Longitudinal section of young ear of corn. O, the fruits; S, the stigmas; SH, the sheath-like leaves; ST, the flower stalk. (After Sargent.)
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 microscope shows us that the starch grains in the endosperm are large and regular in size. When the grain has begun to grow, examination 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. We 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.
Test for grape sugar.
A Test for Grape Sugar.—Place in a test tube the substance to be tested and heat it in a little water so as to dissolve the sugar. Add to the fluid twice its bulk of Fehling's solution,[6] which has been previously prepared. Heat the mixture, which should now have a blue color, in the test tube. If grape sugar is present in considerable quantity, the contents of the tube will turn first a greenish, then yellow, and finally a brick-red color. Smaller amounts will show less decided red. No other substance than sugar will give this reaction. If Benedict's test[7] is used, a colored precipitate will appear in the test tube after boiling.
Starch changed to Grape Sugar in the Corn.—That starch is being changed to grape sugar in the germinating corn grain can easily be shown if we cut lengthwise through the embryos of half a dozen grains of corn that have just begun to germinate, place them in a test tube with some Fehling's solution, and heat almost to the boiling point. They will be found to give a reaction showing the presence of sugar along the edge of the cotyledon and between it and the endosperm.
Digestion.—This change of starch to grape sugar in the corn is a process of digestion. If you chew a bit of unsweetened cracker in the mouth for a little time, it will begin to taste sweet, and if the chewed cracker, which we know contains starch, is tested with Fehling's solution, some of the starch will be found to have changed to grape sugar. Here, again, a process of digestion has taken place. In both the corn and in the mouth, the change is brought about by the action of peculiar substances known as digestive ferments, or enzymes. Such substances have the power under certain conditions to change insoluble foods—solids—into soluble substances—liquids. The result is that substances which before digestion would not dissolve in water now will dissolve.
A germinating corn grain. C, cotyledon; H, growing root (hypocotyl); P, growing stem (plumule); S, endosperm; d.s., digested starch; p.r., primary root; s.r., secondary root; r.h., root hairs.
The Action of Diastase on Starch.—The enzyme found in the cotyledon of the corn, which changes starch to grape sugar, is called diastase. It may be separated from the cotyledon and used in the form of a powder.