1922] McDOUGALL—SYMBIOSIS 209 
the woods except in the highest and lowest parts. At the same time 
Impatiens biflora is subdominant in the lower parts of the elm 
consociatién, while I. palida is only a little less prominent, and keeps 
largely to the somewhat higher parts of the same consociation. 
During the serotinal season several composites are conspicuous, 
but perhaps the most characteristic plant is Campanula americana, 
which occurs nearly everywhere in the woods, but usually not 
abundantly enough to become subdominant. Finally, during the 
autumnal season species of Aster and Eupatorium urticaefolium are 
the principal subdominants. 
SOCIAL DISJUNCTIVE SYMBIOSIS 
This is the type of symbiosis in which the organisms concerned 
are not in actual contact, at least not all of the time, and in which 
there is no direct food relation. It includes, therefore, all of the 
ordinary interrelations of dominant, subdominant, and secondary 
species in a plant community. These interrelations in deciduous 
forests have been studied and described by numerous authors. 
It will suffice here, therefore, merely to mention the salient features 
of the subject, and to point out their relative importance in the 
community under consideration. ‘The dominant plants of a com- 
munity, which in a forest are trees, are those which largely control 
the environment and so determine what other species may grow in 
the community. They have very important symbiotic relations, 
therefore, 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 materials. 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 
