166 ALTERNATION 



reactions of the living plants. At the last point, it is in 

 direct connection with piant-competition. 



Much uncertainty, as well as diversity of opinion, seems 

 still to exist in regard to the precise nature of the compe- 

 tition between plants that occupy the same area. It has 

 long been admitted that the phrase, "struggle for ex- 

 istence", is true of this relation only in the most figurative 

 sense, but the feeling still prevails that, since plants live in 

 associations, there must be something mysterious and vital- 

 istic in their relation. No one has been able to discover 

 anything of this nature, but nevertheless the impresssion 

 remains. Such a direct relation exists only between para- 

 sites, epiphytes and lianes, and the plants which serve to 

 nourish or support them. In the case of plants growing on 

 the same sbratum, actual competition between plant and 

 plant does not occur. One individual can affect another 

 only in as much as it changes the physical factors that in- 

 fluence the latter. Competition is a question of the reaction 

 of a plant upon the physical factors which encompass it, and 

 of the effect of these modified factors upon the adjacent 

 plants. In the 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 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 unfavor- 

 able effoct 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 de- 



