THE ROOT-SYSTEMS OF DESERT PLANTS 95 



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 espe- 

 cially 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 char- 

 acter 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 environ- 

 ment 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 diffi- 

 cult 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 



