434 BOTANICAL. GAZETTE | DECEMBER 
Considering now the relation of this to the other associations, 
it is very important to note that it is the natural association for 
its situation, and has not to be brought into its typical condition 
nor kept there by the cultivation so necessary on the upland hay 
meadows. When a piece of marsh is diked and drained, there 
follows, as we shall later note, a perfectly natural succession of 
plants, from the Staticetum to the Phleumetum, without any care 
or seeding, and the Phleumetum therefore represents the best 
adapted type of vegetation in this region to the conditions of 
the reclaimed marsh. And it is important to notice that the 
timothy, and no doubt also the couch, are not native, but intro- 
duced forms. There were in this forested region no mesophytic 
native plants so well adapted to this new field as the open- 
ground hay grasses from Europe, a point in perfect harmony 
with the general principles controlling the relations of introduced 
to native plants as set forth by Gray in his essay ‘‘ On the perti- 
nacity and predominance of weeds.” As long as the drains are 
kept up, and until by long years of cropping the soil begins to 
weaken, this association holds its own against all comers of 
every sort. There is no tendency here for forest to come in, as 
on the upland, for reasons already explained (p. 291), nor can 
the ordinary weeds gain a footing until the timothy weakens 
through exhaustion of the soil or other cause, in which case, 
some scanty approach to forest may occur (p. 293). On these 
marshes, therefore, these European hay-grasses find an even 
more congenial and competition-free field than upon the upland 
meadows. When, however, the drainage becomes imperfect, the 
brown top rises to prominence, and that in turn gives way to 
the broadleaf as the water becomes more abundant. In these 
phenomena of replacement we see illustrated the first principle 
of competition, that a form can hold its own only in the vicinity 
of its optimum, and beyond that it goes down readily before 
another form whose optimum is being approached. 
5. THE ROADSIDE WEED ASSOCIATION, OR CNICETUM. 
In addition to the “weeds” associated naturally with the 
Phleumetum, there occurs a distinct association of upland weeds 
in certain places on the marshes. As this association is by no 
cick Pe 
