CHAPTER I 

 SOCIAL LIFE AMONG PLANTS 



The kinds of communal relationships among plants are manifold, 

 but not all have sociaUstic value. They are fundamentally different 

 from the relationships among animals as described by Deegener 

 (1918) (following Espinas, 1875). A division into "accidental," 

 without advantage to the individual organisms, and "essential," for 

 the benefit of all the individuals or of some of them (Deegener), 

 cannot be considered in the case of plants. The principles of useful- 

 ness, of division of labor, of conscious support, of marshaling all 

 resources for the accomplishment of a common purpose do not exist 

 in the plant world. The struggle for existence rules here undisturbed. 

 It regulates directly or indirectly all the unconscious expressions of the 

 social life of plants. Herein lies the deep and fundamental difference 

 between the vital relations of plant and those of animal communities. 



Inanimate nature furnishes to most plant communities the necessi- 

 ties of life, whereas most animal communities are conditioned by the 

 vegetation and therefore are dependent upon it. 



According to their nature we distinguish two principal types of 

 social relation of plants: dependent unions and commensal unions. 



Dependent Unions. — As dependent unions we designate all commu- 

 nal relations of plants in which the members are in any way dependent 

 upon one another; parasites, epiphytes, humus plants, and plants 

 requiring mechanical support or protection must be regarded as 

 dependent. 



The most intimate form of communal life of plants is parasitism, 

 a nutritive symbiosis in the sense of McDougall (1918). In one-sided 

 parasitism the parasite is dependent upon the host, while the latter 

 suffers only harm. The grim "destroyers in the plant world" from the 

 families Loranthaceae, Rafflesiaceae, Orobanchaceae, etc., are well- 

 known examples of one-sided plant parasitism. And there are all 

 transitions from complete independence to obhgate parasitism (Fig. 1). 



In mutual parasitism each of the united organisms furnishes food 

 or food material for the other. As examples of mutual parasitism 

 McDougall cites ectotrophic mycorhiza, the nodule bacteria of 

 Leguminosae, and the union of algae and fungi in lichens. 



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