Structure and Function of Seeds

Ben Stark                      Ill. Inst. of Technology - Division of Biology
                               Dept. of Chemical and Biological Sciences 
                               IIT Center - Chicago IL 60616 
                               (312)567-3488

Objectives: 

     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. 

Materials Needed: 

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 
   suffice). 
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 
   level)

Strategy:

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
       individually.

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 
follows: 
    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, 
       etc.). 
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 
emergence. 

Performance Assessment/Conclusions:

     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?