Examine a dry kernel of corn and notice that on one side there is a slight oval-shaped depression (Fig. 41-1). Now take a soaked kernel and cut it in two pieces making the cut lengthwise from the top of the kernel through the centre of the oval depression and examine the cut surface. A more or less triangular-shaped body will be found on the concave side of the kernel (see Figs. 41-2 and 41-3). This is the one cotyledon of the corn. Besides this will be found quite a mass of starchy material packed in the coverings of the kernel and in close contact with one side of the cotyledon. This is sometimes called the endosperm.
Within the cotyledon will be found a little growing shoot pointed toward the top of the kernel. This is the epicotyl, and another growing tip pointed toward the lower end of the kernel; this is the hypocotyl or the part which penetrates the soil and forms roots.
Now examine the seeds that were planted in succession. Some will be just starting a growing point down into the soil. Some of them have probably come up and others are at intermediate stages.
After sending down a root the hypocotyl began to develop into a strong stem which crooked itself until it reached the surface of the soil and then pulled the cotyledons or seed-leaves after it (Fig. 42). These turn green and after a time shrink and fall off.
The pea cotyledons were left down in the soil, the epicotyl alone pushing up to the surface. The corn pushed a slender growing point to the surface leaving the cotyledon and endosperm behind in the soil but still attached to the little plant (Fig. 43).
=Experiment.=--Plant some beans in a pot or box of soil and as soon as they come up cut the seed-leaves from some of them and watch their growth for several days. It will soon be seen that the plants on which the seed-leaves were left increase in size much more rapidly than those from which the seed-leaves were removed (see Figs. 43 and 44). Sprout some corn in the seed tester. When the seedlings are two or three inches long, get a wide-mouthed bottle or a tumbler of water and a piece of pasteboard large enough to cover the top. Cut a slit about an eighth of an inch wide from the margin to the centre of the pasteboard disk. Take one of the seedlings, insert it in the slit, with the kernel under the pasteboard so that it just touches the water. Take another seedling of the same size, carefully remove the kernel from it without injuring the root, and place this seedling in the slit beside the first one (Fig. 45). Watch the growth of these two seedlings for a few days. Repeat this with sprouted peas. In each case it will be found that the removal of the seed-leaves or the kernel checks the growth of the seedling. Therefore, it must be that the seed-leaves which appear above ground, as in the case of the bean, or the kernel of the corn which remains below the surface of the soil, furnish the little plant with food until its roots have grown strong enough to take sufficient food from the soil.