ix, c, 6 Gates: Swamp Vegetation in Hot Springs Areas 505 
THE SCIRPUS GROSSUS ASSOCIATION 
An association, dominated by Scirpus grossus L. f., a tall, stout, 
rank, 3-angled sedge, common along the shore of Lake Bay, is 
found in the hot springs region in but one limited area. This 
is just in front of the Barringtonia woods where the Casilihan 
River enters the swamp area. The density of the Scirpus is not 
sufficient to exclude secondary species, among 'which Polygonum 
tomentosum Willd. was especially abundant. The Scirpus was 
readily succeeded by Phragmites, which formed a narrow fringe 
immediately in front of the forest. 
THE PHRAGMITES ASSOCIATION 
A swamp association, dominated by a tall reed grass, Phrag- 
mites vulgaris Trim, covers more area than any other association 
in the swamp. It thoroughly dominates the central part of the 
Tarlac area, bordering the stream where Typha does not. It 
extends from the immediate vicinity of the hot springs nearly 
to Lake Bay, as well as a considerable distance back from the 
stream on both sides. When in bloom, the tall flowering stems 
of Phragmites are easily the most conspicuous feature of the area. 
Normally, Phragmites grows in water from a few centimeters 
to a meter in depth. Although it can grow rather close to the 
hot springs it is more easily killed by hot water than several 
other plants. Thus, considerable Phragmites in the central part 
of the Tarlac swamp was killed by the high hot water in Sep- 
tember, 1914, with the result that during October, in place of an 
extensive area of Phragmites in flower, there was merely a 
fringe at the edge of the swamp. As soon as the water cooled, 
a reinvasion of Phragmites set in. Those rhizomes which had 
not been killed sent up new shoots. On the surface of the water, 
forming a part of the abundant debris, were many short pieces 
of Phragmites stems which were sprouting as they floated. As 
the water receded they came nearer to the ground, with cor- 
respondingly greater chances of establishment. This of course 
shortened the invasion time. The setback has given the Ses- 
bomia a chance materially to extend its range, and this extension 
has rapidly taken place. 
The Phragmites association is very simple in structure, rarely 
containing more than the dominant species, closely in control 
of the environment. The tension lines on both sides are sharp. 
In the Tarlac area, vines, such as Merremia gemella Hallier f., 
Ipomoea sp., and Cissus trifolia K. Sch. may sometimes occur 
in profusion near the margin with the Sesbania association. 
