Ne ESS ee EN ee Te Cn ae me A et mn Sey 
649 
a few days during the dry season was completely deciduous. Another 
plot, on a level bench of the main ridge at the same altitude and 
in the same physiographic situation as that of A, Table XIX, has 
a much more xerophytic and open vegetation than it would be expected 
to have, the difference apparently being caused by its greater exposure 
to the wind. Again, on a slope of 25° on the main ridge a vegeta- 
tion exists which reminds one very much of that of the Dinochloa 
parang of the lower altitudes. Here Dinochloa tjankorreh is found 
growing together with species of Calamus, other procumbent lianas, 
and small trees such as Casearia solida, Dysoxylon turczinanowii and 
others. Plot B, Table XIX, has the same slope and depth of. soil 
and although at the same altitude, is much less exposed. Plot C, 
Table XX, compares very favorably with Plot B, Table XIX, in all 
particulars excepting that the former is much more subject to the 
sweep of the winds; as a consequence it has a less mesophytic vegetation 
and members of the Dipterocarpacee are almost absent. In Plot (©, 
Table XX, Anisoptera vidaliana reaches its highest altitude noted on 
the Reserve. The examples which have been cited will suffice to 
show that mere exposure has a tendency to make a less mesophytie and 
a more open vegetation irrespective of soil conditions. Other things 
being equal, the steeper the slope the more unstable and shallow will 
be the soil and as a consequence the greater the paucity of arborescent 
growth. A comparison of the plots analyzed: in Table XIX emphasizes 
this fact. In this instance the situations of the plots are approximately 
the same in regard to exposure and altitude, but the slopes vary in degree 
from a level ridge-bench to those having an ‘inclination of from 25° to 
over 45°. In those up to 40°, the members of the Dipterocarpacee 
together with other trees of the same crown class maintain the ascendency 
with the associated shade species. In slopes of over 40°, these trees 
become of less importance and when they do exist they seldom have 
developed beyond the sapling stage. ‘They are associated with smaller 
and, during the dry season at least, less mesophytic or light-loving tree 
species, such as Casearia solida, Buchanania florida, Semecarpus perro- 
tetti, tea macrophylla, and others, all of which are nearly absent where 
the slope is less steep. Crypteronia cumingiana reaches its best develop- 
ment on the edges of overhanging cliffs and probably because of a deep 
and wide spreading root system it is peculiarly fitted to maintain a 
foothold on unstable soil, although this tree is not found in places where 
the exposure to wind is great. (Pl. XXIX.)  Precipitous slopes having 
an inclination of over 45°, contain but little arborescent vegetation ; they 
usually form the sides of box canyons and are consequently more or less 
protected from drying influences of direct insolation. . Ridges such as have 
been described, alternate with deep ravines the sides of which may be suf- 
ficiently steep to be classed as gorges. The slopes of the former are 
sparsely covered with forests and even when they are steeper than 50°, 
