COMPETITION AND ECESIS 257 



competition, i.e., the latter increases as the distance diminishes 

 and the reverse. Their arrangement, i.e., whether in families 

 or communities, and whether the individuals are scattered or 

 grouped by species, exerts a marked influence by determining 

 that the contest shall be between like forms or unlike forms. 

 Position is controlled primarily by seed production and dissem- 

 ination, though it is influenced in a large degree by the location 

 of the first points occupied by an invader. The individuals of 

 species which produce many seeds and are relatively immobile usually 

 occur in dense stands. In these the competition is intense for 

 the two reasons of similarity of form and density of arrangement. 

 As a result the plants fall below the normal in height and width. 

 When the seed production is small or the mobility great, the 

 individuals are scattered among those of other species, and the 

 closeness of the competition depends largely upon the similarity 

 existing between them. 



272. Vegetation pressure. Masses of vegetation are often 

 said to force the weaker plants or species toward the edge, thus 

 producing an outward or forward pressure. Plants are likewise 

 said to have been driven into the water or into unfavorable habitats 

 by the pressure of stronger ones. The movement concerned is 

 merely migration, which may or may not be followed by ecesis. 

 It is determined by the nature of seed or fruit, and has no con- 

 nection with stronger or weaker species, or the presence of a vital 

 pressure. The direction taken by the migrants is largely inde- 

 terminate. Migration is outward, or away from the mass, because 

 it is radiate. The chances of ecesis are greatest at the edge, where 

 the similarity between the plant forms is less, and the competition 

 correspondingly diminished. Hence the actual movement or 

 invasion is outward. 



In the same way, the question of water plants is mere!}' one 

 of migration and ecesis. The plants that grow at the edge of a 

 pond or stream are not forced into the water by stronger neighbors. 

 They can become water plants only in one of two ways, both of 

 which are equally possible for strong and weak species. Their 

 seeds may be carried into water, either at one flight, or by agraflual 

 movement year by year, and those that adjust themselves become 

 water plants. On the other hand, a gradual increase in the water 

 of a habitat to the maximum may result in the extinction of some 

 forms, and the adjustment of others. 



