ECOLOGY OF WOODLAND PLANTS, 365 
betwecn the two, there is not infrequently an overlap—the 
Xerophytes being carried further down the slopes, due partly to 
the surface being strewn with fallen blocks of sandstone and 
soils resulting from their denudation. As we have seen, the 
more resistant sandstones usually occupy the higher ground, and 
the conditions of soil and climate there favour Xerophytes; 
while shales occupy the lower levels and sheltered slopes, where 
soils and climatic conditions favour the development of 
Mesophytes. 
As we follow the species over the escarpments, we find that, 
on reaching the shales and clays, Ling is the first to die out, 
which it does very quickly. The Bilberry often extends further 
down the slope, but, as with Ling, it dies out as the humus 
becomes deficient, while Deschampsia holds on when both its 
usual associates have disappeared. These features can be well 
shown in a limited area, as in the case of the Armitage Bridge 
Woods (fig. 8, p. 349) or other woods skirting the Millstone-Grit 
Plateau, where, on the steep slopes, we get a rapid transition 
from the dry exposed grit area above to the moist sheltered 
slopes of the shales and clays below. These results fully bear 
out Warming's observations that the distribution of plants is 
determined largely by available water. 
A comparison of the four maps (Trees, Undergrowth, Geology, 
and Xerophytes and Mesophytes) will show the effect of the 
dominant factors on plant-distribution in this area, viz.: soils, 
moisture, exposure, light, and shade. 
In the field I found it very convenient to use special terms 
for these soil types, and had intended suggesting names which 
seemed to express well the associations in relation to soil- 
conditions; but, eonsidering the unsettled state of Ecological 
Nomenclature, and that it will soon be considered by an inter- 
national committee, I have withheld them. As one may gather 
from a perusal of Clement's paper (16), the study of ecology 
may very easily be burdened with many cumbrous names which 
to me it seems well to avoid if possible. The terms I have used, 
Xero-pteridetum and Meso-pteridetum, are easily understood, 
and indieate suffieiently well the two important associations 
determined by soil-conditions in this district. 
Livingstone (60) has evidently been working on similar lines 
in a glaciated area in Michigan, but paying special attention to 
the distribution of trees, and has obtained corresponding results. 
LINN. JOURN.—BOTANY, VOL. XXXVII, 2D 
