1st. _The woodland formation._—This formation is characterized by trees and shrubs, and it is what is called a _close_ formation. By this it is meant that so far as the climate is concerned the conditions are favorable for the development of trees and shrubs in such abundance that they become the dominant vegetation type of the region and grow close together. Other plants, as herbs, grasses, etc., occur, but they grow as subordinate elements of the general vegetation type, and as undergrowth. The land portion of the globe, therefore, outside of arctic and alpine regions, where the annual precipitation is 40 to 60 or more inches, is the area for woodland formation. In some places, the eastern part of England, for example, the annual precipitation is 25 to 30 inches, but the cool temperature permits a forest growth. It is true there are places where forests do not grow,—where man cuts them down, for example. But if cultivated lands in this region were allowed to go to waste, they would in time grow up to forest again. So there are swamps where the soil is too wet for trees, or sandy or rocky areas where there is not a sufficient amount of soil or water to support forest trees. But here it is the soil conditions, not climatic conditions, which prevent the development of the forest. But we know that swamps are being filled in and the ground gradually becoming higher and drier, and that soil is slowly accumulating in rocky areas, so that in time if left to natural forces these places would become forested. So this area of heavy annual rainfall is a _potential_ forest area. These areas are determined by warm currents of moisture-laden air from the ocean moving over cooler land areas where the moisture is precipitated. In general these areas are along the coasts of great continents and on mountains. Therefore the interior of a continent is apt to be dry because most of the moisture has been precipitated before it reaches the interior. Deserts or steppes are therefore usually near the interior of continents. Some exceptions to this general rule are found: central South America, which is a region of exceptional rainfall because the moisture-laden winds here come from the warmest part of the ocean; the desert region west of the Andes mountains, where the winds are not favorable; southern California, where the winds come chiefly from a cooler portion of the Pacific ocean and move over an area of high temperature, etc. [Illustration: Fig. 486. Typical prairie scene, a few miles west of Lincoln, Nebraska. (Bot. Dept., Univ. Nebraska.)]