88 BOTANICAL FEATURES OF NORTH AMERICAN DESERTS. 
A marked feature of the distribution of desert forms is the peculiarly 
isolated position of the separate individuals, especially in the perennials. 
Each shrub will occupy, to the exclusion of others of its kind, an area of 
many square feet or square yards. With this wide separation one is 
quite prepared to find a great horizontal distribution of the roots, and this 
does occur in many species, but is not to be taken for granted in any 
instance not confirmed by actualobservation. Long lateral rootsof Yucca 
have been recorded on 
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species, but the char- 
acter is wholly a spe- 
cific one. Dr. W. A. 
Cannon has found that 
distinct types of root- 
systems exist among 
species inhabiting the 
arid areas in which a 
marked increase in 
the proportion of soil 
moisture does not 
occur until a great 
depth is reached. 
The root-system of a 
specimen of Echinocactus 
wislizent which was 60 
cm. high and 35 cm. in 
diameter, growing about 
75 meters north of the 
laboratory, was carefully 
exposed and the course 
of its roots mapped (fig. 
3). The roots, as the 
figure indicates, were 
branched very freely. 
There were three main 










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(A= 
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the base of the plant not 
Fic. 2.—Root-system of Cereus giganteus. Scale: 1 far from 10 cm. from the 
unit = 4 inches, surface of the ground,and 
which so directed their 
growth, and that of the branches, that the area compassed by them was about 
equally apportioned and well covered. Asa rule, the roots were slender. At a 
distance of 15 cm. from the plant one of the largest of them was 7.6 mm. in diam- 
eter, and 1 meter from the plant it was 4.6 mm. in diameter. The roots ran 
about 6 cm. below the surface, in places which were free of stones, but when a 
stone was encountered the root dipped beneath it and availed itself of the better 
water-supply to be found there. The most deeply placed root, however, was 


