502 BOTANICAL GAZETTE [JUNE 
across the valley at a comparatively slow rate, as is witnessed by 
the formation of peat on the recent levels. 
Occasionally sedge moor may precede willow thicket on stream 
banks, and it is not at all uncommon to find stream banks of shingle 
remain xerophytic for a long period. ‘The small isolated dry grass- 
land patches throughout the willow thicket association are undoubt- 
edly of this type. 
Where the meander approaches its maximum curvature, the 
force of the stream on the inside of the curve is so slight that fine 
material is freely deposited. A good illustration of this is to be 
seen in oxbow 20 (fig. 5). The main current flows through the 
cut-off channel. A small portion of the stream with only slight 
carrying power still flows through the oxbow. It has built up 
a sandy and muddy stream flat. Such a habitat as this has a 
varied vegetative history. Usually, an association of Eleocharis- 
Ranunculus is the first to become established. This is the char- 
acteristic amphibious community of the Park. The principal 
species are Eleocharis acicularis (L.) R. and S., E. palustris (L.) 
R. and S., Ranunculus reptans L., R. natans L., Allocarya scopu- 
lorum Greene, and Alopecurus fulous (L.) R. and S. Eleocharis 
acicularis builds a dense turf or mat. Allocarya scopulorum may 
also grow so thickly as to form a rather close growth over the soil 
surface. Alopecurus fulous is a constant principal species of the 
community. Eleocharis acicularis often grows into several inches 
of water; such plants are sterile. However, by a slight lowering 
of the water level, the plants spread rapidly both by the under- 
ground parts and by seed, and in one season may make good head- 
way toward reclamation of the mud flat exposed. Eleocharis 
palustris (fig. 6) finds its best expression in some of the oxbow lakes, 
especially those that have a flat, stony, or gravelly bottom and 
possess water only a part of the year. In oxbow lakes 8 and 9, for 
example, almost the entire area over which water stands, for a time 
at least, is covered with Eleocharis palustris and Ranunculus 
reptans. Rare associates are Glyceria borealis (Nash) A. Nels. and 
G. grandis Wats. 
The Eleocharis-Ranunculus association is followed usually by 
sedge moor, in which Carex variabilis is the predominant species, 
