3] PHYSIOLOGICAL REACTIONS 8l 



suitable conditions are found, and, within such regions, naturally to 

 those habitats themselves. Consequently plants in nature are limited 

 not only to areas of particular climate but more precisely to special 

 habitats within these areas, the final limitation being ecological. In 

 such cases as oases in a desert or islands in an ocean, this limitation 

 may be extreme. 



The subject of modification by, or adaptation to, various conditions 

 is dealt with in the next section. Here we should mention the 

 manner in which, quite apart from any special antagonism, sheer 

 physical competition among plants for the requisites of life may 

 limit the habitat and actual range of a particular species or even 

 strain. Especially may root-competition for water and aerial com- 

 petition for light prove veritable struggles for existence in which 

 the weaker individuals succumb. Generally speaking, the closer any 

 two types are in their ecological requirements, the keener will be 

 the competition between them : in such even contests the slightest 

 advantage to one competitor can swing the balance in its favour. 



As most plants living on land need soil in which to root and some 

 well-lit space in which to grow, it is particularly in ' open ' areas 

 not yet covered with higher vegetation that competition is least and 

 plants can enter and establish themselves successfully. Most such 

 areas tend to be colonized by successive waves of plants that usually 

 start with primitive or other lowly types but in favourable regions 

 normally lead up to forest. This progressive colonization is called 

 ' succession ', and is described in Chapter XI. The farther it 

 proceeds, the less space there is left for new colonists and the more 

 tendency there is for former colonists to be ousted by coarser 

 competitors. Meanwhile animals, including Man, are continually 

 opening up new habitats and abandoning old ones — often after 

 destroying the natural vegetation, and rarely without disturbing it. 

 For these and other reasons the geographical areas of plants and 

 plant communities are rarely if ever static. 



Structural ' Adaptations ' of Vegetative Parts 



Numerous features help plants to oflFset the eflFects of unfavourable 

 conditions and consequently widen their potential ranges. Having 

 noted already such functional modifications as acclimatization of 

 various sorts, and physiological ' adaptations ' such as the ability of 

 many plants to evade unfavourable periods of cold or drought, we 

 shall deal here with changes o^ form that appear to be developed in 



