Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 285 



dependent solely upon seedage for its dissemination. A thin fringe 

 of approximately equally spaced individuals of this species is more 

 commonly seen than isolated patches or clumps. The comparatively 

 low frequence of Zizania is probably due to the absence of the peren- 

 nial creeping stem as well as to the fact that the fruit is so eagerly 

 sought after by myriads of waterfowl. 



The reed-grass commonly invades in solid phalanx. Widely 

 separated individuals or clumps of this species are seldom seen in 

 the open water. This habit is responsible for the "bank" or wall 

 effect produced by Phragmites as one views these colonies from the 

 water. Sometimes such a bank as this lies between the open water 

 and a belt of Scirpus toward the shore. A very pronounced double 

 belt of tall plants is thus produced, the two portions of which are 

 clearly differentiated by the color of the respective species. In the 

 deeper water, however, this sequence is usually reversed. When 

 Scirpus and Phragmites are associated together in dense stands the 

 latter is usually in the minority. Under these conditions of mixture 

 the reed-grass commonly leads out into more open water along with 

 the bulrush. 



The reed canary-grass, Phalaris arundinacea, is a frequent 

 member of the older portions of this association. Societies of this 

 species are especially noticeable in midsummer after the species has 

 ripened its seed and the whole plant has become golden yellow. The 

 broad belts with it in dominant mixture look somewhat like fields of 

 ripened wheat at this time. The contrast with the surrounding 

 dark-colored members of the association is marked. So also the 

 individuals of Scirpus or Phragmites that are scattered through the 

 Phalaris areas stand out in bold contrast. 



The bulrush-reed-grass association becomes considerably modi- 

 fied in and about the more strongly alkaline lakes. This modifica- 

 tion may appear both in the composition and the density of the asso- 

 ciation. The reed-grass and the wild rice are greatly reduced in 

 frequency and abundance in such habitats. Both of these species 

 appear to be quite sensitive to an alkaline substratum above a certain 

 concentration which has not been determined. The bulrush is not 

 nearly so conspicuous a halophobe, but even this species, Scirpus 

 validus, reveals much lower form and is less densely aggregated 

 under the more highly concentrated conditions. The relative open- 

 ness of the association is a noteworthy feature whenever the more 

 halophilous situations are invaded. Scirpus americanus is the most 



