ROOT VARIATION IN DESERT PLANTS 



331 



outward as well as downward, but, in addition, there were a 

 few short laterals that, given appropriate water relations might 

 develop into radiating laterals characteristic of the species (fig. 

 3, B). The root-system of the irrigated form, in other words, 

 was a generalized one, since differentiation into absorbing and 

 anchoring roots, which is so well marked a featue of the species 

 in nature, under the cultural conditions, was lost. 



Opuntia vivipara 



Opuntia vivipara is a cylindropuntia the distribution of which, 

 so far as known at present, is confined to the bottom of a small 

 wash not far from the domain of the Desert Laboratory. The 

 soil conditions, and the water relations of the habitat are unlike 

 those where cacti usually occur in that the soil is deep and the 

 water relations, relatively good. The root-system of the species 

 consists of an anchoring and of an absorbing portion, as is usual 

 with the cacti, but, unlike most other opuntias, part of the 

 roots are fleshy and part are fibrous. The fleshy roots are slender 

 fusiform and, like the fleshy roots of 0. neoarbuscula, may give 

 rise to shoots. 



'In the garden cultures the roots of 0. vivipara exhibited not 

 a little range in variation which was related to soil differences. 

 Some of the specimens were growing in the garden soil, and some 

 of them were in sand. Root development was unlike in the two 

 media. In the garden soil a brush of roots was organized and 

 a few laterals were formed. The anchoring roots penetrated 

 about 25 cm. while the laterals lay near the sm-face of the soil. 

 In the sand, on the other hand, only a brush of anchoring roots 

 was formed and these roots remained close to the surface of the 

 soil. WTiether growing in the garden soil, or in the sand a part 

 of the roots were fleshj'- and a part were fibrous, as in the proper 

 habitat. It also is noteworthy, whatever may be the significance, 

 that, as compared with the other species of Opuntia growing in 

 the garden, the number of roots of vivipara were especially abun- 

 dant. The root habit of a garden-grown specimen of 0. vivipara 

 is shown in figure 3, C. 



