PLANT FORMATIONS. 517 



The entire region is a potential woodland formation, i.e., if left 

 to the operation of natural forces, in time a forest would develop 

 over the entire area unless interrupted by great climatic or 

 physiographic changes. It is the region of normal to excessive 

 rainfall, and in North America covers practically the Hud- 

 sonian and Canadian zones and the Alleghanian, Carolinian, 

 Austroriparian, and Pacific Coast transition areas, the Austral 

 region and wooded slopes of the Rocky Mountains, as well as 

 the humid tropical area. 



988. (3.) The Grassland Formation. This is also a close 

 formation, since the climatic factor, humidity, is still favorable 

 to the abundant growth of a vegetation type. Here the vegetation 

 consists of tufted and perennial grasses as the dominant and 

 potential element, although other herbage may exist here and 

 there as an accidental or subordinante element. This is the 

 region of less rainfall and high winds, a transition region between 

 the woods type and the desert. In the evolution of the woods 

 and prairie types they have come into sharp competition for the 

 occupation of the debatable border territory with wide invasions 

 of each type into the interior territory of the other, like the armies 

 of two nations struggling for the occupancy of territory where 

 the border line is as yet ill-defined. Climatic conditions finally 

 turn the balance in favor of the woods or prairie, according 

 as the degree of humidity of the region aids one above the other, 

 and the border line becomes established. This is not sharply 

 defined unless there is some natural physical barrier, as a chair? 

 of mountains, while on the plain or level the transition line 

 shows a commingling of the two types. 



989. (4.) The Desert Formation. Here the moisture of the 

 soil as well as the humidity of the atmosphere is very low and 

 the conditions of life become so austere that plants cease their 

 conflict with neighbors for territory because the conflict with 

 climate is so great as to keep down the numbers and leave un- 

 occupied spaces. Here the two general vegetation types (wood- 

 land and grassland) mingle because there is plenty of room 

 for all that, can survive. Millions of seeds are scattered which 



