92 ROOT HABITS OI? DESERT PLANTS. 



the hard pan which occurs within 50 cm., or less, of the surface, and on 

 Tumamoc Hill by the underlying rock. On the flood-plain, where the soil is 

 sand and adobe, as by West Wash, the penetration is probably about 2 

 meters, but on the Santa Cruz flood-plain water from the rains is thought 

 not to penetrate the ground over i meter, although it probably goes below 

 that depth in places. 



The penetration of the roots of the cactus type apparently presents 

 another problem, since certain of the cactus TOot-systems, Opuntiaarbuscula 

 particularly, lie within 2 to 5 cm. of the surface or well within the depth 

 attained by water either during summer, when the most active growth 

 occurs, or in winter. The relation in this instance may be with a proper 

 air supply, rather than with the penetration of the rains alone. 



Where there is considerable depth of soil and the water table is close to 

 the surface, as in places near the Santa Cruz river, the conditions are most 

 favorable for the deep penetration of roots. Under such conditions the 

 water chain during the rainy season may be continuous from the water 

 table to the surface and so favor deep penetration. It is here that Prosopis 

 vehUina and Populus sp. may be in position to obtain perennial water and 

 attain a large growth. The roots of Prosopis have here been seen to reach 

 5 to 8 meters below the surface, which is apparently the greatest depth 

 reached bv any plant in this vicinity. 



RELATIONS OF THE ROOTS OF NEIGHBORING PLANTS. 



The desert plants vary greatly in their abundance. It is well known 

 that shrubs inhabiting the bajada are comparatively remote from one 

 another. On the flood-plain the same species may form a fairly dense 

 thicket. In favorable seasons the annuals, both of the bajada and the 

 flood-plains, are occasionally so numerous as to completely hide the ground. 

 It is evident, therefore, that the roots of the different classes of plants, 

 annuals or perennials, hold var3'ing relations to each other, which are dif- 

 ferent in accordance with the kind of habitat they occupy. 



The mutual relations of the roots of annuals and of perennials vary also 

 wdth the character of the perennial roots, but only as the roots of annuals 

 find sufficient room for full development in all of the habitats. 



On the bajada and Tumamoc Hill the roots of such annuals as Anoda 

 thurheri, Kallstroeniia grandiflora, Solanum elcBagnifolium, Erodium cicu- 

 tarium, and others which have a deeply penetrating root-system, may 

 reach as deep as the roots of the perennials having a generalized type of 

 root-system. But on the flood-plain the roots of the two classes of plants 

 do not occupy the same horizon due in part to the fact that the roots of the 

 annuals differ little if any in habit from those on the bajada, but mainly 

 because of the deep penetration of the perennials. This can be illustrated 

 by a single example. The roots of Covillea on the bajada may extend 

 away from the main axis as far as 4 meters, while on the flood-plain the 



