plant one or two seeds (of one or both types as desired, each in a separate
cup) into the potting soil. Water (with tap water) the pots as needed to keep
the soil moist, but don't overwater them. Also, remember to water all pots
exactly the same way (same days, same volume of water, etc.). Don't feed this
set of plants with the plant food. After a few days the seeds will germinate.
The first two oval shaped, leaf-like parts of the plant you will see are the
cotyledons; the real leaves will appear later. Record data as follows:
1. Record date of planting.
2. Record watering schedule (days, volumes, etc.).
3. Record date that germinating plant is first visible.
4. At each date following emergence of the plant record both the diameters
of the cotyledons and the overall height of the plants. If more than
one plant is being measured, be sure to keep measurements for each plant
3. Choose three seeds of the same type. Plant each in a separate cup filled
with vermiculite, not potting soil. Fill the three gallon containers as
1. One with just tap water.
2. One with miracle grow made up according to the directions on the box.
3. One with miracle grow made up to only one fourth of the strength listed
on the box (e.g., if full strength miracle grow uses one teaspoon per
gallon, use only one fourth of a teaspoon per gallon for this container,
One plant will be watered with water only, one with full strength miracle grow
only, and one with one quarter strength miracle grow only. It is very important
that each plant get watered only from one of the containers during the course of
the experiment and also that all plants get the same volume of liquid on the
same days. Watering should be done to keep the soil moist. Record the
watering/feeding schedule for each plant, the date of emergence of each plant,
and the height of the plant and diameter of the cotyledons at each date after
I have done these exercises with fifth and sixth graders and even as a
science fair experiment for a bright third grader, but they could probably be
adapted for high schoolers as well. The data analysis can be more sophisticated
as the kids get older, but all grades should be able to draw the seed and embryo
as seen through the hand lens. All grades should be able to make a simple graph
of the data, specifically height of the plants and cotyledon size versus time
(it is useful to plot the height and cotyledon size versus time on the same set
of axes). The older grades might do more sophisticated graphs such as
(especially for experiment number 2 above) plotting the ratio of plant height to
cotyledon diameter versus time. In addition, data from the entire class can be
pooled; this allows for cases in which one of an individual student's plants
didn't germinate and also allows for lessons in averages.
Things to discuss are the structure of the embryo and the cotyledons
(experiment 1), how the sizes and relative sizes of the cotyledons and plants
change with time (experiments 2 and 3), and whether giving the plants miracle
grow enhances their growth and whether this enhancement occurs throughout the
course of the experiment or only after a certain time (experiment 3). Implicit
in these discussions is that the cotyledons supply the nourishment for plant
growth for a time, but then the plant must get its nourishment on its own. How
do the data from experiments 2 and 3 illustrate this? For example, in
experiment 2 does the cotyledon size remain constant or shrink while the plant
size keeps increasing? In experiment 3 do the three plants grow at the same
rate for a while, but eventually do the fed plants grow faster, and does the
plant with higher strength miracle grow do the best?