EM CERIN A Oy ee ey Pest) Oy Re RPE Co voters eile 2 
We I UE By 
PAPERS ON BOTANY 123 
xerophytic vegetation may persist indefinitely, even though 
there is a slow lateral migration as the valley widens. It is 
probable that the dryness occasioned by the slope to the south 
is in most places not in itself sufficient to preserve the prairie 
from forest encroachment, for forest is able to establish itself 
in quite xerophytic habitats in the vicinity, and has in fact 
done so over most of the south-facing ravine slopes. Other 
physical factors aid the original exposure afforded by direction 
of slope. One is instability of surface, due partly to steepness, 
partly to the meagerness of protection against erosion afforded 
by the open and sparse ground-cover.* Others are accidental 
and artificial factors which destroy or check forest growth, 
such as fire, cutting, grazing, and trampling. These operate in 
places only temporarily, but in other places recur frequently 
enough to permit the continued existence within the forest of 
small but rather numerous patches of prairie, with more or 
less shifting boundaries, wherever the basic condition of south- 
ward exposure is fairly extensive. The spur-tops and other 
small upland areas adjoining the slopes may also show prairie 
vegetation, especially in those ends and corners of uplands 
which are too small or too narrow to tempt the farmer to in- 
vade them with the plow, even if there are no trees. This type 
of situation is a distinct geographic or topographic entity and 
should have a name. Upland salient or simply spur-top might 
answer this need. 
The locality which shows these local prairie inclusions best is 
hardly more than a mile east of the southern part of Charles- 
ton, along a large ravine (‘“Endsley’s ravine” or “Endsley’s 
Hollow”) which runs east and a little south to the Embarras 
river. A topographic map showing some of these patches of 
prairie was made so that situation and extent could be quite 
definitely shown (Fig. 1). Most of the prairie is below the crest 
*It is of interest that even in flat areas like east- central Illinois, the topographic divers- 
ity of the broken country of the valley-slopes and ravines results in vegetation-diversity, in 
a manner entirely comparable to that which prevails over the general area in a mountzinous 
region. Thus in the rock exposures along the Embarras river three miles east of Charles- 
ton, and in the various slopes and valleys of the district, tnere can be found many of the 
tvpes of physical habitats common in mountain areas. One thus can learn much of the re- 
lations of topography and vegetation as developed in mountains without visiting the moun- 
tains themselves. 
