102 A Study of the Vegetation of 
of the shallowed water, while through the agency of further 
humus accumulation, a suitable foothold is eventually afforded 
for the development of cattails, sedges, rushes, and other mem- 
bers of the marsh community. , 
About the margins of shallow ponds and streams and often 
growing in shallow water are to be found Mimulus langsdorfu, 
Veronica spp., Radicula spp., Ranunculus alismaefolius, or Steiro- 
nema ciliatum. If the ponds are larger, the shallower water near 
the margins is often dominated by plants of the following com- 
munity : ; 
The N ymphaea-Potamogeton Associes 
Typically this associes is composed of plants rooted in the mud 
below but with a part or all of the leaves floating on the water 
surface. Their general position in the sequence of succession is 
following that of the submerged forms, whose reaction upon the 
habitat, as previously pointed out, is to shallow the water and 
thus make it less fit for themselves and more fit for the anchored 
floating forms. 
The Marsilea community is well represented in certain small 
ponds but is most conspicuous in the rock pockets in the beds of 
streams during the dry season. The ability of Marsilea vestita 
to float on the surface of water of varying depth or to live rooted 
in the mud or even in rather dry soil fits it to occupy just such a 
changing habitat. However, it is far less important than mem- 
bers of the following consocies. 
The Potamogeton consocies consists of various species of Pota- 
mogeton of which P. americanus often is dominant (Fig. 29). 
Frequently associated with it are the submerged P. pectinatus 
and P. californicus. In late summer ponds containing the latter 
species are sometimes margined for considerable distances from 
the shore with the broadly expanded leaves and the dense spikes 
of P. americanus. Associated with it are Polygonum amphibium 
and the duckweeds, Lemna minor, L. trisulca, and locally, Spiro- 
dela polyrhiza. 
Other ponds and especially lagoons along the upper Palouse 
River are characterized by an abundance of Nymphaea poly- 
102 
