The question is 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 clayey soil, where it is held so long that the ground is saturated for weeks or months at a time. South of the tropical portion of South America the vast pampas of Argentina closely resemble the North American prairies and the drier plains to the west of them. Grain in the east and cattle in the west are fast causing the disappearance of those great tussocks of tufted grasses eight or nine feet high which hold among grasses a position analogous to that of the Big Trees of California among trees of lower growth.