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 tlte 
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 prespers 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 differences, 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. 
