362 VESTAL: A BLACK-SOIL PRAIRIE STATION 
abundant than invading species: it is probable that change in 
floristic composition lags behind changes in ecological conditions, 
due to greater or less plasticity of environmental relations in most 
of the plant species. 
The forest border—The forest of the area mapped is not in 
itself part of the subject-matter of the present discussion. Much 
of it is a mixed tree growth, in which basswood is very prominent. 
Other trees are elm, walnut, oaks (several species), hackberry, wild 
crabapple, choke-cherry, and wild plum. The undergrowth is 
made up of characteristic species of mesophytic forest, with black- 
berry and raspberry shrubs in more open spots. 
Parts of the forest border are in apparently original condition. 
The undisturbed growth is of two types, first that of exposed sunny 
borders, best seen on south and west edges of wooded areas, and 
second that of shaded borders, on north edges. No east-facing 
borders in good condition are to be seen at the county line. 
The exposed borders show very distinct zonation. In some 
places a low-tree zone is seen at the edge of the forest proper. This 
is composed of wild crab (Pyrus coronaria) or of plum (Prunus 
americana) or of thorn-apple (Crataegus sp.). These trees are 
usually from nine to fourteen feet in height, and form a zone of 
varying width. A shrub zone is seen just outside the trees. A 
dogwood (Cornus Amomum) dominates; hazel (Corylus americana) 
and elder (Sambucus canadensis) are locally abundant. The 
dogwood is usually three and a half feet high, the hazel is about 
seven feet high; the shrub zone is four to ten feet wide. Grape- 
vines (Vitis vulpina) cover some of the outer shrubs and trees. 
The outermost zone is almost a pure growth of sunflowers (Helian- 
thus decapetalus and H. divaricatus), locally replaced by a species 
of Verbesina.* The height of the sunflowers is two to four feet; 
the zone is two to eight feet in width. The prairie just outside 
the sunflower zone is strikingly uniform with that farther from the 
forest. 
The shaded forest borders are less regular in composition and 
structure. The border of one grove, which abuts at its northern 
edge on a moist grassland (Agrostis zone, see map), was observed 
* Dr. ft. 
D. A, Cockerell has examined the specimens, which are alternate-leaved, 
resembling Verbesina helianthoides, but differing from the description of that species 
in that the leaves are not sessile 
