Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 277 



lakes. Modesitt dug a pit on the shore of one of these lakes over 

 the top of which a crust was formed. He found that there were 

 47.72 g. of soluble salts in each 100 cc. of this crust when it was 

 dried at 105° C. 



THE PONDWEED ASSOCIATION 



This association which is very frequent throughout the sandhill 

 lake regions is characterized by the presence of species which are, 

 in the main, similar to those of Warming's "limnaea-formations" 

 or of the "Pondweed Formation" of Pound and Clements. Repre- 

 sented by one or more of the dominant species in an endlessly 

 variable mixture, the association is found in greater or less degree 

 in practically all of the ponds, lakes, and streams of the region. 

 Notwithstanding the salinity of the water in some cases the species 

 of this association are all typical of fresh waters. 



As has been said the lakes are all shallow, and so as a rule the 

 individuals of the various species are rooted at least a part of 

 the time in a loose, sandy, or mucky soil at the bottom of the lakes. 

 Typically this association is composed of plants which are com- 

 pletely submerged or at most possess floating leaves, as in the case 

 of Potamogeton, some species of which are wholly submerged while 

 others possess both submerged and floating leaves. Various methods 

 of propagation characteristic of this kind of plant life are revealed 

 by the species of this association, but these aspects will not be 

 treated at length in this paper. The common types of stem and 

 leaf morphology exhibited by the dominant aquatics need not be 

 indicated in this study. 



Composition of the Pondweed x\ssociation 

 dominant species 

 Batrachium trichophyllum Potamogeton heterophyllus 



Myriophyllum spicatum Potamogeton richardsonii 



Potamogeton foliosus 



PRINCIPAL AND SECONDARY SPECIES 



Ceratophyllum demersum Potamogeton natans 



Lemna trisulca Potamogeton pectinatus 



Polygonum amphibium Potamogeton pusillus 



Potamogeton amphifolius Potamogeton zosteraefolius 



Potamogeton lonchites 



Although no very careful detailed studies have been made of the 

 distribution of these species, it appears from observations extending 



