COOPERATION 151 



mcnt, protection, spore production, and dissemination arc all more or 

 less specialized functions of the community. 



With respect to attached plants in general, cooperation is chiefly 

 concerned with reaction, by means of which the community modifies 

 the habitat in some degree to its advantage. This exists in small 

 measure at least with some lichens and most mosses wuth respect to 

 water relations, but is much more important in ferns and flowering 

 plants, which may modify practically any one of the factors of the 

 habitat. Such effects have been considered in some detail in the 

 chapter on reactions, and hence it is necessary here merely to empha- 

 size the cooperative nature of the process. Cooperation also plays a 

 role in dominance and hence in the layering of communities, though 

 the original selection is made by competition. It likewise operates in 

 mass migration and invasion, and its effect is to be seen in both climax 

 and serai stages. To a more limited degree, it is involved in local 

 migration wherever tlie parent plant takes some concern for the fate 

 of its offspring by such devices as catapult fruits or stolons and 

 runners. 



In symbiotic relations between plants of two species, cooperation 

 is present in varying degrees, but it is rare that two respective 

 families are concerned. In some instances, a single individual of each 

 is involved, as is probably the case in many mycorhizas of trees and 

 the fungi of orchids; in others, the microscopic organism is present in 

 vast numbers in the tissues of a single symbiont, as with Nostoc and 

 cycad, or clover and the nodule bacterium. This may prove to be 

 the rule with lichens, though the fungus element is often derived from 

 more than one germule. There are a few striking instances of sym- 

 biosis between a plant and an animal community, but these can best 

 be treated in connection with similar relations between animals. 



Cooperation in Animals below the Social Level. In considering 

 the beneficial effects of aggregation, Allee has discussed in several 

 chapters the results of the past decade that bear upon the stimulation 

 of growth and reproduction by crowding, and the effect of crowding 

 upon survival, as well as upon sex determination and morphology 

 (1931, a: 147-334). As a consequence, he reaches the conclusion that 

 interdependence or automatic cooperation is so widespread among 

 animals as to rank as a basic property of animal protoplasm, and 

 probably of all organisms, an opinion supported by what has been said 

 above as to plants. Such a type of cooperation has been more or less 

 definitely demonstrated for tropistic aggregation in more than a hun- 

 dred different groups, from bacteria and Infusoria to fishes and rep- 

 tiles, the great majority of which are far below the level where dis- 



