62 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
Where the larger of the superficial roots leave the main root they are 
relatively heavy, but taper rapidly until a small size is reached, which 
is maintained fairly well for long distances. A few measurements will 
indicate the character of these roots. One of the larger ones was 1.6 cm. 
in diameter at the base; another was 7 mm. in diameter 30 cm. from 
its base; another root measured 1 cm. in diameter 45 cm. from the main 
root, and it extended 2.5 m. farther from the place where the measure- 
ment was made. Roots not above 50 cm. in length were about 3 mm. in 
diameter at the base, but those longer, as, for example, 5:5 m., are some- 
what heavier, but still not very large. This one was 2 cm. in diameter 
at 30 cm. from the main root. 
In the area included within reach of the superficial roots were two 
perennials, namely, Parkinsoma microphylla and Larrea tridentata, and 
one of the longer cactus roots reached to the base of another Parkinsonia, 
not included here. During the rainy seasons the area is well covered 
with annuals, but as there were none present when the cactus was studied, 
their number and kinds are not known. 
The specimen of Parkinsonia was a small plant, but probably a rather 
old one, about 50 cm. high and 1.3 m. distant from the cactus. It gave 
indication of a severe struggle for existence, presumably owing to an 
insufficient water-supply. The following points were noted in the char- 
acter and distribution of its roots. 
A tap-root measuring 3.5 cm. in diameter at the level of the ground 
went straight down through the malpais and terminated abruptly on 
reaching the caliche. Several slender roots left the main one at distances 
varying from 5 to 10 cm. from the surface and ran in a fairly horizontal 
direction for approximately 10 cm. At a depth of 8 cm. a lateral root 
8 mm. in diameter was put out, which ran 20 cm. nearly horizontally, 
and then dipped rapidly, so that at a point 50 cm. from its base it was 
45 cm. beneath the surface of the ground. Thus it ran through the caliche 
and was lost in crevices of the underlying rock. At the tip of the tap-root 
there were several roots about 10 cm. in length. 
As opposed to the Parkinsonia, the specimen of Larrea was very vigor- 
ous and apparently perfectly normal. ‘The roots of this plant were traced 
as far as practicable, but owing to the age of the plant (it was evidently 
much older than the cactus) and to the consequent large extent of its 
roots, and further, as will appear directly, to the character of the root- 
system, it was not practicable to make so exact a study as was done 
with Cereus; however, the relations of the root-systems were followed out 
as carefully and as completely as possible. | 
The Larrea was about 50 cm. high and was much branched. It was 
50 cm. away from the cactus and 1.3 m. from the Parkinsonia. In the 
Larrea a relatively large mass composed of about three branched roots 
penetrated through the malpais to the caliche and there stopped (fig. 2). 
