with all sorts of water-loving plants. The deserts also contain a
few valleys where the larger streams keep the ground moist at all
seasons. In such places the variety of trees is as great as in
many forests. Moreover almost all deserts have short periods of
abundant moisture.

At such times the seeds of all sorts of little annual plants,
including grasses, daisies, lupines, and a host of others, sprout
quickly, and give rise to a carpet of vegetation as varied and
beautiful as that of the prairie. Thus the desert has not only
its own peculiar bushes and succulents but many of the products
of vegetation in swamps, grasslands, and forests. Though much of
the ground is bare in the desert, the plants are actually crowded
together as closely as possible. The showers of such regions are
usually so brief that they merely wet the surface. At a depth of
a foot or more the soil of many deserts never becomes moist from
year's end to year's end. It is useless for plants to send their
roots deep down under such circumstances, for they might not
reach water for a hundred feet. Their only recourse is to spread
horizontally. The farther they spread, the more water they can
absorb after the scanty showers. Hence the plants of the desert
throttle one another by extending their roots horizontally, just
as those of the forest kill one another by springing rapidly
upward and shutting out the light.

Vegetation, whether in forests, grasslands, or deserts, is the
primary source of human sustenance. Without it man would perish
miserably; and where it is deficient, he cannot rise to great
heights in the scale of civilization. Yet strangely enough the
scantiness of the vegetation of the deserts was a great help in