470 GENERAL MORPHOLOGY OF PLANTS 



food, or shorten the path from humus to available food, or the 

 humus plants feed on the humus directly. Nor should we leave 

 out of account the myriads of nitrite and nitrate bacteria (see para- 

 graph 202) which make certain substances in the soil available 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 collection or gathering of multitudinous 

 forms, where processes, influences, evolutions, degenerations, and 

 regenerations are at work. 



FOREST SOCIETIES. 



639. Different kinds of forests. We know that the mem- 

 bers of a plant community vary. Not only is there variation in 

 different years or periods, but also in different regions. Regions 

 which are so widely separated as to show great climatic differ- 

 ences show great differences in the character of plant societies. 

 The same is true of the forest. Each different climatic belt or 

 region has its characteristic forest. For example, the northern- 

 most forests are chiefly firs and balsams with here and there 

 colonies of birches and aspens. Next to these come extensive 

 forests of the white pine in North America, and also hemlock. 

 These forests of firs and balsams and pines extend southward on 

 the higher mountains because it is an extension of the same cli- 

 matic belt. Next to this great belt of conifers is a great belt of 

 the hardwoods, including the oaks, hickory, beech, maples, etc. 

 The forests of the Rocky Mountains are different from those of 

 the Alleghanies, because of the great barrier of the plains be- 

 tween them which has prevented the migration of species. 

 Tropical forests are different from those of the temperate re- 

 gions. The character of these forests depends largely on climatic 

 factors. The character of the forest varies, however, even in 

 * See Darwin, Vegetable Mould and Earth-worms. 



