47 RELATION TO ENVIRONMENT. 



fusion.* It is sometimes applied to an association of individuals 

 of a species, or of several species occupying a rather definite area 

 of ground where the soil conditions are not greatly different 

 (individual formation); by others it is applied to the plants of 

 a definite physiographic area, as a swamp, moor, strand, or 

 beach, bank, rock hill, clay hill, ravine, bluff, etc. (principal for- 

 mation) ; and in a broad sense it is applied to the plants of climatic 

 regions, of those in bodies of water, etc. (general formations). 

 Space here is too limited to discuss all these kinds of formations, 

 but the nature of the general formations will be pointed out. 

 The general formations may be grouped into four divisions: 



i st. Climatic formations. 



2d. Edaphic formations. 



3d. Aquatic formations. 



4th. Culture formations. 



912. Climatic formations. Climatic influences extend over 

 wide regions, so that climate controls the general type of vegeta- 

 tion of a region. In the sense of control there are two climatic 

 factors, temperature and moisture, especially soil moisture. 

 Temperature exerts a controlling influence over the vegetation 

 type only where the total heat during the period of growth and 

 reproduction is very low. This occurs in polar lands and at 

 high elevations where the climate is alpine. In the temperate 

 and tropical regions of the globe moisture, not heat, controls 

 the general vegetation type. These vegetation types in general 

 are coincident with rainfall distribution, and Schimper recognizes 

 here three types, which with the arctic-alpine type would make 

 four climatic formations as follows: 



i st. 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 



* See the author's "College Text-book of Botany." Chapter XLIX. 



