Forests, grass-lands, deserts, represent the three chief types of
vegetation on the surface of the earth. Each is a response to
certain well-defined conditions of climate. Forests demand an
abundance of moisture throughout the entire season of growth.
Where this season lasts only three months the forest is very
different from where it lasts twelve. But no forest can be
vigorous if the ground habitually becomes dry for a considerable
period during which the weather is warm enough for growth. Desert
vegetation, on the other hand, which consists primarily of bushes
with small, drought-resistant leaves, needs only a few irregular
and infrequent showers in order to endure long periods of heat
and drought. Discontinuity of moisture is the cause of deserts,
just as continuity is the necessary condition of forest growth.
Grasses prevail where the climatic conditions are intermediate
between those of the forest and the desert. Their primary
requisite is a short period of fairly abundant moisture with
warmth enough to ripen their seeds. Unlike the trees of the
forests, they thrive even though the wet period be only a
fraction of the entire time that is warm enough for growth.
Unlike the bushes of the desert, they rarely thrive unless the
ground is well soaked for at least a few weeks. Most people think
of forests as offering far more variety than either deserts or
grass-lands. To them grass is just grass, while trees seem to
possess individuality. In reality, however, the short turfy grass
of the far north differs from the four-foot fronds of the bunchy
saccaton grass of Arizona, and from the far taller tufts of the
plumed pampas grass, much more than the pine tree differs from
the palm. Deserts vary even more than either forests or
grass-lands. The traveler in the Arizona desert, for example, has