SEEDS AND SEEDLINGS 73 



Endosperm the Food Supply of Corn. — We do not find that the 

 one cotyledon of the corn grain serves the same purpose to the 

 young plant as did 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 

 microscope shows us that the starch grains in the outer part of 

 the endosperm are large and regular in size. Those 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. We shall later prove that, in order to 

 make use of starchy food, a plant or animal must in 

 some manner change it over to sugar. That starch 

 is being changed to grape sugar in the germinating 

 corn grain can easily be shown by the following 

 experiment : — 



Cut lengthwise through the embryo half a dozen grains A grain of corn, 



of corn that have just begun to germinate. Place them in wlL- C cot 



a test tube with a little Fehling's solution and heat almost yiedon;' ^V 



to the boihng point. On examination, the corn grains will endosperm ; 



be seen to give a slight reaction for the sugar test, especially h, hypoco- 



along the edge of the cotyledon and between it and the tyl; P, plu- 



endosperm. mule. 



Digestion. — This change of starch to grape sugar is a process 

 of digestion. It is performed by means of a substance found in 

 the cotyledon known as a digestive ferment or enzyme. The enzyme 

 found in the cotyledon of the corn, which changes starch to grape 

 sugar, is called diastase. 



Demonstration. The action of diastase on starch. — Diastase can be sepa- 

 rated from the cotyledon. It is here used in the form of a powder. 



To 1 c.c. of starch in 100 c.c. of water, add a very little (1 gram) of dias- 

 tase. Place the vessel containing the mixture in a warm place, \yhere the 

 temperature will remain nearly constant at about 98° Fahrenheit. Test 

 part of the contents at the end of half an hour, and the remainder the next 

 morning, for starch and grape sugar. The starch in the latter test will be 

 found to be completely changed to grape sugar. 



Experiment. — Select nine germinating grains of corn; remove the en- 

 dosperm from six of them. Replace the endosperm in three of the grains 

 by a little corn-starch paste. Place all nine grains on netting over a cup of 

 water, so that the roots reach the water. Keep them moist. Watch from 

 day to day to see which seedlings do the best. Explain the experiment- 



