THE PLANT FORMATION 



231 



ship to each other beyond the superficial one of location. Forma- 

 tions, especially contiguous ones, sometimes show more or less 

 similarity in composition, i.e., they have a certain number of 

 species in common. With respect to the grouping of their indi- 

 viduals, they are distinguished as open when the plants and plant 

 groups are scattered, and closed when they are so crowded that 



!i?i^*^"'"'r'^-.t- 



vtt-S ^-^ 



Fig. 90. The gravel slide a xerophytic formation of the Rocky Mountains. 



invasion is very difficult. Finally, a formation is said to be 7nixed 

 when, owing to position or development, it is really a mixture of 

 two or more distinct ones. 



245. Classification by habitats. We have already found that 

 the most important differences of habitats so far as plants are 

 concerned are due to water and light. What is true of plants 

 must hold as well for the formations which they compose. In 

 consequence, three great groups of formations are recognized 

 corresponding respectively to hydrophytes, mesophytes, and xero- 

 phytes. Upon the basis of light, mesophytic formations are sub- 

 divided into sun and shade formations. Within each group par- 

 ticular formations are arranged according to the type of habitat, 

 i.e., pond, forest, meadow, dune, etc. Meadow formations, for 



