1x,¢,6 Gates: Swamp Vegetation in Hot Springs Areas 509 
Sesbania area. There are no secondary species with it except 
the ever-present vines, Merremia gemella Hallier f., Ipomoea 
sp., and Cissus trifolia K. Sch. The first of these is the most 
noteworthy, for under the hot water swamp conditions it pos- 
sesses a woody stem which may reach a diameter of 20 mm in 
place of its normal herbaceous stem. The stem is larger above 
the surface of the water than below it. When festoons of vines 
entirely cover Sesbania, they interfere with its development. 
The tight woody coils on the stem also interrupt the development 
of aerenchyma. 
Sesbania keeps its place in the area because of the fact that 
the shrubs which would naturally succeed it do not grow in 
standing water. Whenever conditions are at all favorable for 
parang shrubs they readily replace Sesbania. 
Around certain of the hot springs in the Tarlac area there 
is a zone of Panicum repens L. between the parang and Sesbania. 
This is due to the complex local conditions attendant upon the 
fluctuations of the hot water. 
Eight plants of Sesbania cannabina Pers., invading Typha in 
a ditch along the railroad, is the total extent of this association 
in the vicinity of the Los Bajios springs. The plants are poorly 
developed and badly overrun with vines. 
THE CYNODON ASSOCIATION 
A wet-ground and strand association, dominated by a grass, 
Cynodon dactylon Pers., occurs sparingly in the immediate 
vicinity of the Los Bafos hot springs in certain low areas 
which are wet to the surface even during the greater part of 
the dry season. 
There are no secondary species with it. It is invaded by both 
Fimbristylis spathacea Roth and Panicum repens L. When 
Fimbristylis miliacea Vahl, in very wet places, is too closely 
cropped by cattle or goats, Cynodon will sometimes appear and 
become established. 
THE PANICUM REPENS ASSOCIATION 
The Panicum repens association is present in both hot-spring 
areas. This wet-ground association of grasses is typical of cer- 
tain parts of the strand and also develops on flat wet areas. 
Along Lake Bay this association is present before Sesbania and 
gives place to it. Around certain of the hot springs this asso- 
ciation is present as a tension-line association between the hot 
springs and the parang shrubs on the dry land. There it is 
fairly well developed, containing also a number of secondary 
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