234 



PLANT PHYSIOLOGY AND ECOLOGY 



in vegetation. The basis is entirely different from that of habitat 

 classification and in consequence the two supplement each other. 

 Both necessarily deal with the same formations, and are used to 

 give different points of view of the vegetation of a region. The 

 habitat classification is simpler in that it considers only those 

 formations actually on the ground, while development usually 

 has to take account of formations that have disappeared. 



248. Regional classification. The grouping together of forma- 

 tions that occur in the same region is warranted by the fact that 



# 



H.- 



<. 



M 





m^^ 



''^S 



Fig. 93. An open formation of pines. 



they have a larger or smaller number of species and genera in 

 common. This has arisen from the mutual invasion constantly 

 taking place between adjacent formations. An additional reason 

 for such a grouping is furnished by climatic factors, which are 

 essentially uniform in each region. Classification with respect to 

 general vegetation regions or topographic differences is frequently 

 used. Its value lies in furnishing a simple summary of vegetation 

 rather than in pointing out an essential relationship. In mountain 

 countries, the grouping is a topographic one, determined by 



