4/6 RELATION TO ENVIRONMENT. 



" principal formation" but is a more popular expression, and 

 besides includes all the plants growing on the area, while in the 

 use of the term "principal formation" we have reference mainly 

 to the dominant plants and the most conspicuous subordinate 

 species. 



917. Complex character of plant societies. In their broadest 

 analysis all plant societies are complex. Every plant society 

 has one or several dominant species, the individuals of which, 

 because of their number and size, give it its peculiar character. 

 The society may be so nearly pure that it appears to consist of 

 the individuals of a single species. But even in those cases 

 there are small and conspicuous plants of other species which 

 occupy spaces between the dominant ones. Usually there are 

 several or more kinds in the same society. The larger individuals 

 come into competition for first place in regard to ground and 

 light, the smaUer ones come into competition for the intervening 

 spaces for shade, and so on down in the scale of size and shade 

 tolerance. Then climbing plants (lianas) and epiphytes (lichens, 

 algae, mosses, ferns, tree orchids, etc.) gain access to light and sup- 

 port by growing on other larger and stouter members of the society. 



Parasites (dodder, mistletoes, rusts, smuts, mildews, bacteria, 

 etc.) are present, either actually or potentially, in all societies, 

 and in their methods of obtaining food sap the life and health 

 of their hosts. Then come the scavenger members, whose 

 work it is to clean house, as it were, the great army of saprophytic 

 fungi (molds, mushrooms, etc.), and bacteria ready to lay hold 

 on dead and dying leaves, branches, trunks, roots, etc., disin- 

 tegrate them, and reduce them to humus, where other fungi 

 change them into a form in which the larger members of the 

 plant society can utilize them as plant food and thus continue 

 the cycle of matter through life, death, decay, and into life again. 

 Mycorhiza (see Chapter IX) or other forms of mutualistic 

 symbiosis occur which make atmospheric 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 



