1x,c,6 Gates: Swamp Vegetation in Hot Springs Areas 507 
layer of rubbish collects. Often a layer nearly 20 cm thick is 
thus formed, which entirely excludes light from the ground. 
THE ACROSTICHUM ASSOCIATION 
A stage of vegetation, somewhat intermediate between herbs 
and shrubs, dominated by a fern, Acrostichum aureum L., is 
present in both areas. This fern is characteristic of salt mar- 
shes, but although this swamp area is a fresh-water habitat, the 
hot water issuing from the hot springs contains salts. The fern 
might be considered as a miniature tree fern, for the stem de- 
velops upright, although seldom reaching a height of more than 
30 cm. From the top of the thick stem, the leaves radiate 
obliquely upward in all directions to a height of from 1 to 2 m. 
When well developed the spreading leaves interlock, and from 
above the vegetation appears to be a solid mass of fern. A cross 
section, however, shows that the plants are well spaced, from 
0.5 to 15 m apart, the ground between being bare of plants 
and in the rainy season usually covered with water. 
This association invades all the genetically lower associations 
which are not too near the hot water. Of the associations 
which may succeed, the Acacia consocies of the parang associa- 
tion is the more usual. Individual trees of Acacia farnesiana 
Willd. occur here and there throughout much of the area domi- 
nated by the fern. The first response to the presence of Acacia 
is the larger size of the leaves of Acrostichum in a circle around 
the Acacia plant, even before the Acacia is as high as the fern. 
The effect extends beyond the area shaded when shading begins 
to take place. This better development is probably due to the 
local presence of more available nitrogen associated with the 
leguminous Acacia. 
When Acacia becomes sufficiently numerous to give the char- 
acter to the general impression as seen from the outside, the 
upper story of vegetation is formed by the spreading branches 
of Acacia, although, per unit area, individuals of Acrostichum 
are much more numerous. The Acrostichum grows very dense 
and the numerous long leaves extend up into the crown of the 
Acacia. A radically different idea of the area is thus given 
from the inside. Acrostichum appears almost entirely prevalent, 
there being but relatively few Acacia stems. The ground is 
bare. 
Acrostichum shows excellently the change in size of leaves 
in its réles of invader, dominant species, and relic. From in- 
vader to dominant species the change is gradual, the leaves under 
