PLANT COMPETITION 163 



cept for the early stages in succession, crowding is a regular feature 

 of the plant matrix, and both individual and species regularly bear 

 the impress of competition in some measure. The process begins with 

 increasing aggregation so that shoots or roots occupy the same space 

 to a certain degree and thus make joint demands in excess of the 

 available or immediate supply. However, this effect is really exerted 

 through reaction, each leafy shoot reducing the light intensity and 

 thus affecting its neighbor, while each root system reacts similarly 

 upon the water content and solutes of the soil. No direct coaction is 

 involved except for those rare instances where growth leads to the 

 compression or heaving of tuberous roots, chiefly under cultivation. A 

 similar passivity characterizes the competition among flowers for in- 

 sect visitors, relative success or failure depending upon the form, size, 

 and color of the competing flowers. 



The experimental study of plant competition as a process was dis- 

 cussed by Clements (1905) and later w^as developed on a comprehen- 

 sive scale in the prairie climax with Weaver (1924) and with Weaver 

 and Hansen (1928). This concerned itself with transplant cultures, 

 sod transplants, and denuded quadrats in subclimax and true prairie, 

 and utilized a number of dominants, subdominants, and ruderals as 

 paired competitors. These represented a variety of life forms and a 

 large number of species, namely, tall, mid, and short grasses, annual 

 and perennial forbs, shrubs, and trees. Similar studies were carried 

 out in the ecotone between woodland and prairie in order to disclose 

 the essential relations between forest, scrub, and prairie, and the 

 nature of competition between individuals of the same species was 

 analyzed in several field crops. The course and outcome of competi- 

 tion were traced in terms of measured reactions, of functions, and of 

 changes in form and structure to afford a detailed and coherent pic- 

 ture of the entire process. 



The Factors in Competition. AYhile such indirect factors as 

 humidity, pressure, and wind may have some effect upon the process, 

 plants can actually compete only for energy or materials, namely, for 

 light, water, nutrients, oxygen, or carbon dioxide. However, compe- 

 tition for the last two is more or less exceptional, oxygen being at a 

 deficit chiefly in saturated soil and carbon dioxide in pond and lake, 

 the small amount in tlie air being kept fairly uniform through air 

 currents. Of the three major factors, water is regularly first in im- 

 portance in natural communities, light second, and nutrients last, 

 though nutrients may stand first for intensive field crops. However, 

 in temperate humid regions especially, water content may become of 

 relatively little importance, while soil air or nutrients may become 



