24 MANGROVE SWAMPS 
Skirting the inland portions of the water channels, through 
which the tide ebbs and flows, is often found a strip of nipa 
palm (Nipa fruticans), usually narrow, although sometimes 
it occupies areas of considerable extent (Plates IX, XLV). In 
Pangil Bay, Mindanao, there is a single area of nipa covering 
9,000 hectares. Nipa grows farther up the streams flowing 
through the mangrove forests than do the trees forming them, 
being found along streams where the effect of tide is barely 
noticeable. In some places the mangrove trees have been killed 
or cut out and nipa planted over wide areas of swamp. Such is 
the case north of Manila Bay, where much of the original tree 
growth has been entirely replaced by nipa. 
Nipa has a large, branching, horizontal rhizome, or under- 
ground stem, which grows just below, or on the surface of, the 
soil and sends up short branches with a cluster of pinnate 
leaves, which rise 7 meters or more above the ground. Nipa 
frequently forms a dense mass of vegetation which is difficult 
to penetrate. 
Undergrowth in a heavy virgin swamp is usually scanty, but 
in places where stands are light, in cut-over areas, and along 
the outer edges of the swamp, a fairly heavy undergrowth of 
vines, shrubs, ferns, and herbs is developed. Very noticeable 
in this are a swamp fern, Acrostichum aureum, and two spiny- 
leaved undershrubs, Acanthus ilicifolius and A. ebracteatus. 
Among the commonest woody vines are Derris trifoliata 
Lour. (D. uliginosa Benth.) , Tristellateia australasiae L. C. Rich., 
Dalbergia candenatensis Prain, Caesalpinia nuga Ait., Caesal- 
pinia crista Linn., and Finlaysonia obovata Wall. Herbaceous 
vines are represented by the epiphytes Hoya and Dischidia. 
Epiphytes are fairly numerous throughout the swamps. Per- 
haps the most conspicuous elements are the orchids, especially 
species of Cymbidium and Dendrobium. Epiphytic ferns are 
represented by Drynaria quercifolia J. Sm., Polypodium sinua- 
tum Wall., and sometimes Asplenium nidus L. The most pecu- 
liar epiphytes are those containing cavities which are inhabited . 
by ants. These are very abundant and are represented by 
Myrmecodia, Hydnophytum, and Polypodium sinuatum Wall. 
The bases of the stems of Hydnophytum and Myrmecodia are 
greatly enlarged and contain labyrinthine cavities in which ants 
are found in large numbers (Plates VI, VII). The stems of 
Polypodium sinuatum are swollen and hollow, the cavities being 
inhabited by ants (Plate VII). Dischidia saccata Warb. is 
