INVASION 223 



The influence of habitat upon germination is of primary importance, 

 though the manner in which its influence is exerted is by no means as . 

 evident as might be supposed. In the case of seeds sown in the planthouse, 

 it is ahnost universally the case that germination is less than in nature, 

 notwithstanding the fact that temperature and moisture appear to be 

 optimum. In nature, the seeds of the species may be carried into a number 

 of dift'erent formations, any one or all of which may present conditions un- 

 favorable to germination. With respect to probability of germination, 

 habitats are of two sorts : those which are denuded and those which bear 

 vegetation. It is impossible to lay down general' propositions with respect 

 to either group, since germination will vary with the character of the in- 

 vading species, the annual distribution of heat and moisture in the habitat, 

 etc. In a general way, however, it may be stated that the chances for 

 germination are greater in vegetation than in denuded areas, chiefly because 

 the latter are usually xerophytic. On the other hand, the lack of competi- 

 tion in the denuded area tends to make ultimate establishment much more 

 certain. Here, as elsewhere when exact statistical results are desired, the 

 use of the quadrat, and especially of the permanent quadrat, is necessary 

 to determine the comparative germination of the invading species in relation 

 . to denudation and vegetation. 



271. Adjustment to the habitat. The seedling once established by 

 germination, the probability of its growing and maturing will depend upon 

 its habitat form, plasticity, and vegetation form. Even though it may 

 germinate under opposite conditions, a typical hylophyte, such as Impatiens 

 for example, will not thrive in an open meadow, nor will characteristic 

 poophytes, such as most grasses, grow in deep shade. In the same way, 

 xerophytes do not adapt themselves to hydrophytic habitats, nor hydrophytes 

 to xerophytic conditions. Many mesophytes, however, possess to a certain 

 degree the ability to adjust themselves to somewhat xerophytic or hydro- 

 phytic situations, while, woodland plants often invade either forest or 

 meadow. This capability for adjustment, i. e., plasticity, is greatest in in- 

 termediate species, those that grow in habitats not characterized by great 

 excess or deficiency of some factor, and it is least in forms highly specialized 

 in respect to water-content, shade, etc. It may then be established as a 

 fundamental rule that ecesis is determined very largely by the essential physi- 

 cal 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 



