286 MINNESOTA BOTANICAL STUDIES 



distinctly halophilous of all of the Scirpi within our limits. This 

 species often produces extensive belts of closely grown plants which 

 in many instances become absolutely socially exclusive. The regu- 

 lar position of the belts of 6'. americanus is upon a saturated beach 

 or in the shallower water near the shore. This species sometimes 

 mixes with 5". validus on about equal proportions. 



Typha latifolia is the least frequent of the dominant species of 

 this association as it occurs in the sandhills. The degree of alkali- 

 tolerance of this species appears to be low and this fact may be an 

 important reason for its relative in frequency. This plant is, how- 

 ever, one of the last to give way before later invaders and so it is 

 seen as a late relict of the marsh formation after the areas have been 

 transformed into a later association. 



Secondary species are to be found in considerable abundance 

 between the scattered clumps or thinly distributed individuals of the 

 dominant species of this association as we find it towards the open 

 water. The open bays within the denser stands also reveal the pres- 

 ence of similar species. Along the more open water front relicts of 

 a former association are especially common, among which Nymphaea 

 advena and Potamogeton natans may be noted. Pelagic species 

 such as Lemna, Spirodela, IVoMa, and Asolla are common among 

 the erect stems of protected bays. A green or purple surface is 

 sometimes seen in these places according as to whether the cover is 

 composed of Lemnaceons species or Azolla. Certain algal species 

 such as Clathrocystis and Anahaena are often of such abundance as 

 to produce the well-known water-bloom. 



The shallower open areas nearer shore are sometimes filled with 

 Sagittaria arifolia, S. latifolia, or Alisma plantago-aqiiatica which 

 occur in large stands of closely grouped individuals. Here also is 

 found Sparganium eiirycarpum as scattered individuals or more 

 commonly in open aggregations. So also the less watery or older 

 portions of the association toward the land are regularly invaded by 

 manv of the species of succeeding associations. Among these plants 

 are Spartina cynosuroides, Stachys palustris, Asclepias incarnata, 

 Berula erecta, Cicuta occidentalis, and Teucrium occidentale. 



THE SMARTWEED ASSOCIATION 



As the water recedes beneath the bulrush-reed-grass associa- 

 tion, conditions arise that make possible the invasion of additional 

 numbers of helophytic species. Commonly this is a slow process as 



