8o ROOT HABITS OF DESERT PLANTS. 



own, or by maintaining its union with the parent and deriving the needed 

 supply through the parent root. It is this requirement that confines the 

 species, apparently of the extreme xerophytic type, to places where the 

 soil is of considerable depth and the water relations relatively favorable. 



Prosopis Velutina. 



Prosopis shows a very marked power of accommodation to varied con- 

 ditions of soil and water, especially the former, and when grown under 

 extreme conditions, exhibits differences in growth habit so great as nearly 

 to defy recognition. For instance, on Tumamoc Hill and on the bajada, the 

 species occurring to a limited extent in both places, Prosopis assumes the 

 form of an irregular bush, but on the flood-plain, its proper habitat, it may 

 become a tree 15 meters or more in height, with a well-defined bole. Fre- 

 quently, however, along the river-bottoms the species takes the form of 

 the cultivated eastern apple. By an earlier obser\'er (Havard, The Mezquit, 

 American Naturalist, vol. 18, page 450, 1884) the variation in the size of 

 Prosopis was taken as an index of the depth of the water table : if the tree 

 was large, the water lay close to the surface ; if it was small, the water table 

 was very deep. That the species is an indication of the presence of peren- 

 nial water is said to be the belief among native ranchers of southern Arizona, 

 who, it is said, may even follow the roots of mesquite in digging wells — a 

 "wa ter- witch " which points unerringh^ to subterranean water. Although 

 these beliefs are largely fanciful, they nevertheless have some foundation ; 

 where the mesquite grows large the perennial supply of water is relatively 

 close to the surface, and where it is small the water-supply is limited but 

 probably confined to the surface water. 



How deep the roots of Prosopis may penetrate the soil is difficult to 

 learn, but it is conditioned on the character of the soil, the depth of the water 

 table, and the penetration of the rains. Where a substratum, as hard caliche 

 and possibly fine-grained adobe clay, makes difficult or prevents deep perco- 

 lation of flood waters, roots of plants will not strike deep ; but where it is 

 such as to permit the deep sinking of the rains, or the rise of water from 

 the perennial water suppl}^ the plant roots may also penetrate to great 

 depth. The most deeply placed roots of Prosopis known to the writer are 

 those of plants growing by the Santa Cruz, which penetrate at least 5 meters, 

 but I have been informed by a reliable observer that the roots of mesquite 

 growing by a tributary of the Santa Cruz have been seen to reach to a depth 

 of 8 meters. As one leaves the river and goes toward the sides of the flood- 

 plain, Prosopis becomes smaller until, at the edges of the plain, it is little 

 more than a large bush. The water table also is deeper at the sides of the 

 flood-plain than near the river, and it is believed, although not actually 

 demonstrated, that the roots of the mesquite reach perennial water only 

 where the water table is relatively close to the surface. 



Where the surface soils do not permit the deep penetration of the roots of 

 Prosopis, as on Tumamoc Hill and the bajada, the plant derives all of its 



