LOCAL DISTRIBUTION OF SPECIES. 63 
From this central root-complex about 5 laterals arose at a depth averaging 
15 cm. from the surface; these were traced from 1.5 to 3 m. without 
arriving at the tips. The course of all of these roots was a gradual descent. 
They passed on a gentle slope through the malpais and entered and passed 
through the caliche on the same slope, after which they reached the under- 
lying rock and ran along its surface. Where left, the roots were 60 cm. 
more or less beneath the surface of the ground. Besides these roots the 
main root-mass gave off numerous roots from 10 to 25 cm. in length. 
In general character, the roots of Larrea were quite unlike those of 
the Cereus. They were of a dark color, brittle, and rather large. One 
root, which was traced for a distance of 3 m., was at a point 60 cm. from 
its base 1.5 cm. in diameter; another 90 cm. from its place of origin and 
45 cm. beneath the surface was 5 mm. in diameter; and another root 
at a point 2.8 m. away from its base was 1.2 cm. in cross-section. At the 
“base all of the longer roots were about 2.6 cm. in diameter. 
We may now summarize the leading characteristics of the root-systems 
and the roots of Cereus giganteus, Larrea, and Parkinsonia and note more 
specifically their mutual relations. 
Cereus.—The main root penetrates through the malpais to the caliche, 
where it stops. In the young plant it serves both as an anchor for the 
plant and as an organ for the absorption of water. As the plant becomes 
larger, laterals which in a plant 1.2 m. high may extend as far as 5 m. away 
from the main root are pushed out near the surface of the ground. These 
roots are confined to the uppermost layer of soil, the malpais, and for the 
most part run within 10 cm. of the surface; they constitute the leading 
absorbing system. ‘They are of a light color and are tough and rope-like. 
In yet larger plants the bases of these roots are enlarged, and they func- 
tion in a mechanical way as important supports of the plant. 
Parkinsonia.—The main root goes through the malpais and stops when 
it reaches the caliche. The laterals leave the main root at a level 
somewhat deeper than in Cereus, and after running a short distance 
on a rapid decline pass into and run through the caliche and finally reach 
and run along the surface of the underlying bed-rock. Only the shorter 
roots, which branch out from the main root, remain near the surface of 
the ground. 
Larrea.—The main root complex enters the malpais and goes straight 
down to the caliche, where, asin Parkinsonia and Cereus, it abruptly stops. 
The laterals run for a short distance in the malpais, at no place nearer 
the surface than 15 cm., and enter the caliche, which they pass through 
on a slope, and reach and run along the surface of the rock beneath. As 
a whole, the roots of Larrea were larger than those of Cereus, but this 
may have been due wholly to the difference in age of the two forms. In 
both Parkinsonia and Larrea the main root-system was the leading sup- 
