155] FLORA OF COLUMBIA AND VICINITY 1 3 



that of the slope which has been denuded of its soil. 



In that portion of a region that is of economic use, man is 

 the dominant factor in plant life, and ruderal and cultural types 

 take the place of the old indigenous societies. 

 7. THE PLANT-SOCIETY. 



A plant-society is a community of species living together 

 under like conditions, the members of which may be mutually 

 helpful — the larger forming shade for the smaller — or there may 

 be intense competition for control or even for existence. A 

 stable society is one in which the various species have found 

 their appropriate habitats and the competition is restricted to 

 narrow limits. An unstable society is one in which the various 

 members, either through some important change in the environ- 

 ment, or on account of the coming in of new floral elements, 

 are undergoing a process of breaking apart or of coming to- 

 gether. The felling and underbrushing of a forest break up a 

 sylvan association. In the new adjustment some plants tend to 

 become extinct, and plants, fitted to open conditions come in. 

 In some cases a plant-community consists wholly of one species 

 or of several closely related species. Thus whole flats are oc- 

 cupied by the giant ragweed (Ambrosia triMa), and the June- 

 grass {Poa pratensis) covers pastures many acres in extent. 

 The smartweed communities include usually six or more species 

 of Polygonum. 



If certain species are gregarious, others occur singly and at 

 distant intervals. Thus orchids are seldom found in any abund- 

 ance. As plants become rare, they verge upon extinction, for in 

 general an abundance of individuals of a species in a region 

 gives that species a momentum, a power of survival, that the 

 singly growing species do not have. Fertilization is more surely 

 effected, and reproduction more certainly insured. 



