506 The Philippine Journal of Science ww 
When the vines become very dense, they may so load down the 
Phragmites that it falls, sometimes submerging and thus destroy- 
ing the vines. In the center of the association, vines are much 
less common, although seldom entirely absent. In addition to 
those mentioned in the Tarlac area, Merremia hastata Hallier f., 
Ipomoea cairica Sweet, and Streptocaulon baumii Dene, are com- 
mon in the Los Banos area. In both cases the vines are so 
promiscuous in distribution that they do not determine any line 
of succession. 
On account of drainage ditches in the Los Banos area, the 
water level is lower and here Phragmites is losing its dominance. 
During the rainy season the ground is water-soaked, but in the 
dry season it does not contain a great deal of water. The large 
rootstocks of Phragmites build up the ground in their immediate 
vicinity. The abundant aerenchyma on the roots testifies to the 
usual water-soaked condition of the soil. The roots permeate 
the surface soil in all directions so thoroughly that there is very 
little room for other plants. The numerous culms, each about 
1 cm in diameter and from 1 to 3 m high, grow close together. 
In this rather intolerant species, because of crowding, the 
leaves are limited to a bunch at the top. Individual plants grow- 
ing in full illumination are leafy to the base. Viewed from 
within, the vegetation appears like a miniature forest of leaf- 
surmounted canes, and from without as a very dense mass of 
broad-leaved grass. Only rarely can even etiolated plants be 
found on the ground floor beneath Phragmites. 
The transition into the Phragmites association is very abrupt, 
as Phragmites grows higher than any other herb in the swamp. 
Phragmites successfully invades Typha in standing water. The 
tension line is very sharp. Phragmites is invaded by the Premna 
odorata consocies of the parang association. As Premna is 
tolerant, especially as a seedling, its seeds can germinate under 
Phragmites and send up spindling stocks through it. At the 
level of the grass leaves the stems of Premna branch widely, 
forming a canopy over Phragmites. Thus shaded, Phragmites 
must give way. 
At present a considerable part of the Los Banos area is in 
this condition. Phragmites is surely being replaced by the taller 
Premna. Although the number of plants of Phragmites is so 
much greater in proportion, the spreading canopies of Premna 
above give the character to the general appearance from the 
outside. Under cover of the shade, at the level where the dead 
or dried stocks of Phragmites are bent over in the wind, a dense 
