Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 289 



usually found as more or less scattered patches or slender mar- 

 ginal belts upon sandy soil, which, as Pound and Clements indicated 

 (57), is commonly somewhat shaded and is constantly bathed by the 

 waters of near-by springs or streams. The appearance of this asso- 

 ciation is thus seen to be quite different from the broad belts or 

 extensive tracts of low valley commonly controlled by the bulrush- 

 reed-grass association which covers hundreds of acres of the sand- 

 hill lowlands. 



I have chosen to unite in this association the "spring marsh 

 type," "characteristic of the sandy marshes about the sources of 

 streams," and the "spring brook type" of the "wet sandy margins of 

 spring branches" of the above authors, because after an extensive 

 reconnaissance of the region these two "types" appear to be but 

 slight modifications of a single unit that may be properly designated 

 as an association. 



As might be expected from the close relationship of this asso- 

 ciation to the other marsh associations, and to the earlier members of 

 the meadow formation, it has been found that in addition to a num- 

 ber of species quite distinctive of the streamside marsh there are 

 many species from these related associations that wander in and often 

 become locally dominant. 



Beneath the shade furnished by a more or less well-developed 

 arborescent cover we find the conditions which really make desirable 

 the delimitation of this association. In the saturated soil of such 

 situations we often find such species as Impatiens hiHora, Epilobium 

 lineare, and E. adenocaulon associated with some of the species 

 already noted in the formation. The touch-me-not dominates wide 

 areas in such sites and is to be considered the greatest modifying 

 factor of the marsh aspect as represented in this association. This 

 plant is more commonly a pronounced mesophyte in other regions, 

 but here it partakes strongly of the helophilous condition. Further- 

 more, the dense aggregation that characterizes this species as a 

 mesophyte is not the commonest aspect of its development in this 

 association. The aspect here is much more open so that one finds 

 mixed with the Impatiens such species as Helenium autumnale, 

 Rumex hritannica, Eupatorium perfoliatum, E. piirpureiim, Mimu- 

 lus jamesii, and Berula erecta. The two latter species sometimes 

 develop a marked secondary layer beneath an open touch-me-not 

 cover. Helenium is often seen in dense clusters. Bidens tricho- 

 sperma, B. levis, B. frondosa, Alsine longifolia, and Homalocenclirus 



