504 27ie Philippine Journal of Science i9u 
the water level is much lowered Pistia may rest on the ground 
and under these conditions its roots extend into the soil. Any 
other plants that become established there can easily shade Pistia 
out of existence. 
This association is not present in the Los Banos area. 
THE CASTALIA-NYMPHAEA ASSOCIATION 
The Castalia-Nymphaea, an association of aquatic plants rooted 
in mud with leaves floating on the surface of the water, is now 
sparingly represented near the upper part of the swamp, rather 
near some of the hot springs. Castalia pubescens Willd. was 
the only species present. The appearance of the association was 
entirely typical. With it were no regular secondary species. 
In the Tarlac area it was often the association bordering the 
Casilihan River, where it was succeeded either by Typhia or by 
Panicum amplexicaule Rudge and Panicum repens L. 
This association does not occur in the Los Banos area. 
THE TYPHA ASSOCIATION 
This marsh association, dominated by Typhia angustifolia java- 
nica Schindl., occurs in a few of the ditches in the Los Banos 
area and borders pools and streams running from the hot springs 
in both areas, but does not occur in hot water. Aside from 
Polygonum tomentosum Willd. and convolvulaceous vines which 
may be locally present, there are seldom any secondary species. 
From the front of this association a mat of Bacopa may some- 
times extend out over pools of open water. In the ditches, 
Typha is as a rule not replaced. The exception was in a single 
pool in the Los Banos area where eight plants of Sesbania can- 
nabina Pers. became established in 1913. Around pools and 
along streams Typhia readily gives place to Phragmites and less 
readily to Acrostichum. 
In the Tarlac area the high water of 1914 submerged much 
of the Typhia. As September is the normal resting season of 
Typhia, the dead leaves could not be taken to indicate that they 
were killed by the high water, especially as seeds were found 
germinating without having fallen from the spike. 
In the level central part of the Tarlac area, Typha was suc- 
ceeded by wide areas of Panicum repens L. This grass is period- 
ically harvested for forage, as well as pastured by carabaos, both 
of which tend to retard natural succession. Elsewhere Typhia 
is succeeded by the Phragmites association, which is very abund- 
ant and wide spread in this area. 
