72 CARNEGIE INSTITUTION OF WASHINGTON. 



uppermost soil-levels, which are wetted by rains only, do not provide 

 water in suitable amount to support forests. It is in the uppermost 

 soils that most of the roots of mesophytic trees are confined. The 

 root-soil-moisture relation of mesophytic trees is now well estab- 

 lished for the semiarid Southwest. Here, having given suitable soils, 

 such resistant species as the mesquite may assume the tree habit if 

 the water-table lies within 40 feet of the surface, and, under such 

 favoring soil-moisture conditions, fairly dense forests are formed. 

 Very similar conditions appear to obtain in the less arid middle West, 

 as well as in other portions of the United States. In Kansas and 

 Nebraska, for example, the depth to water where mesophytic forests 

 are to be found is less than 40 feet. This is along the flood-plains of 

 streams. On the adjacent higher lands the water-table lies at from 

 60 to several hundred feet, and although the soils may be favorable, 

 forests are wanting. In other regions, as the Coastal Plain of Texas, 

 the higher lands are occupied by xerophytic species of trees and there 

 may not be an intimate relation between the roots of such species and 

 the water-table. 



Root Variation in Desert Plants, by W. A. Cannon. 



Field studies and experimental work on the variation of the roots 

 of perennials have been carried on for several years with the following 

 as the leading results : 



Root-systems of desert perennials may be roughly classed as being 

 either generalized or specialized. The former type, of which the 

 roots of Franseria dumosa are given as an example, is capable of 

 greater variation than those of the latter. When grown in shallow 

 soils the roots of Franseria are necessarily shallowly placed, and, in 

 this regard resemble those of the cacti. When grown in deep soil, on 

 the other hand, the roots of this species penetrate deeply and do not 

 radiate widely, and thus resemble the deeply penetrating roots of 

 Zizyphus. 



The cacti and a few other plants have highly specialized roots 

 which lie near the surface of the ground. Such root-systems are dif- 

 ferentiated into anchoring roots and widely reaching absorbing roots. 

 When the soil is coarse, as in sand, the absorbing roots either fail 

 to develop or they develop only to a small extent, and the entire sys- 

 tem is composed of the anchoring roots. A similar result is derived 

 from an especially favorable water-supply. Thus variation in the 

 cactus type, a highly specialized root-system, is toward less differen- 

 tiation, that is, toward the generalized root-type, as in Franseria. ^ 



The garden cultures were confined to several species of native 

 cacti which were given a good supply of water for 24 months and were 

 grown in deep soil, a part of which was sand and a part garden soil. 



