650 
may have shelves which contain trees the presence of which prevents 
more rapid erosion; the latter if unobstructed would render the slope so 
unstable as to become entirely treeless. The rocky, dry bed of these 
ravines is almost destitute of soil and correspondingly arboreal vegetation 
is entirely absent. Indeed, save for a scanty growth of ferns and mosses, 
vegetation of all kinds is wanting, although lichens are abundant on the 
rocks. A number of causes combine to bring about this condition. The 
lack of sufficient soil alone would operate against the invasion of plants 
from the slopes; during the dry season strong insolation increases the 
xerophytic condition of the habitat, and in the rainy season the torrents 
which periodically sweep through the bed of the ravine make the 
situation an exceedingly unstable one. However, small side ravines 
opening into the larger ones give a habitat of sufficient stability to 
support a rich vegetation. (See Pl. XXX.) 
Nearly perpendicular eroding banks of the Lamao and other rivers 
show distinct types. If, as often is the case, these banks are exposed 
to strong insolation from one side, the vegetation during the dry season 
is confined to the small trees such as Casearia solida, Brucea luzoniensis, 
Villebrunea trinervis, Leucosyke capitellata, and the ferns, Polystichum 
coniifolium, Polybotrya appendiculata, Asplenium  laserpitiifolium, 
and others. Other herbaceous plants are scattered specimens of Bego- 
nia rhombicarpa and species of Selaginella. During the rainy season 
these become represented by a greater number of specimens; indeed, 
in the more stable places on these walls, species of Selaginella and 
Begonia form more or less dense growths. In every instance where 
such banks are moistened by the seepage of underground water, the 
habitat is favorable for the development of mesophytic vegetation which 
is only slightly less luxuriant during the dry season. It is noteworthy 
that the vegetation occupying such habitats is ecologically remarkably 
uniform from the base to the top of the mountain. Plates XX XI and 
XXXII show two such situations at different altitudes, one in the 
Anisoptera-Strombosia, the other in the Shorea-Plectroma formation. 
The araceous Schizmatoglottis rupestris and Begonia rhombicarpa are 
common to both situations. Plate XX XIII shows two banks of the 
Lamao River at 500 meters’ altitude; on the left there is no seepage and 
consequently but little vegetation excepting lichens; on the right there 
is seepage, with a resulting luxuriant growth of Schizmatoglottis and 
the urticaceous Elatostema whitfordu. The boulders in the river bed 
of the larger streams (see Pl. XX XIII) are entirely destitute of any- 
thing except crustaceous lichens and occasionally the fern, Gymnopteris 
inconstans. This latter with the creeping rubiaceous Geophila herba- 
cea, Trichomanes javanicum, and a species of Selaginella constitute the 
vegetation of the wet rocks of the small but persistent streams, or in 
situations where the seepage is sufficient to maintain a wet atmosphere 
as well as a wet soil during the entire year. On the walls of narrow 
