508 T/ie Philippine Journal of Science am 
the latter condition being larger. The change from dominant 
species to a relic is very abrupt wherever the Acacia has a sharp 
boundary. Leaves of Acrostichum in Acacia may be twice as 
large as those in the Acrostichum association. In nonlegumin- 
ous associations in which Acrostichum occurs it may be much 
dwarfed and readily gives place under shading. 
In the Tarlac area this association is poorly represented. It 
occurs sparingly toward the upper and mountain side of the 
area. The ferns are scattered and do not dominate the situa- 
tion. Hot water readily kills this fern. By October, 1914, all 
the fern outposts had been killed, the only living ferns being 
those near the edge of the thickets where they were not reached 
by the hot water. Thus, Acrostichum is of minor importance 
in this area, hardly being able to hold its own as an association. 
THE SESBANIA ASSOCIATION 
The Sesbania association is common on the strand of Lake Bay, 
and is also plentifully represented in the Tarlac hot springs 
area. The rather open, sprawling, shrubby, very intolerant 
growth of this plant would appear to make it a poor plant to 
succeed in this area, where the vegetation in general is rather 
dense. On the strand, Sesbania cannabina Pers. grows under 
beach conditions. Its root system is submerged at least during 
the rainy season. In the hot springs area it regularly grows 
in water, or where the soil is very wet throughout the year. 
It will grow in water 2 m in depth. Whenever the stem is 
submerged for any length of time abundant aerenchyma is de- 
veloped. On the main stem the aerenchyma is seldom more 
than 3 mm in thickness, and by its development the epidermis 
is split into long shreds. Root clusters are developed from 
nodes of the stem under water, and on these roots aerenchyma 
is usually exceedingly well developed. In extreme cases, 80 
per cent of the diameter of a root from 6 to 9 mm thick may 
be aerenchyma. On many of these root clusters there are prom- 
inent root tubercles, which are developed entirely under water. 
Roots that are exposed to the air dry up as the water recedes. 
Sesbania invades either Phragmites or Typha, particularly 
the former. When crowded, Sesbania grows higher than either. 
Under the dense shade of Phragmites it appears that Sesbania 
cannot start, but by taking advantage of resting periods and 
of conditions less favorable to Phragmites, Sesbania obtains a 
start and often gains the upper hand on Phragmites. The kill- 
ing of so much Phragmites during the high hot water of Sep- 
tember, 1914, has already resulted in a marked increase in the 
