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 almost tniversally 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 different 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 cpen 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 hydrophvtes 
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 
