COMPETITION 169 



the consequent numerical reduction or disappearance of one 

 or more species. Dissimilarity, on the other hand, tends 

 to eliminate competition, and to preserve the advantage of 

 the superior form. Species of trees compete sharply with 

 each other when found together : the same is true of shrubs, 

 of rosettes, etc. The relation of the shrubs to the trees, or 

 of the rosettes to the shrubs of a formation is one of sub- 

 ordination rather than of competition. The matter of 

 height and width often enters here also to such a degree 

 that the tallest herbs compete with the bushes and shrubs, 

 and rosettes with mats or grasses. The amount and dispo- 

 sition of the leaf surface are decisive factors in the compe- 

 tition between species of the same vegetation form, in so far 

 as this is governed by liaht. In those plants in which the 

 leaves are usually erect, notably the grasses and sedges, the 

 competition between the aerial parts is relatively slight, 

 and the result is determined by the reactions of the under- 

 ground stems and roots. 



The position of the competing individuals is of the 

 greatest importance. The distance between the plants 

 affects directly the degree of competition while their 

 arrangement, whether in groups according to species or 

 singly, exerts a marked influence by determining that the 

 contest shall be between like forms, or unlike forms. 

 Position is controlled primarily by the relation existing be- 

 tween seed-production and dissemination. It is of course 

 influenced in large measure by the initial position taken by 

 the invaders into a nudate area, but this is itself a result of 

 the same phenomena. The individuals of species with great 

 seed-production and little or no mobility usually occur in 

 dense stands. In these, the competition is fierce, for the 

 two reasons of similarity and density, and the result is that 

 the plants fall far below the normal in height and width. 

 This is an extreme example of the group arrangement. 

 When the seed-production is small, the mobility may be 

 great or little without seriously affectiner the result. The 

 individuals of a species of this kind will be scattered among 

 those of other species, and the closeness of competition will 



