PLANT GROWTH AND NUTRITION 



67 



A grain cut lengthwise perpendicular to the flat side and then 

 dipped in weak iodine shows two distinct parts, an area containing 

 considerable starch, the endosperm, and the embryo or young 

 plant. Careful inspection shows the hypo- 

 cotyl and plumule (the latter pointing toward 

 the free end of the grain) and a part surround- 

 ing them, the single cotyledon (see Figure). 

 Here again we have an example of a fitting 

 for future needs, for in this fruit the one seed 

 has at hand all the food material necessary 

 for rapid growth, although the food is here 

 outside the embryo. 



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 cotyledons 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 grain has begun to grow, examina- 

 tion shows that the starch grains near the 

 edge of the cotyledon are much smaller and 

 quite irregular, having large holes in them. 

 We know that the germinating grain has a 

 much sweeter taste than that which is not 

 growing. This is noticed in sprouting barley 

 or malt. W T e shall later find that, in order 

 to make use of starchy food, a plant or animal 

 must in some manner change it over to sugar. 

 This change is necessary, because starch will 

 not dissolve in water, while sugar will ; in this form substances 

 can pass from cell to cell in the plant and thus distribute the food 

 where it is needed. 



Longitudinal section of 

 young car of corn. 

 O, the fruits; S, the 

 stigmas ; SH, the 

 sheath-like leaves : 

 ST, the flower stalk. 

 (After Sargent.) 



