LOCAL DISTRIBUTION OF SPECIES. 61 
distance of 25 cm. ‘The main root, therefore, runs through the malpais 
to the caliche beneath it, where it abruptly stops (fig. 2). 
The main root gives off 6 laterals near the surface of the ground, which 
radiate from the stem in such a fashion that the ground included in their 
reach is fairly equally divided between them (plate 24). They extend 
from 1.5 to 5 m. from the stem; they are tough and rope-like and they 
branch but little. Four of these roots were not observed to give off 
branches for 1.5 m. of their course, and one root that branched freely 
at the tip gave off only 4 laterals in the course of 3 m. of its length. The 
superficial roots end in a tuft of delicate rootlets, nearly all of which die 
with the advent of the drier seasons. This habit of forming roots of 
limited growth and of shedding them when useless has been observed in 
other cacti also, notably in Opuntia versicolor, and in other 
plants of the desert. These are not confined to the tips of 
the roots, but may appear apparently at any place through- 
out its course. It is an interesting adjustment to desert 
conditions, inasmuch as it permits the plant to rework 
the ground which is not continuously moist but only inter- 
mittently so. By this means the area covered by the roots 
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Vertical extension, semi-diagrammatic, of the root-system of an individual of Cerews giganteus 
which was 1.2 meters in height (solid lines), and that of a neighboring Larrea tridentata 
(broken lines). In the soil section the adobe is represented by the vertical broken lines; 
the hard pan, caliche, underlying it by the stippling, and the bed-rock by the slanting 
broken lines. One-twentieth natural size. 
is greatly restricted, but at the same time it is thoroughly exploited for 
its water. 
The superficial root-system penetrated the ground to a depth which 
was fairly uniform. For example, one root left the main root at the 
surface of the ground, and both it and its three main branches, with little 
deviation, ran 7 cm. from the surface. Another root ran from 3 to 10 
cm. from the surface of the ground, but gave off a branch which went 
straight down to a depth of 17 cm. Others of the superficial roots varied 
from 5 to 15 cm. in depth, and one dipped under a large bowlder, whose 
lower surface was 30 cm. deep, after which it ascended to about the depth 
it ran between the bowlder and the parent root. 
