528 RELATION TO ENVIRONMENT. 



life, death, decay, and into life again. Mycorhiza (see Chapter IX) 

 or other forms of mutualistic symbiosis occur which make atmos- 

 pheric nitrogen available for 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 nitrate and 

 nitrite bacteria (see Chapter IX) 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 cosmos like the Universe 

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

 influences, evolutions, degenerations, and regenerations are at 

 work. 



