PLANT SOCIF.TfRS. 477 



(see Chapter IX) which make certain substances in the soil avail- 

 able to the higher members of the society. Most plant societies 

 are also benefited or profoundly influenced in other ways by 

 animals, as the flower-visiting insects, birds which feed on 

 injurious insects, the worms which mellow up the soil and cover 

 dead organic matter so that it may more thoroughly decay. In 

 short, every plant society is a great cosmos like the universe 

 itself of which it is a part, where multitudinous forms, processes, 

 influences, evolutions, degenerations, and regenerations are at 

 work. 



918. Forest Societies.* Each different climatic belt or region 

 has its characteristic forest. For example, the forests of the 

 Hudsonian zone in North America are different from those of 

 the Canadian zone, and these in turn different from those in 

 the transition zone (mainly in northern United States). The 

 forests of the Rocky mountains and of the Pacific coast differ 

 from those of the Alleghanian, Carolinian (mainly middle United 

 States) or Austroriparian (southern United States) areas. 

 Finally, tropical forests are strikingly different from those of 

 other regions. Similar variations occur in the forests of other 

 regions of the globe. The character of these forests depends 

 largely on climatic factors. The character of the forest varies, 

 however, even in the same climatic area, dependent on soil 

 conditions, or success in seeding and ground-gaining of the 

 different species in competition, etc. 



919. General structure of the forest. Structurally the forest 

 possesses three subdivisions: the floor, the canopy, and the 

 interior. The floor is the surface soil, which holds the rootage 

 of the trees, with its covering of leaf-mold and carpet of leaves, 

 mosses, or other low, more or less compact vegetation. The 

 canopy is formed by the spreading foliage of the tree crowns, 

 which, in a forest of an even and regular stand, meet and form 

 a continuous mass of foliage through which some light filters 

 down into the interior. Where the stand is irregular, i.e., the 



* For a full discussion of forest societies see Chapter L in the author's 

 "College Text-book of Botany." 



