ECOLOGY — INTERPRETATION OF ENVIRONMENT 323 



The competition which takes place has been well described 

 by Clements, who says: "Competition is a question of the 

 reaction of a plant upon the physical factors which encompass 

 it, and of the effect of those modified factors upon the adjacent 

 plants. In an exact sense, two plants do not compete with 

 each other as long as the water-content and nutrition, the heat 

 and light, are in excess of the needs of both. The moment, 

 however, that the roots of one enter the area from which the 

 other draws its water supply, or the fohage of one begins to 

 overshade the leaves of the other, the reaction of the former 

 modifies unfavorably the factors controlling the latter, and 

 the competition is at once initiated. The same relation exists 

 throughout the process; the stronger, taller, and more branched, 

 or the better rooted plant reacts upon the habitat, and the 

 latter immediately exerts an unfavorable effect upon the weaker, 

 shorter, less branched, or more poorly rooted plant. This 

 action of plant upon habitat, and of habitat upon plant, is 

 cumulative, however. An increase in the leaf surface of a 

 plant not only reduces the amount of light and heat available 

 for the plant near or beneath it, but it also renders necessary 

 the absorption of more water and other nutritive material 

 and correspondingly decreases the amount available. 



"In the competition between parents and offspring of the same 

 perennial species, the former usually have so much the advantage 

 that the younger plants are often unable to thrive, or even 

 germinate, and they disappear, leaving a free space beneath 

 and about the stronger parents. This illustrates the primary 

 law of competition, viz.: that this is closest where individuals 

 are most similar." 



The effect of distance, or the interval between individuals, 

 upon competition is fundamental. The competition increases 

 as the interval diminishes, and the reverse. Masses of vegeta- 

 tion are commonly thought to force the weaker species towards 

 the edge, thus initiating an outward or forward pressure, but 

 in all probability competition is purely physical in nature. 

 The movement outward that does occur is> due to simple migra- 

 tion, followed by the germination and establishment of invaders, 

 taking an indeterminate course in their operation of competition. 

 Two common plant associations occurring in the sand areas 



