282 MINNESOTA BOTANICAL STUDIES 



The peculiar restricted distribution and composition of this 

 association makes it very uncertain as to how the association should 

 be considered in relation to the other aquatic and helophytic vegeta- 

 tion of the region. It appears to play little or no particular role in 

 the regular successional relation between the associations of the open 

 water or marsh. The slight humus-gathering ability possessed by 

 the tufted species may be of some importance in relation to the 

 invasion of these areas by Sagittaria, Alisma, and Plantago. 



The Marsh Formation 



The marsh formation is the most extensive and conspicuous 

 vegetation unit of the sandhill lowlands, characterized by a satu- 

 rated soil and often extending into the open water of lakes and 

 ponds. Such areas, often many acres in extent, are always con- 

 trolled either by Scirpus validus, Phragmites phragmites, Zizania 

 aquatica, or Typha latifolia, or by mixtures of these dominant species. 

 The most typical aspect of the formation, because of the relation of 

 the formation to lake and pond phenomena, is found in the several 

 lake centers of the region. 



The regular position of the marsh formation is between the 

 open water with the preceding formation and the meadow associa- 

 tions which lie between the marshes and the lower limits of upland 

 vegetation. The transition from marsh to wet meadow, to rela- 

 tively dry meadows, and finally to one of the upland associations is 

 very easily appreciated in many places because of the regular belt- 

 ing that so often characterizes these various groups. Many areas 

 occupied by this formation are to be seen that doubtless were for- 

 merly covered by open water and more or less filled with the pond- 

 weed formation. The common bulrush, Scirpus validus, is the most 

 frequent of the dominant species. This plant occurs in characteris- 

 tic dense, dark green stands about lakes and ponds, in wet valleys, 

 and less abundantly along streams throughout the sandhill region. 

 The formation as here regarded is similar in part to the "reed-grass- 

 rush formation" of Pound and Clements. 



THE HULRUSH-REED-GRASS ASSOCIATION 



An association composed mainly of Scirpus validus and Phrag- 

 mites phragmites is the most common subdivision of the marsh 

 formation. These two dominant species often occur in extensive, 



