PLANT FORMATIONS. 471 



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 suf- 

 ficient amount of soil or water to support forest trees. But 

 here it is the soil conditions, not climatic conditions, which pre- 

 vent 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. There- 

 fore 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 

 nnd move over an area of high temperature, etc. 



2d. Grassland formation. Grasses form the dominant vege- 

 lation type where the annual rainfall is approximately 15 to 25 

 inches. In true grasslands the formation is a dose one since 



