We therefore conclude from what has just been said that the most arid portions of such deserts as those in the southwest of the United States are on the higher lands, and the less arid portions in the lower lands—the flood-plains or the washes—and that it is only in the less arid areas that plants with pronounced tap roots occur.
It should be definitely pointed out that the foregoing classification of roots is applicable only to such deserts as that of the Tucson region, where a portion of the flora consists of plants with a water balance. In the more arid regions, such, for example, as southern Algeria, fleshy plants are almost entirely absent, and root-systems characteristic of such plants are consequently not to be found. We therefore have in the most intensely arid desert plants with two general types of root-systems only, namely, the generalized type and that form which has a well-developed tap root. In southern Algeria, for example, species of the genus Haloxylon have a modified generalized type of root-system, and this species occupies the plains—the reg or hamada—where the soil is least abundant and hence where the water relations are least favorable. In the hollows of the plains where soil has accumulated to some extent, and along the washes or oueds, we find plants with the main root especially well developed. In fact, it is only where the soil is actually or relatively deep that such forms as Tamarix, Zizyphus or other relatively large forms all having long deep roots, are to be found. From the character of the roots of plants from the plains of southern Algeria, as well as the roots of plants from southwestern United States it is to be seen, therefore, that if any type of root is entitled to be called the xerophytic type, it is the generalized form, and not the deeply penetrating tap-root form which is thus seen to be the peculiarity of plants which grow where conditions are relatively favorable.
Turning now to consider briefly the environment of the roots of desert plants, we should note, in the first place, that the root environment of these plants is not at all well understood. This, of course, comes partly from the fact, as before pointed out, that the soil is difficult to study. However, certain features of the soil, such as the water content, the temperature, and certain other features, which are best known, can be treated briefly.
As a general thing the rains of the desert do not penetrate the soil to any considerable depth. In the Tucson region, where the rainfall does not exceed 30 cm., the penetration of the ground is usually not over 50 cm., although this varies with the variation in the character of the soil. The water table usually lies so deep that the water is not available to the plants. On the mesa, in the vicinity of Tucson, for example, the water table is frequently 25 meters, or more, deep, but on the flood-plain of the Santa Cruz River, it varies from 3 to 10 meters. Under earlier conditions, which need not be described in this
