often asked why, in a state of nature, trees are so
scarce on the prairies--in Iowa, for instance--although they thrive
when planted. In answer we are often told that up to the middle of the
nineteenth century such vast herds of buffaloes roamed the prairies that
seedling trees could never get a chance to grow. It is also said that
prairie fires sweeping across the plains destroyed the little trees
whenever they sprouted. Doubtless the buffaloes and the fires helped
to prevent forest growth, but another factor appears to be still more
important. All the States between the Mississippi River and the Rocky
Mountains receive much more rain in summer than in winter. But as the
soil is comparatively dry in the spring when the trees begin their
growth, they are handicapped. They could grow if nothing else interfered
with them, just as peas will grow in a garden if the weeds are kept out.
If peas, however, are left uncared for, the weeds gain the upper hand
and there are no peas the second year. If the weeds are left to contend
with grass, the grass in the end prevails. In the eastern forest region,
if the grass be left to itself, small trees soon spring up in its midst.
In half a century a field of grass goes back to forest because trees
are especially favored by the climate. In the same way in the prairies,
grass is especially favored, for it is not weakened by the spring
drought, and it grows abundantly until it forms the wonderful stretches
of waving green where the buffalo once grew fat. Moreover the fine
glacial soil of the prairies is so clayey and compact that the roots of
trees cannot easily penetrate it. Since grasses send their roots only
into the more friable upper layers of soil, they possess another great
advantage over the trees.

Far to the south of the prairies lie the grass-lands of tropical
America, of which the Banos of the Orinoco furnish a good example.
Almost everywhere their plumed grasses have been left to grow
undisturbed by the plough, and even grazing animals are scarce. These
extremely flat plains are flooded for months in the rainy season from
May to October and are parched in the dry season that follows. As
trees cannot endure such extremes, grasses are the prevailing growth.
Elsewhere the nature of the soil causes many other grassy tracts to
be scattered among the tropical jungle and forest. Trees are at a
disadvantage both in porous, sandy soils, where the water drains away
too rapidly, and in c