IO2 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 langsdorfii, 

 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 Nymphaea^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. calif ornicus. 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- 



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