56 INVASION 



xerophytes do not adapt themselves to hydrophytic habitats, 

 nor hydrophytes to xerophytic conditions. Many meso- 

 phytes, however, possess to a certain degree the ability to 

 adjust themselves to somewhat xerophytic or hydrophytic 

 situations, while woodland plants often invade either forest 

 or meadow. This capability for adjustment, i.e., plasticity, 

 is greatest in intermediate species, those that grow in 

 habitats not characterised by great excess or deficiency of 

 some factor, and it is least in forms highly specialised in 

 respect to water-content, shade, etc. It may then be estab- 

 lished as a fundamental rule that ecesis is determined very 

 largely by the essential physical similarity of the old and the 

 new habitat, except in the case of plastic forms, which admit of 

 a wider range of accomodation. The plasticity of a plant is 

 not necessarily indicated by structural modification, though 

 such adjustment is usually typical of plastic species, but it 

 may sometimes arise from a functional adaptation, which for 

 some reason does not produce concomitant structural 

 changes. The former explains such various habitat forms 

 of the same species as are found in Galium boreale, Gentiaria 

 acuta, etc., and the latter the morphological constancy of 

 plants like Ghamaenerium, which grow in very diverse 

 habitats. 



The vegetation form of the invading species is often of 

 the greatest importance in determining whether it will 

 become established. The vegetation form represents those 

 modifications, which, produced in the original home by com- 

 petition,!, e., the struggle for existence, are primarily of value 

 in securing and maintaining a foot hold. These comprise 

 all structures by means of which the plant occupies a defi- 

 nite space in the air, through which the necessary light and 

 heat reach it, and in the soil, from which it draws its food 

 supply. These structures are all organs of duration or of 

 perennation, such as root, rootstalk, bulb, tuber, woody 

 stem, etc., which find their greatest development among 

 trees and shrubs, and their least among annual herbs. But 

 while the invaders are aided in securing possession by the 

 proper vegetation form, the occupation of the plant already 



