LOCAL DISTRIBUTION OF SPECIES. a9 
THE ROOT-SYSTEM OF CEREUS GIGANTEUS.! 
The character and extent of the root-systems of desert plants, as well 
as the réle which they play in the distribution of these plants, are in the 
main not known. What their relations may be to the physical environ- 
ment, to the kind of soil, to the air in the soil and its temperature, and 
what to the varying amounts of available water of the soil, as well as to 
the root-systems of adjacent plants, and the influence of these and other 
factors on the presence of plants in their peculiar habitats are among the 
most pressing problems of desert botany that await studious inquiry. 
Something of these relations and something of their significance so far 
as field observations can point them, in one representative of the most 
perfectly adapted of all desert plants, will be summarized in the following 
paragraphs. 
Cereus giganteus has a fairly highly specialized root-system, which 
throughout the life of the individual plant has a close relation to its leading 
needs, namely, of adequate support, of sufficient water, and of proper 
aeration. The portions of the roots which respond to these needs are 
not fixed, but suffer gradual change, so that what is characteristic of the 
youth of the plant no longer holds for the mature form. In the young 
plant, the main root-system is that associated with the tap-root, which 
functions both for anchorage and for absorbing water. But with the 
growth of the plant, laterals arise which come to extend far from the 
parent root and which take over the function of absorbing water and 
leave that of supporting the stem to-the tap-root. This is the condition 
in plants about 20 cm. long and may be found in plants as high as 1.2 m. 
But in the larger forms the exposure to the impact of winds, which at 
times are of great violence, often, or perhaps always, makes such anchor- 
age as that afforded by the tap-root insufficient, so that later the final 
adjustment of the roots to external agencies is effected. This is the 
enlargement of at least the bases of the lateral roots, so that efficient 
braces are provided by which strains and stresses set up in the high sub- 
aerial portion are rendered harmless. We therefore find in a cactus 
6.8 m. high, for example, the following condition of the roots, which 
is the final one: In place of a single branched tap-root there are numerous 
rather slender and straight roots which form a brush or tuft (plate 21). 
In the case in question these penetrated the ground 77 cm., or a distance 
which was less than one-ninth the height of the main stem of the plant, 
or, including the branches in the estimate, one-fourteenth the entire 
length of that portion which was exposed to the wind. ‘The large laterals 
are the bases of the superficial or absorbing system of the plant, here 
functioning as a means of support as well. 
ee 
1Prepared by request and contributed by Dr. W. A. Cannon, Member of Staff of 
Desert Laboratory. 

