The roots serve at least three distinct uses or purposes: First, they give the plant a foothold in the earth; secondly, they enable the plant to secure from the soil the large amount of water needed in plant growth, and, thirdly, they enable the plant to secure the indispensable mineral foods which can be obtained only from the soil. So important is the proper supply of water and food in the growth of a plant that, in a given soil, the crop yield is usually in direct proportion to the development of the root system. Whenever the roots are hindered in their development, the growth of the plant above ground is likewise retarded, and crop failure may result. The importance of roots is not fully appreciated because they are hidden from direct view. Successful dry-farming consists, largely in the adoption of practices that facilitate a full and free development-of plant roots. Were it not that the nature of arid soils, as explained in preceding chapters, is such that full root development is comparatively easy, it would probably be useless to attempt to establish a system of dry-farming. Kinds of roots The root is the part of the plant that is found underground. It has numerous branches, twigs, and filaments. The root which first forms when the seed bursts is known as the primary root. From this primary root other roots develop, which are known as secondary roots. When the primary root grows more rapidly than the secondary roots, the so-called taproot, characteristic of lucerne, clover, and similar plants, is formed. When, on the other hand, the taproot grows slowly or ceases its growth, and the numerous secondary roots grow long, a fibrous root system results, which is characteristic of the cereals, grasses, corn, and other similar plants. With any type of root, the tendency of growth is downward; though under conditions that are not favorable for the downward penetration of the roots the lateral extensions may be very large and near the surface Extent of roots