182 PLANT COMMUNITIES 



is no greater degree of difference between a tree and a plant com- 

 munity, such as a forest, than there is between a tree and a one-celled 

 alga, and we shall find that the forest community has a life cycle and 

 can do practically everything that the tree can do. It has some- 

 times been said that we should not consider plant communities as 

 individual organisms because it is often nearly impossible to tell 

 where one community ends and another begins. It is quite true that 

 certain kinds of plant communities overlap at their margins in such 

 a way that their delimitation is not easy but if this fact were to be 

 accepted as a valid argument against considering plant communities 

 as individuals the argument would have equal force against con- 

 sidering certain kinds of plants as individuals. For example a single 

 plant of wild ginger {Asarum canadense) may reproduce by means of 

 rhizomes until it forms a circular patch several feet in diameter. 

 No one would consider this patch a single plant and yet it is practic- 

 ally impossible to tell how many plants there are or where one plant 

 ends and another begins. The same thing is true of many other 

 plants that spread by rhizomes or runners. But this difficulty in 

 delimiting individuals need cause us no worry. In the majority of 

 plant communities, as in the majority of plants, the difficulty is not 

 encountered. 



In every plant commimity there are certain species of plants 

 that are called dominant plants because they largely control the 

 environment and so determine what other species may grow in the 

 community. In a forest the dominant species are trees. They have 

 very important symbiotic relations with all other members of the 

 community through their direct or indirect control of light, space 

 relations, water supply, and to a certain extent available food ma- 

 terials. From this point of view it is of interest to compare a plant 

 community with a human community. In a human community 

 man is the dominant species. As the dominant species he controls 

 the environment to such an extent as to determine what other species 

 may live in the community. Some of the other species usually found 

 in a human community are the horse, dog, cat, mouse, fly, etc. Some 

 of these are not present because man wants them to be, but because 

 man is present and is controlling the environment in such a way as 

 to make it possible for the other species to live in the community. 

 These facts are just as true of the plant community. The presence 

 of some of the species is distinctly advantageous to the dominant 

 plants, while that of others is just as distinctly disadvantageous, as 



