1905] BROWN—THE PLANT SOCIETIES AT YPSILANTI 273 
abandoned because the bur oak was found to occur not alone in moist 
situations, but with equal frequency under much more xerophytic 
conditions, such as prevail in the oak openings a few miles west 
from Ann Arbor. However, the bur oak of these drier situations 
differs in appearance from the walnut association form, which has 
drooping branches with heavy corky ridges. The walnut society 
occurs in the better drained parts of the floodplain and for a short 
distance up from the base of the slopes, as shown in the map. It 
grows best of all in the coves. Along the tension line nearest the 
water Carpinus caroliniana is common; along the drier tension line 
Osirya virginiana is rarely absent. The most common associated 
herbaceous species are Houstonia ciliolata, Mesadena atriplicijolia, 
Allium canadense, Verbesina alternifolia, Homalocenchrus virginicus, 
Falcata comosa, and F. Pitcheri. 
As a whole the vegetation of the floodplain is mesophytic. Its 
growth differs from that of the peat-forming group in being mainly 
forest, in the tendency of its species to grow in mixture, in possessing a 
greater number of southern forms in its composition, and in the greater 
shade endurarce of its species. The composition does not vary much 
in similar parts of the valley, but certain elements occur which serve 
to distinguish the stream-course type from that which occurs in the 
More remote parts of the valley, where mesophytic associations are 
found. Platanus occidentalis, Juglans nigra, Hicoria laciniosa, 
Houstonia ciliolata, Quercus acuminata, Meibomia canadensis, Fraxi- » 
nus pennsylvanica, F. lanceolata, and Acer saccharinum occur char- 
Scteristically along stream; but Fagus americana, Acer Saccharum, 
Asimina ‘riloba, and associated species, forming a distinct maple- 
beech society in some other parts of the valley, are rarely met with 
on the floodplain. 
_3- BLurr socreties.—Like the floodplain, the blufis are covered 
with forest, but of a quite different character. The woods are more 
ses share in common with other vegetation numerous xerophytic 
“Hons. Four societies need to be distinguished in an account 
of existing relations. 
he oo? society.—Beyond the few hickories at the north, 
sia y 1s little represented at the bayou. This may be partly 
ined by the absence of the more moist till slopes, upon which 
