286 THE FORMATION 



factors upon the adjacent plants. In the exact sense, two plants do not com- 

 pete 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 foliage of one begins to overshade the leaves of the other, the reaction 

 of the former modifies unfavorably the factors controlling the latter, and 

 competition is at once initiated. The same relation exists throughout the 

 process; the stronger, taller, the 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 merely reduces the 

 amount of light and heat available for the plant near it or beneath it, but 

 it also renders necessary the absorption of more water and other nutritive 

 material, and correspondingly decreases the amount available. The inevit- 

 able result is that the successful individual prospers more and more, while 

 the less successful one loses ground in the same degree. As a consequence, 

 the latter disappears entirely, or it is handicapped to such an extent that it 

 fails to produce seeds, or these are reduced in number or vitality. 



Competition in vegetation furnishes few instances as simple as the above, 

 but this will serve to make clear the simplest case of ordinary competition, 

 i. e., that in which the individuals belong to a single species. The various 

 individuals of one species which grow together in a patch show relatively 

 slight diflFerences, in height, width, leaf expanse, or root surface. Still, some 

 will have the largest surfaces for the impact of water, heat, and light, while 

 others will have the smallest ; the majority, perhaps, will occupy different 

 places between the extremes. The former will receive more than their share 

 of one or more factors. The reaction thus produced will operate upon the 

 plants subject to it inversely as the amount of surface impinged upon. The 

 usual expression of such competition is seen in the great variation in height, 

 branching, etc., of the different individuals, and in the inability of many to 

 produce flowers. This is particularly true of annuals, and of perennials of 

 the same generation. 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 

 disappear, leaving a free space beneath and about the stronger parents. 

 This illustrates the primary law of competition, viz., that this is closest 

 when the individuals are most similar. Similar individuals make nearly the 

 same demands upon the habitat, and adjust themselves least readily to their 

 mutual reactions. The more unlike plants are, the greater the difference in 

 their needs, and some are able to adjust themselves to the reactions of others 

 with little or no disadvantage. 



