3d. _Desert formations._—These occur where the annual rainfall is still lower, 10 to 4 inches or even less, 2 to 3 inches, while in one place in Chili it is as low as ½ inch. In the great Sahara desert it is about 8 inches, while in the Sonora Nevada desert in the southwestern United States it is 4 to 8 inches. Here the formation is an open one. In the forest and prairie formations the plants compete with each other for occupancy of the ground, since climatic conditions are favorable, so that the struggle against climate is not severe. But in the desert plants do not compete with each other; since the climate is so austere, the struggle is against the climate. Hence plants stand at some distance from each other because the roots need the moisture from the ground for some distance around them. There is not enough moisture for all the plants that begin, and those which get the start take the moisture away from the intervening ones, which then die. Since the struggle is against the adverse conditions of climate and not a competition between plants to occupy the ground, no one floristic type dominates as in the case of the grasses and forests of the grassland and woodland formations, but grassland and woodland types grow together. So we find grasses, trees, and shrubs growing without competition in the desert. The dominant vegetation type is xerophytic. [Illustration: Fig. 488. Northern limit of tree growth, Alaska. (Copyright, 1899, by E. H. Harriman.)] 4th. _Arctic-alpine formation._ This formation extends from the limit of tree growth to the region of perpetual ice and snow. The forest here comes in competition with climate, with the severe cold of the long winter night, so that tree growth is limited, and on the border line with the woodland formation the trees are stunted, bent to one side by the heavy snows, or the tops are killed by the cold wind. The arctic zone of plant growth is sometimes spoken of as the “cold waste,” since conditions here are somewhat similar to those in the desert, the extreme cold exercising a drying effect on vegetation, and the vegetation type then is largely xerophytic.