64 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
port, which was also true for the relatively young Cereus, and it is prob- 
able that this system in the two former plants always subserves this end. 
The relations of the root-systems of the three perennials are shown 
in the vertical and the horizontal charts. The absorbing roots of Larrea 
and of Parkinsonia were not completely exposed, although they were 
dissected sufficiently to show that the roots of neither occupied the same 
soil horizon as those of the cactus. Indeed, physical contact between 
the three plants was demonstrated in one instance only. In this case 
one of the longer roots of the cactus sent out several smaller roots which 
completely encircled the root crown of Larrea (plate 24). The superfi- 
cial roots of Cereus did not extend in the direction of Parkinsonia, but 
the tips of one of the longest of these were traced to near the base of a 
neighboring Parkinsonia which was on the circumference of the area 
investigated. The course of the roots of this plant was not followed. 
Although it thus appears that the roots of Cereus are so far removed 
from those of the other two plants that the most intense competition 
for water, such as would occur if the roots lay in the same level, probably 
does not take place, this does not mean that the plant is free of compe- 
tition. The annuals, which are to be found in this area at different seasons 
of the year, do not, so far as observed, send their roots more deeply than 
the malpais, and, therefore, although their roots include a greater verti- 
cal range than those of the cactus, they compete actively with its roots 
in the uppermost portion, at least, of the soil. 
The superficial nature of the absorbing root-system of the Cereus brings 
the plant into relations with the temperature of the ground and the rain- 
fall which most of the other perennials do not enjoy. ‘The influence 
of the temperature of the ground is not so obvious as that of the rainfall, 
but nevertheless it may be found to be of considerable moment as a factor 
controlling or limiting the distribution of the Cereus, in the absorption 
of water, and probably in other ways. Reference to the records of the 
temperature of the ground at a depth of 15 cm., within which the entire 
absorbing system of the cactus is placed, which have been made at the . 
Desert Laboratory since 1905, indicate that frequently the summer tem- 
perature of the soil at that depth may reach and exceed 37° C. and that 
the winter temperature may fall to 10° C. or below. ‘Temperatures 
nearer the surface, where the main absorption root-system is placed, are 
not given, but must be higher in summer and lower in winter than the 
extremes given for the greater depth. 
By these conditions of soil temperature, which increase the xerophilous 
conditions, Cereus is subject to an environment quite distinct from that 
affecting either Parkinsonia or Larrea, and which is directly traceable 
to the superficial nature of its root-system. This circumstance, when 
related to the penetration of the rains, is seen to be of even greater impor- 
tance. All heavy falls of rain, as those 1 inch or over, penetrate the soil 
