GROWTH-FORMS. 71 



Competition-forms. — The amount of a particular factor available for any 

 species or individual is either determined by the habitat alone or by com- 

 petition. In the great majority of cases, the major limits are fixed by the 

 habitat, and within these competition determines the amounts available for 

 each plant. Indeed, this is probably true of all communities except those 

 initial ones in which the individuals are widely scattered. In nearly all cases, 

 then, a growth-form is due partly to the nature of the habitat and partly to 

 the modification of this by competition. The part played by each can be 

 determined only by actual experiment or by the comparison of individuals 

 growing in the same habitat but in areas with and without competition. 

 Fortunately, such areas are of sufficient frequence in nature to reveal the 

 normal growth-form of the habitat as well as the growth-form due to com- 

 petition. A study of the chaparral and strand communities of southern 

 California (Clements and Clements, 1916) disclosed an unusually large number 

 of such competition-forms, especially among the annuals, as would be expected. 

 While competition-forms are probably just as frequent among perennials, 

 they are often much less striking. 



As competition may occur in all degrees in accordance with the number 

 and density of individuals, so there may be a complete series of forms from 

 the normal to the extreme in which the plant never develops beyond the 

 seedling stage before it dies. Under somewhat less severe competition, plants 

 develop stems and leaves but fail to form flowers and fruit. In the next 

 degree, reproduction occurs, but the flowers are single or few, while beyond 

 this are more and more perfectly developed forms until the optimum for the 

 habitat is reached. Each form is an index to some degree of competition, but 

 its exact indicator value is more difficult to determine. This is due largely to 

 the fact that competition has as yet received but little attention, especially 

 on the experimental side. The view advanced by Clements (1904 : 166; 

 1905 : 310; 1907 : 251; 1916 : 72) that competition is purely physical seems 

 to be confirmed by recent experiments. While it is perhaps unnecessary to 

 rigidly exclude metaphor in connection with competition, it should be recog- 

 nized that the experimental results so far obtained show that plants do not 

 compete for "room." Competition has to do only with the direct factors of 

 the habitat. Water and light are the factors universally concerned, though 

 soil-air, nutrients, and heat must also be taken into account in particular 

 habitats. In addition, there is often more or less decisive competition between 

 the flowers of a community for pollination agents. Furthermore, the course of 

 competition may be determined by a deleterious substance, especially a solute, 

 which handicaps one species more than another. Such a handicapping influ- 

 ence is even more frequently represented by biotic agents, parasitic plants, 

 rodents, grazing animals, etc. 



The competition-forms commonly met with are due to competition for 

 water or light, or for both together. There has been no experimental study 

 of competition for soil-air or for nutrients, and it is impossible to assert at 

 present that plants do compete for heat. Studies of germination under differ- 

 ent densities of seeding suggest such competition for seedlings at least. No 

 adequate study of competition-forms has been made, and hence it is impossible 

 to relate them to definite quantities of water or light. In fact, it seems 

 increasingly probable that the forms resulting from intense competition are 



