298 BOTANICAL GAZETTE [OCTOBER 
system, thrives in a siliceous and somewhat salt soil, needs a 
constant rather than a great water supply, can spread a meso- 
phytic foliage to the summer sun and retreat to a winter xero- 
phytic condition, can endure exposure to unshaded sun and 
strong winds and not spread too great a surface to their trans- 
piring influence, and can use the wind in dissemination and cross- 
pollination. No one form of vegetation can be found to fit best 
all these conditions, but any ecologist can tell at once what type 
comes the nearest to fitting them in the aggregate; it is the 
grasses and grass-like plants. This is why the vegetation of the 
marshes is so overwhelmingly of that kind. 
The plants of the marshes. 
We turn now to consider in some detail the vegetation of the 
marshland, and naturally ask first what kinds of plants live there. 
No attempt has yet been made to prepare a flora of the marshes, 
and such scanty lists of species as exist have already been men- 
tioned (page 162). From the point of view of ecological plant 
geography, however, their floristic completeness is of slight 
importance, since the character of the vegetation is determined 
by only a few prominent forms, and all of those rarer and less 
conspicuous species, naturally of such interest and importance to 
the floristic student, might be wanting without affecting the 
characteristics of the vegetation as a whole. Further, the spe- 
cies, as such, are not of special concern ecologically. What is 
here important is this, the species as an aggregation of adapta- 
tions, that is, as a vegetation- (or life-, or biologic) form, for 
these vegetation-forms are the unit of the ecologist as the spe- 
cies are of the systematist. In a general way species and vegeta- 
tion-forms may not be coincident. Thus the same vegetation- 
form may be developed under similar ecological conditions 
from very different and widely separate species, as witness 
Agave-Aloe, Calluna-Erica, and Cereus-Euphorbia (some), but 
this does not hold true in minutiae, and such forms as those 
above mentioned, while alike in most characters, differ in many 
particulars. This is of course because plants are not indefi- 
nitely plastic to environmental influences, but are limited much 
