Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 293 



shores. Over the more undulating flats from which surface water 

 has permanently disappeared, the association usually becomes more 

 patchy by the invasion of species from other associations and by a 

 soil-moisture or alkali differentiation among the regular members 

 of the association. This latter condition often results in breaking 

 up the characteristic belted condition and in the production of a 

 patchwork in which locally dominant species alternate in a great 

 variety of combinations. With the drying days of summer the sur- 

 face wafer disappears, and many previously dormant roots and 

 rhizomes send forth myriads of individuals of the numerous princi- 

 pal and secondary species, and for a time at least the uniform dark- 

 green grassy nature of the association is notably interrupted by the 

 blooming of these species. 



The presence of the Jnnci, species of Eleocharis, and other 

 sedges in close aggregations, are to be regarded as the typical expres- 

 sions of this association. These often occur in mixture as well as 

 in almost pure stands which cover the ground quite to the exclusion 

 of all other species ; Eleocharis palustris, ranging from the marsh 

 formation to its greatest expression here, is often seen in extensive 

 stands of such density as to completely, and for a long time, prevent 

 invasion. The number of individual stems in a square yard under 

 these conditions is astounding. 



The other typical aspect of the association is seen more com- 

 monly when the dominant species occur in a much more open stand, 

 thus making easily possible the incoming of many additional species. 

 This more open association might indeed be classified as another 

 association or sub-association if it were not for the fact, that, taking 

 the vegetative season as a whole, the sedges and rushes are control- 

 ling. Aside from this fact the most noteworthy structural character 

 of the association is the presence, at certain seasons of the year, of 

 various social principal and secondary species in such abundance as 

 to mask completely the dominants in localized areas. These species 

 sometimes take the form of remarkably well-developed layered socie- 

 ties. The most frequent and conspicuous examples of this kind of 

 distribution occur with Campanula aparinoides, Mentha canadensis, 

 Lotus americanus, Steironema ciliatum, Naumbergia thyrsiiiora, 

 Caltha palustris, Menyanthes trifoliata, Gerardia hesseyana, Lyth- 

 rum alatmn, Lilium umbellatum, and Halerpestes cymhalaria. Less 

 abundant social species of wide distribution are Lycopus lucidus, 



