two or three score miles from the coast until a point is reached where
mountains begin to obstruct the rain-bearing trade-winds. At once the
rain forest gives place to jungle; in a few miles jungle in its turn
is replaced by scrub; and shortly the scrub degenerates to mere desert
bush. Then in another fifty miles one rises to the main plateau passing
once more through scrub. This time the scrub gives place to grass-lands
diversified by deciduous trees and pines which give the country a
distinctly temperate aspect. On such plateaus the chief civilization of
the tropical Latin-American countries now centers. In the past, however,
the plateaus were far surpassed by the Maya lowlands of Yucatan and
Guatemala.

We are wont to think of deserts as places where the plants are of few
kinds and not much crowded. As a matter of fact, an ordinary desert
supports a much greater variety of plants than does either a forest or
a prairie. The reason is simple. Every desert contains wet spots near
springs or in swamps. Such places abound 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 i