ROOT-SYSTEMS OF PERENNIALS. 6l 



The number of small laterals at the bases of the plant is especially large. 

 The roots are mainly either horizontal or vertical, without being at any 

 intermediate angle, a character so strong in the root-system of Prosopis, 

 to be described later, and of so wide occurrence among the larger plants, 

 where the soil conditions are favorable, that it forms one of the most striking 

 of the general root characters of desert plants. 



The lateral extension of the roots of the Covillca from the flood-plain is 

 considerably less than that of the plant from the bajada. Those of the 

 latter reach out over 4 meters, while the roots of the former, as mentioned 

 above, extend hardly half as far. The root-area, or the area included 

 within the reach of the roots of the bajada-grown Covillea, was about 50 

 square meters, while that of the plant from the ffood-plain was only one- 

 fourth as much. These differences make it imperative that fewer plants 

 occupy a given space on the bajada than on the flood-plain. It is of interest, 

 however, to note that the mass of earth compassed by the root-system of 

 the two specimens of Covillea was approximately equal. 



With the characteristic differences in root development and root posi- 

 tion of the bajada and flood-plain plants are associated differences in the 

 relations of these plants to their neighbors. In excavating the roots of the 

 specimen from the flood -plain no roots of other woody plants were encoun- 

 tered within the root-area of the plant studied. But the condition of the 

 bajada-grown Covillea was quite otherwise. No fewer than 60 roots of 

 neighboring Covilleas were encountered at the same level, and frequently 

 in physical contact with the roots of the plant studied, and ver}^ many 

 more were met between the levels of these roots and the surface of the 

 ground. Therefore the competition between neighboring Covilleas on the 

 bajada, for soil water, is presumably keen, while, on the other hand, com- 

 petition between neighboring Covilleas on the flood-plain, for soil water, is 

 at best indirect, and may be so slight as to be negligible. 



FououiERiA Splendens. 



Foitquieria occurs most abundantly on the northern slope of Tumamoc 

 Hill, on its lower drainage slope, or the upper portion of the bajada, and 

 on the bajada near West Wash. It was not found on the flood-plain of 

 West Wash or of the Santa Cruz, that is, in other words, the species in the 

 vicinity of the Desert Laboratory does not grow where the soil has consider- 

 able depth, so that the "normal" behavior of its roots is not known. 



The soil conditions where the specimens of Fouqtiieria specially studied 

 were growing, the bajada to the north of Tumamoc Hill, are nearly the 

 same as those of the Covillca habitat above described, except that the super- 

 ficial adobe clay is somewhat thicker and there may be a greater admixture 

 of small stones. The superficial soil is underlaid by rotten caliche and the 

 latter by hard cahche, as in the habitat referred to. 



