l8] plant adjustments and applications 565 



Vegetational Adaptation 



Just as different kinds of plants are adapted in form or function 

 for life under particular conditions, and this fact largely limits their 

 distribution on earth, so the communities which plants make up 

 collectively, and which as a whole we term vegetation, are limited 

 in area by local conditions. Thus the general principle of adaptation 

 of plants to particular habitats applies also to vegetation ; and even 

 as evolution has tended in a very general way to be towards better 

 and better adaptation of individuals, so it appears to have been with 

 vegetational change. Most obviously, the dominant species on 

 which so much depends, commonly have their areas prescribed by 

 climatic conditions and, more locally within climatically suitable 

 areas, by edaphic or other immediate considerations. And so it 

 largely is with the rest of a community, though local conditions are 

 often controlled to a considerable degree by the dominants them- 

 selves, or by other plants of similar life-form. Nor is the total 

 effect of the community necessarily by any means the same as the 

 sum of the effects of its components considered individually. 



Perhaps the most noteworthy and general tendency to change in 

 natural plant communities is through the process of succession, 

 which was considered in Chapter XI. The progression from one 

 community to another of more efficient energy-utilization which is 

 succession, and which normally involves domination by higher and 

 higher life-forms as the stages succeed one another in an area, is 

 essentially a matter of the changing environmental demands and 

 adaptational attainments of successive colonists. Thus the largely 

 different plants involved in different stages have usually such different 

 habitat requirements that the species of one stage are commonly 

 ousted by those of the next — because they are not suitably adapted 

 to its attendant conditions. These conditions, admittedly, are 

 largely introduced or controlled by the plants themselves ; but the 

 principle nevertheless holds. The immediate plant geographical 

 implication is that successional changes introduce different conditions 

 to which usually very different plants are adapted, and whose dis- 

 tributions are thereby altered, at least potentially. At the sam.e 

 time succession tends to render areas unsuitable for many previous 

 colonists and their ecological analogues, correspondingly limiting 

 the ranges of such organisms. These and allied changes go on con- 

 tinuously in the world today, as Man or other agencies disturb 

 vegetation and initiate or alter successions in various ways. We 



