xx, c, 6 Gates: Sivamp Vegetation in Hot Springs Areas 503 
thoroughly dominated by a sedge, Fimbristylis miliacea Vahl. 
The individual plants are about 70 cm high and usually form 
a rather dense turf. The waxy green color of the stems of the 
plants make this association stand out distinctly from the sur- 
rounding associations. The area thus occupied is generally burnt 
over during the dry season. On one side it is bordered by cogon, 
which, however, does not replace Fimbristylis as the ground is 
water-soaked during the greater part of the year ; this condition 
precludes cogon. The presence of Acrostichum and Acacia is 
indicative of succession, but very little headway is being made 
on account of the dense turf and recurring fires. There are no 
secondary species of importance. 
This association replaces the Fimbristylis spathacea association 
by crowding Bacopa and Lippia from between the clumps of 
Fimbristylis spathacea Roth and eliminates the latter species by 
shading. 
In the damp ground at the edge of the water behind Diplachne 
in the Tarlac area there was a rather poor representation of 
this association. No particular relation was noticed between 
this association and the Ipomoea reptans association, although 
runners of Ipomoea often passed through the Fimbristylis area 
toward the open water. Sesbania replaces either Fimbristylis 
or Ipomoea under favorable conditions. 
THE VALLISNERIA ASSOCIATION 
The Vallisneria association, an aquatic one in which the plants 
are rooted beneath the surface of the water and the entire plant 
remains submerged, is present in the Casilihan River in the Tarlac 
area, but does not occur in the vicinity of the hot springs. The 
association is represented by Vallisneria gigantea Graebn., Cera- 
tophyllum demersum L., and Potamogeton malainus Miq., none 
of which is abundant. This association also occurs in Lake 
Bay at the mouth of the river. Whenever any of the plants 
reach the surface of the water they catch such floating aquatics 
as Pistia, Lemna, and Jussieua repens L., as well as floating 
debris. 
This association is not present in the Los Banos area. 
THE LEMNA-PISTIA ASSOCIATION 
An association of floating aquatic plants, characterized by 
Lemna trisulca L., and Pistia stratiotes L., is found near the 
mouth of the Casilihan River, as well as in Lake Bay. When 
