Structure of Seeds and Effects of Fertilizer on Plant Growth

Ben Stark                      Illinois Institute of Technology
                               BCPS, IIT Center
                               Chicago IL 60616
                               (312) 567-3488

Objectives:

     There are two main objectives of this Mini-teach.  The first is to learn 
about the structural components of seeds of higher plants and the functions of 
each of these components.  The second is to investigate how fertilizer affects 
the growth of plants, particularly of seedlings, and how this relates to seed 
structure.  An additional objective is to learn to express and interpret 
data/results using graphs.  This lesson is fine for grade levels 3-8, with the 
level of sophistication (particularly regarding the data analysis) scaled 
appropriately. 

Materials Needed:

     Each student will need 10 2 inch (5 cm) pots, enough vermiculite to fill 
the pots, several peanuts (preferably raw) and 12 or so (dry) lima beans (other 
beans will also work).  For the class, there will also be a need for enough 
flats to hold the pots, 10 or more one-gallon plastic jugs, a source of 
distilled water, a box of dry fertilizer (such as Miracle Grow), and razor 
blades (only for dissection of lima beans, if this is desired, and obviously 
only for older students).  A space near a window in the classroom large enough 
for the flats is also needed, and magnifying glasses are a help. 

Strategy:

First Activity: Investigation of Seed Structure

      This activity works particularly well with raw peanut seeds, but can also 
be done with lima beans or other types of beans.  If you use peanut seeds, each 
student should shell a peanut and remove the feathery coat on each seed.  The 
seed (or peanut) can be seen to have two halves, demarcated by a longitudinal 
line all around the seed.  Each student should hold the peanut using both hands 
and gently push the two halves apart at this longitudinal line.  The peanut can 
then be observed with the naked eye, or with the aid of a magnifying glass.  If 
the peanut has been separated successfully, there will be two similarly sized 
pieces; these are the "cotyledons" (there are two cotyledons in the seeds of 
"dicot" plants like peanuts and beans; other plants ("monocots") have seeds with 
only one cotyledon). 

     One cotyledon will have, at one end, a small structure that looks like a 
tiny plant.  In fact, it is a tiny plant (actually a plant embryo).  Close 
examination of the embryo will reveal that it has both leaves and a root.  In 
the seed the embryo is in a quiescent state until the seed germinates, at which 
time it begins to grow.  Until it grows large enough to manufacture its own food 
via photosynthesis, the plant depends on food stored in the cotyledons; the 
initial growth of the seedling thus depends on the food stored in the 
cotyledon(s). 

    The same activity can been done with dry beans, although the cotyledons will 
have to be pried apart with a razor blade.  Alternatively, the beans (e.g., lima 
beans) can be soaked overnight in water, at which time the seed coat will slip 
easily from the seed and the cotyledons will separate easily without need of a 
razor blade. 

Second Activity: Effects of Fertilizer on Seedling Growth

     Each student should take ten pots and fill each with vermiculite.  It is 
important to use a non-nutritive support (i.e., vermiculite) rather than potting 
soil, as potting soil has lots of nutrients, which will serve to complicate 
interpretation of the results.  In other words, we want to control the amount of 
nutrients each plant will get and know what this amount is.  A single dry lima 
bean should then be planted into each pot. 

     Each student (or group of students) should then prepare fertilizer 
solutions for the experiment.  Into each of 5 one-gallon jugs, place one gallon 
of distilled water.  Leave one of the jugs with water only (i.e., do not add any 
fertilizer to it).  To the second jug, add the amount of dry fertilizer 
recommended on the fertilizer box for one gallon of water; mix/dissolve (this 
will be "full-strength" fertilizer).  To the third jug add twice the recommended 
amount of dry fertilizer, and mix/dissolve.  To the fourth jug add one-half the 
recommended amount of dry fertilizer and mix/dissolve.  To the fifth jug add 
one-tenth the recommended amount of dry fertilizer and mix/dissolve. 

     Each student will have two pots/seeds that will be watered throughout the 
course of the experiment with the contents of one, and only one of the five jugs 
described in the previous paragraph (e.g., there will be two pots watered with 
the water only, two pots watered with the one-tenth strength fertilizer only, 
etc.). All pots will be watered on exactly the same schedule and with exactly 
the same volume of liquid.  This will ensure that every one of the ten pots will 
always get the same amount of water; only the amount of fertilizer that each 
gets (from none to double-strength) will vary (because of the varying 
concentration of fertilizer (from none to double-strength) in each of the five 
jugs).  The first watering should occur just after the seeds are planted; 
subsequent waterings should be on an as need basis (i.e., when the vermiculite 
is dry). 

     Students should record their observations in notebooks.  In particular, 
after the seeds have sprouted, they should measure the height of each plant 
(using a ruler).  It is very important when recording data to note the date that 
those data were recorded (it is also very important to record the date of the 
initial planting). 

Performance Assessment:

     Regarding the second activity, all of the seedlings will grow initially to 
a certain point, because of the food stored in the cotyledons.  Beyond this, 
growth is dependent on photosynthesis and nutrients absorbed by the plants' 
roots.  The latter is, in turn, dependent on the concentration of fertilizer 
added to each plant. 

     Thus, we might expect that the seedlings watered with water only (no 
fertilizer added) will grow to a certain height and then stop growing, when the 
food in the cotyledons is exhausted.  But if the water contains fertilizer, it 
should support additional growth. 

     The data (seedling height versus days since planting) should be plotted.  
Students should use the resulting graphs to help in interpreting the results of 
the experiment, particularly in a quantitative way (e.g., How long is growth 
supported by the cotyledons alone?  What percentage of the growth with full-
strength fertilizer is due to the food in the cotyledons?  Is one-tenth strength 
fertilizer enough to support as much growth as full-strength fertilizer, and if 
not, at least some growth above that due to the cotyledons alone?  Does double-
strength fertilizer afford better growth than full-strength fertilizer?  Does 
half-strength fertilizer result in as good growth as double-strength and full-
strength fertilizer?, etc.). 

     It may also be helpful for the students to review the first activity as the 
second activity is progressing, i.e., to remember what the cotyledons and embryo 
look like in the seed before germination, compared to how the seedlings appear 
as they are growing.