ENDOSPERM IN SEEDS 



125 



PROBLEM VI. WHERE IS THE FOOD SUPPLY OF DIFFERENT 



SEEDS? 



Laboratory Exercise. To study the structure and composition of 

 a grain of corn. 



Materials. Soak corn grains, some whole and some cut lengthwise 

 at right angles to the flat surface. 



Method and Observations. In whole corn grain find a light-colored 

 area on one side. This marks the position of the embryo. 



In a grain cut lengthwise at right angles to the flat side find the 

 embryo. Describe its shape, position, and relative size compared 

 with the rest of the corn grain. The area outside of the embryo is 

 known as the endosperm. Place iodine on the surface of the cut 

 corn grain. Describe what happens. Test a grain of corn for protein. 



Conclusion. What nutrients are present in the corn? Where are 

 they found? 



Endosperm the food supply of corn. We find that the one 

 cotyledon of the corn grain does not serve the same purpose to the 

 young plant as do the two cotyle- 

 dons of the bean. Although we 

 find a little starch in the corn 

 cotyledon, still it is evident from 

 our tests that the endosperm is the 

 chief source of food supply. The 

 study of a thin section of the corn 

 grain under the compound micro- 

 scope shows us that the starch 

 grains in the endosperm are large 

 and regular in size. When the 

 embryo has grown a little, an ex- 

 amination shows that the starch 

 grains near the edge of the cotyle- 

 don are much smaller and quite 

 irregular, having large holes in 

 them. This means that the starch 

 is being used by the young plant. 



Seeds with endosperm. In the 

 seeds of the pea and bean we have found that the embryo takes 

 up all the space within the seed coats. There are some plants 

 that have food stored outside of the embryo. Such a plant is the 



Wi 



endosperm 



tA^l edbn, 



-plumule. 



--WpoCotyl 

 root^tAp 



Longitudinal section of a grain of corn. 

 Find the embryo. Is the corn a seed or 

 a fruit? Why? 



