Pool : THE VEGETATION OF THE SANDHILLS OF NEBRASKA 279 



alkaline lakes or ponds. In the shallower water where there are 

 openings between the colonies of water-weeds Lemna trisulca is 

 often developed in very pronounced cushions or beds sometimes 

 three yards across, which because of their light, yellow-green color 

 contrast boldly with the generally dark green or brownish sur- 

 roundings. 



Toward the western portion of the sandhill region Batrachium 

 trichophyllum becomes a frequent, and often a very abundant mem- 

 ber of this association. The water crowfoot groups might well be 

 classed as vernal societies of the association under discussion. The 

 species has usually flowered and passed its height of development 

 long before the other dominants become conspicuous. This species 

 is very common everywhere as a secondary species. When of con- 

 trolling abundance Batrachium is one of the most showy plants of 

 the whole state. I have seen the species so densely aggregated in 

 some ponds and lakes that hundreds of square yards of water sur- 

 face were concealed by the masses of showy, white to cream-colored 

 flowers. The water areas dominated by Batrachium are typically 

 old ecologically, or ponds in which there is a great accumulation of 

 organic matter in the form of mud or muck from which for some 

 reason or other the marsh formation is absent. The species appears) 

 to be rather plastic with reference to its water requirements, since 

 it is found in abundance from the more open water to the relatively 

 low water-content substratum of a late stage in the marsh or wet 

 meadow. With Batrachium trichophyllum in such amphibious 

 situations we commonly find such plants as Roripa nasturtium, Car- 

 damine hirsuta, Veronica americana, Mimulus jamesii, and Mon- 

 niera rotundifolia. 



This association plays a well-known remarkable role in the. 

 preparation of a suitable substratum for later stages in the succes- 

 sion. As a result of the dense aggregation of the individuals of 

 the constituent species, the bodies of water controlled by this asso- 

 ciation rather early exhibit a pronounced accumulation of debris 

 consisting of the remains of former generations of aquatic plants 

 and animals mixed with fine mineral particles brought in by wind 

 and wave and effectively held in position by the expanding tangle. 

 In this manner the accumulation of humus decreases the total 

 amount of water in the substratum and the habitat approaches a 

 little nearer the mesophytic. The soft oozy bottom is slowly raised 

 by the accretion of humus and the substratum becomes relatively 



