As the hypocotyl, or first seed-growth, extends downward into the soil, rootlets start from its sides and rounded point. From the point the main or tap-root extends downward, and from the sides extend the branch or feeding roots. The length of the hypocotyl varies in seeds of different plants and trees. The long list of plants that lift up bodily the halves of the seed or what remains of it, such as the bean, pumpkin, and most of the fruits, make a relatively long growth of the hypocotyl during the process of root-extension. This seems to be a wise provision for forming a foundation for the forcing process of lifting the bulky cotyledons to the surface with a force that often uplifts a hard-baked earth surface.
This class of seeds should be planted shallow. If as deeply planted as peas, they will not be able to force the cotyledons to the surface. At the West a heavy rain followed by dry days often forms a crust that makes it difficult for apple-seed to germinate if planted more than three fourths of an inch deep. If the soil is well firmed in planting, the whole of this numerous class will do best if not covered more than from one half to three fourths of an inch deep (Fig. 1).
On the other hand, all plants that do not lift up the seed-leaves or cotyledons will bear deeper planting. The pea is an example. This forms the two cotyledons, but they remain in the soil and the slender shoot pushes up easily to the surface. In this class of plants the hypocotyl lengthens but little and does not lift up the cotyledons. Hence this class of plants, including those with one cotyledon, such as corn, can be planted five or six inches deep and yet reach the surface in favorable soils. But, as a rule, horticultural seeds should not be planted deeper than is required to secure the needed supply of moisture. As an example, it is often recommended to plant the sweet-pea deep in trenches, gradually filling up to a depth of five or six inches. But experience shows that if planted one inch deep they will succeed much better when planted on heavy soils.