Page:Popular Science Monthly Volume 81.djvu/99

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93
THE ROOT-SYSTEMS OF DESERT PLANTS

determined in part by the character of the soil, in part by the penetration of the rains and in part by the character of the root itself. (3) And roots that not only reach widely, but also penetrate fairly deeply.

The superficial root-system (Type 1) is characteristic of many plants, particularly of the cacti. In some instances all of the roots except the anchoring roots, which, however, may not penetrate more than 50 cm., may not be more deeply placed than from 2 cm. to 5 cm. so that with a cane one can easily remove the root and then with little exertion can strip it from the soil to the base of the stem. Perhaps the root-system of Opuntia arbuscula (?) is the most superficial of any thus far described. In this species the ideal superficial root-system just alluded to finds complete expression. But the giant cactus PSM V81 D099 Carnegia gigantea.pngGiant cactus (Carnegiea gigantea), A, Vertical Extension of Giant Cactus. growing in association with a creosote bush (Covillea tridentata), B. The anchoring roots of the cactus and the superficially placed absorbing roots are shown in position. It will be seen that the roots of the creosote bush, which are of the generalized type, occupy a lower position in the soil than those of the cactus. Between the surface of the soil and the dotted horizontal line is the adobe soil, here about 30 cm. in thickness. Below the dotted line is the hardpan, caliche, which is impervious to water and is not penetrated by the roots. also, although it is now reckoned among the trees, has a rootsystem which is essentially superficial. The accompanying figure gives a good idea of the position occupied in the ground by the root-system of a small giant cactus. The plant referred to was 1.2 meters high. The supporting system consisted of a stout root crown from which proceeded a few relatively slender branches, and the main absorption system consisted of long, slender branches and superficial roots which extended as far as three meters from the base of the plant. It may be said, in passing, however, that as the giant cactus becomes large, the anchoring system, sufficient in its younger stages, is no longer strong enough, and the bases of the superficial laterals increase greatly in thickness and form props by which the upright position of the cactus is maintained.

There are several plants which illustrate the pronounced forms of the tap root, among which, in southwestern Algeria, may be cited the Tamarix, and certain other small shrubs, and in our own southwest such a form as palo christi, or Christ's thorn. Zizyphus also, which occurs both in southern Algeria and in the southwestern part of the United States, has a pronounced tap root. I will refer especially to the rootsystem of palo christi (Koerberlinia spinosa). The Koerberlinia spinosa is a close-growing, spinous shrub without leaves at any stage,