340 



W. A. CANNON 



deep soil, on the other hand, the roots of this species penetrate 

 deeply, and do not radiate widely, and thus resemble the deeply 

 peneti'ating 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 

 differentiated into anchoring roots and widely reaching absorb- 

 ing 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 system is composed of the anchoring roots. 

 A similar result follows an especially favorable water supply. 

 Thus variation in the cactus type, a highly specialized root-sys- 

 tem, is toward less differentiation, that is, toward the generalized 

 root type as in Franseria. 



Variation Under Cultural Conditions. The garden cul- 

 tures 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 portion of which was sand and a portion adobe garden 

 soil. 



Two cylindropuntias, 0. arbuscula and 0. neoarhuscula, of 

 nearly similar subaerial habit, were found to possess, the former 

 fibrous roots, and the lattter fleshy roots, under natural con- 

 ditions. The two species were treated in a similar manner and 

 both had unaccustomed depth of soil and supply of water. The 

 development of both species, however, was not changed as re- 

 gards the qualities named. 



Opuntia spinosior, 0. vivipara, and 0. discata were grown under 

 relatively favorable conditions as regards soil depth and water 

 supply. Certain specimens, also, were placed where their roots 

 projected in sand. The most striking results were a nearly, or 

 total suppression of the absorbing system of the roots in certain 

 cases, by which the root-system was brought to closely resemble 

 the generalized type. An abundant supply of water operates, 

 thus, to lessen the differentiation of the roots, and, in addition, 

 to bring about a somewhat deeper placing of the root-system. 



Variation Experimentally Induced. For the purpose of 

 bringing about the deepest possible root penetration of normally 

 shallowly rooted species, mainly the cacti, several plants were 



