1905] BROWN—THE PLANT SOCIETIES AT YPSILANTI 269 
water from the base of the bluffs to the central ponds, giving rise to one 
of the main differences between the conditions existing in the bayou 
and the usual kettle-hole swamp or bog in which water is more or 
less standing. The ridge along the west is usually free from surface 
water, although always moist. 
THE PLANT SOCIETIES. 
The vegetation of the bayou and bluffs consists of a number of 
plant societies, which may be treated under three heads. 
1. PEAT-FORMING socreTIES.—Under this head are included the 
societies commencing with the deepest submerged vegetation and 
reaching landward as far as the elms, red and green ash, and silver 
maple of the floodplain. A connected account of each in its order 
is given because the vertical succession of societies which occurs 
here seems to have been a common one on the wide floodplain areas 
below Ypsilanti, and would appear to possess some unusual features in 
the light of what has been written regarding the floodplains of other 
river valleys. This group comprises 131 species, or 32 per cent. of 
all species found upon the area surveyed. Seven distinct societies 
may be distinguished in order as follows, the number of species being 
given in parenthesis: 1, pondweed (7); 2, waterlily (5); 3, water 
smartweed (6); 4, Typha-Sparganium (10); 5, water sedge (19); 
6, willow (71); 7, tamarack (13); total 131 species. 
The first society is submerged, consisting chiefly of species of 
Potamogeton, and occurs only in the stream. The growth offers 
resistance to the flow of water, checking the current, and causes 
deposit of suspended particles in addition to that arising from vege- 
table decay. Following upon the submerged society, there appears 
one of floating plants, which still further checks the current. The 
Waterlilies secure a firm anchorage. The white waterlily (Castalia 
aia) is more common in the current. The yellow pond lily 
(Nymphaea advena) commences just back of this and occupies the 
bays where the current moves more slowly or is scarcely perceptible. 
the oy of species in relation to habitat is best shown between 
bay and the current (map). With the presence of the waterlily 
“oclety, there is a marked tendency to secure room neat the water 
Surface. The potamogetons appear to rely on the anchorage offered 
