Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 281 

 THE STONEWORT- NAIAD ASSOCIATION 



This is a peculiar association revealed in some of the lakes and 

 ponds of the region which are characterized by a broad belt of shal- 

 low water above an open sandy bottom. The association, composed 

 of species of Chara, Naias Uexilis, and Zannichellia palustris, some- 

 times covers the sandy bottom in such places with an open or more 

 or less perfect carpet of tufted vegetation. The characteristic posi- 

 tion of the association is in those quiet waters that sometimes lie 

 between the shore line and a belt of rushes or reed-grass standing 

 farther toward the open water of the lake. The association also 

 invades to a certain degree the open bulrush-reed-grass association 

 after the humus has for some cause been washed out thus exposing 

 a comparatively bare sandy lake bed beneath clear shallow water. 

 The tufted colonies of the above species are often very conspicuous 

 as a distinct layer in such places. As a rule, also, there are quanti- 

 ties of algae of a number of species which coat the sand particles 

 and pieces of debris that lie between the tufts of the dominant 

 species with a rich color tone varying from deep blue-green to light 

 yellow-green. Species of Nostoc and certain calcareous algae are 

 often very noticeable here. Against this shallow, pebbled cover are 

 seen the low tufts of the dark olive-green Naias or Zannichellia with 

 their cricoid leaves. These tufts, composed of a few stems with a 

 height of a few inches, occur scattered over the alga-strewn bottom 

 at intervals varying in width from a yard down to a few inches 

 where the densest aggregation was seen. 



Other species that are occasionally seen with the above are 

 Ruppia occidentalis and a peculiar submerged form of Eleocharis 

 acicularis. The tufts of the latter are more strict than those of the 

 other members of the association. The stiff, needle-like leaves' 

 gathered into close tufts stand up in a decidedly bristling manner. 

 The growth of this species beneath the water is very different from 

 that which typifies its habit upon the wet beach near the water line 

 where it frequently forms local areas of dense sod. 



A very different aspect of this association is seen in some of the 

 small lakes and ponds in certain portions of the sandhills where the 

 Chara phase of the association becomes so extremely abundant and 

 voluminous as not only to control the situation, but to pack the 

 waters with a dense wirelike tangle of harsh, calcified stems. This 

 condition presents a peculiar type of closed alga association com- 

 posed of a number of species of Chara. 



