256 PLANT PHYSIOLOGY AND ECOLOGY 



species which are so unUke the occupants that they enter at a 

 clear advantage or disadvantage usually establish themselves 

 readily. This principle lies at the base of the changes in suc- 

 cession which give a peculiar stamp to each stage or formation. 

 A reaction sufficient to bring about the disappearance of one 

 stage can be produced only by the entrance of invaders so differ- 

 ent in form or nature that they change the impress of the for- 

 mation materially or entirely. A formation becomes stable 

 when the entrance of such invaders is no longer possible. For 

 example, while many vegetation forms can still enter a forest, 

 none of these are able to place the trees at a disadvantage. As 

 a consequence the final forest stage, though it may change in 

 composition, can not be displaced by another. 



270. Influence of vegetation form and habitat form. The 

 course and the result of competition are primarily dependent 

 upon the vegetation form and the habitat form of the competing 

 species. Species of the same vegetation form, i.e., two or more 

 kinds of shrubs, compete closely with each other, and the result 

 is a reduction in the number or size of the individuals, or the 

 entire disappearance of one or more species. On the other hand, 

 dissimilarity in vegetation form tends to diminish competition 

 and to maintain the advantage of the superior form. Species 

 of trees, as a rule, compete sharply with each other when found 

 together. The same is true of other vegetation forms, shrubs, 

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

 rosettes to the shrubs of a formation is one of subordination and 

 not of competition. The matter of height and width often plays 

 an important part in deciding this question. The amount and 

 disposition of leaf surface are decisive factors in competition 

 between species of the same vegetation form, in so far as this is 

 governed by light. In plants in which the leaves are usually 

 erect, e.g., grasses and sedges, competition between aerial parts 

 is slight, and the result is chiefly determined by the roots. The 

 effect of a difference in habitat form is unusually marked, since 

 the invader must then adjust itself to the more or less unfavor- 

 able conditions of a new habitat, in addition to meeting the test 

 of competition. 



271. The effect of position. The position of the competing 

 individuals is of the greatest importance, as already indicated. 

 The distance between the plants directly affects the degree of 



