232 



PLANT PHYSIOLOGY AND ECOLOGY 



example, may differ from each other considerably or entirely in 

 the species which compose them, but they are essentially alike 

 in character and structure and hence belong to the same type. 



The names of the particular formations of each type are ob- 

 tained by adding the name of one or more facies to the general 



Fig. 91. The aspen forest, a mesophytic formation. Pines appear on the 



drier ridges. 



name of the type. Thus, an oak-hickory forest is essentially 

 different from a balsam-spruce forest, though both belong to the 

 group of forest formations. The buffalo-grass prairie is similarly 

 distinct from the grama-bluestem prairie, etc. The same formation 

 may occur in two or more separate localities, and in this case 

 geographical terms are used to distinguish the different examples. 

 246. Types of formations. The following list shows the 

 arrangement of the more common habitats. 

 I. Hydrophytic formations 



(1) pond (5) ditch 



(2) marsh, bog (6) meadow thicket 



(3) stream (7) bank 



(4) spring (8) sandbar 



