Vegetation of America
No part of the world can be truly understood without a knowledge of its
garment of vegetation, for this determines not only the nature of the
animal inhabitants but also the occupations of the majority of human
beings. Although the soil has much to do with the character of vegetation,
climate has infinitely more. It is temperature which causes the moss and
lichens of the barren tundras in the far north to be replaced by orchids,
twining vines, and mahogany trees near the equator. It is rainfall which
determines that vigorous forests shall grow in the Appalachians in
latitudes where grasslands prevail in the plains and deserts in the
western cordillera.
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 been jogging across a gravelly plain studded at
intervals of a few yards with little bushes a foot high. The scenery is so
monotonous and the noon sunshine so warm that he almost falls asleep. When
he wakes from his daydream, so weird are his surroundings that he thinks
he must be in one of the places to which Sindbad was carried by the roc.
The trail has entered an open forest of Joshua's, as the big tree yuccas
are called in Arizona. Their shaggy trunks and uncouth branches are
rendered doubly unkempt by sword like, ashy-yellow dead leaves that double
back on the trunk but refuse to fall to the ground. At a height of from
twelve to twenty feet each arm of the many-branched candelabrum ends in a
stiff rosette of gray-green spiky leaves as tough as hemp. Equally bizarre
and much more imposing is a desert "stand" of giant suhuaros, great fluted
tree-cacti thirty feet or more high. In spite of their size the suhuaros
are desert types as truly as is sagebrush.
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 the ascent of man. Only in dry regions could primitive man compete with nature in fostering the right kind of vegetation. In such regions arose the nations which first practiced agriculture. There man became comparatively civilized while his contemporaries were still nomadic hunters in the grasslands and the forests.
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