Structure of a Grain of Corn.--Examination of a well-soaked grain of corn discloses a difference in the two flat sides of the grain. A light-colored area found on one surface marks the position of the embryo; the rest of the grain contains the food supply. The interesting thing to remember here is that the food supply is _outside_ of the embryo. 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 hypocotyl and plumule (the latter pointing toward the free end of the grain) and a part surrounding 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. [Illustration: 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. [Illustration: 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.