THE BOOT-SYSTEMS OF DESERT PLANTS 



93 



determined in part by the character of the soil, in part by the penetra- 

 tion of the rains and in part by the character of the root itself. (3) 

 And roots that not only reach widely, but also penetrate fairly deeply. 



The superficial root-system (Type 1) is characteristic of many 

 plants, particularly of the cacti. In some instances all of the roots 

 except the anchoring roots, which, however, may not penetrate more than 

 50 cm., may not be more deeply placed than from 2 cm. to 5 cm. so 

 that with a cane one can easily remove the root and then with little 

 exertion can strip it from the soil to the base of the stem. Perhaps the 

 root-system of Opuntia arbuscula (?) is the most superficial of any 

 thus far described. In this species the ideal superficial root-system 

 just alluded to finds complete 

 expression. But the giant cactus 

 also, although it is now reck- 

 oned among the trees, has a root- 

 system which is essentially super- 

 ficial. The accompanying figure 

 gives a good idea of the position 

 occupied in the ground by the 

 root-system of a small giant 

 cactus. The plant referred to 

 was 1.2 meters high. The sup- 

 porting system consisted of a 

 stout root crown from which 

 proceeded a few relatively slender 

 branches, and the main absorp- 

 tion system consisted of long, 

 slender branches and superficial 

 roots which extended as far as 

 three meters from the base of 

 the plant. It may be said, in 

 passing, however, that as the 

 giant cactus becomes large, the anchoring system, sufficient in its 

 younger stages, is no longer strong enough, and the bases of the super- 

 ficial laterals increase greatly in thickness and form props by which the 

 upright position of the cactus is maintained. 



There are several plants which illustrate the pronounced forms of 

 the tap root, among which, in southwestern Algeria, may be cited the 

 Tamarix, and certain other small shrubs, and in our own southwest such 

 a form as palo christi, or Christ's thorn. Zizyphus also, which occurs 

 both in southern Algeria and in the southwestern part of the United 

 States, has a pronounced tap root. I will refer especially to the root- 

 system of palo christi (Kocrberlinia spinosa). The Koerberlinia 

 spinosa is a close-growing, spinous shrub without leaves at any stage, 



Giant cactus (Carnegiea gigantea), A, 

 Vertical Extension of Giant Cactus. 

 growing in association with a creosote 

 bush (Covillea tridentata) , B. The 

 anchoring roots of the cactus and the 

 superficially placed absorbing roots are 

 shown in position. It will be seen that 

 the roots of the creosote bush, which 

 are of the generalized type, occupy a 

 lower position in the soil than those of 

 the cactus. Between the surface of the 

 soil and the dotted horizontal line is the 

 adobe soil, here about 30 cm. in thick- 

 ness. Below the dotted line is the 

 hardpan, caliche, which is impervious to 

 water and is not penetrated by the 

 roots. 



