CHAPTER XXXIX. 

 PLANT SOCIETIES. 



623. Plant societies are associations of the plants of an area, 

 over which the conditions are similar. 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 inconspicuous 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 smaller 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 

 support 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., disintegrate 

 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. 

 Mycorhizas (see paragraph 205) or other forms of mutualistic 

 symbiosis occur, which make atmospheric nitrogen available for 



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