THE roots of the desert plants are of interest, in part because of their relation to the physiological activities of the shoot, and in part because of their own physiology. There is a close relation between the character of the roots of the desert plants and the distribution of the plants, and probably with many other activities of the plants, as, for example, the formation of the leaves, of the flowers and the taking on of new growth. What the precise relation may be between the root systems and the adaptation of desert plants to desert surroundings is not known, nor, for that matter, the relation of the roots of the plants of the more humid regions to their distribution, or to their origin. Also the special relation of the roots of plants to the substratum has not been extensively investigated, as, for example, the character of root development as related to the precise per cent, of water content, or to the temperature. The lack of quantitative experimental studies on roots in soil is to be attributed in large part to the difficulty in studying the soils. If certain activities of the roots, or the significance of root character to many features of the plants' activities, are to be understood, it will be necessary to do quantitative experimental work on plants growing in the soil, and not, as heretofore extensively done, growing under highly artificial conditions.
It is popularly supposed that the roots of the desert plants are very long—that is, that they penetrate the ground to great depths, and from this that the length of a root-system is in some way a measure of the aridity of a locality. It is difficult to say how this idea arose, which really is without adequate foundation, because a relatively small amount of work has been done on the roots of the plants either of the humid regions or of the deserts, in the field. It is probable, however, that the few excavations that have been made have been carried on in those places where it chanced that the roots penetrated to great depth. But it is in exactly these places where the most favorable moisture conditions of the given locality are to be found, namely, where the soil is deep, giving an opportunity for the penetration of water to a great depth, as in the bottoms, or along the banks of stream ways—arroyas in our southwest, ouedes in southern Algeria, or weds in the eastern Sahara.