288 MINNESOTA BOTANICAL STUDIES 



almost even- other plant is excluded. The ground cover produced 

 by the low plants is perfect in many instances. In the open areas 

 where the Polygonums are found in more open aggregation the 

 association admits a number of secondary species. Of these the 

 commonest are Polygonum hydropiper and P. persicarioides. The 

 latter species is sometimes seen in dense patches which alternate 

 with the dominant species. Rumex persicarioides is also an occa- 

 sional invader as scattered individuals or in small clusters where 

 the association passes over directly to the rush-meadow association. 



Pound and Clements have pointed out (57) certain other varia- 

 tions of this association (called by them the "Smartweed Forma- 

 tion") that are sometimes seen in the sandhills. The commonest 

 variations are produced by the presence of P. lapathifolium and P. 

 incarnatum sometimes associated in marshy situations, and in other 

 cases by P. punctatum and P. hydropiperoides. The development 

 of these various aspects is probably due to chance dissemination 

 rather than to any significant habitat change. 



Still another phase of this association is seen in the sandhills, 

 as well as in every other region of the state, where P. pennsylvani- 

 cum takes exclusive possession of marshy habitats as well as rela- 

 tively drier ruderal situations. 



The rapid drying-up of some of the shallower lakes and ponds 

 sometimes results in the production of rather sudden changes in 

 edaphic conditions. These changes commonly accelerate the com- 

 plete disappearance of water-plant species and tend at the same 

 time to reduce the optimum for the marsh series, and also to favor 

 the invasion of such areas by numerous species of later associa- 

 tions. A peculiar heterogeneous type of vegetation may^thus pre- 

 vail for a time in such shallow depressions where practically all 

 species of the mixed population may become dominant in local 

 patches. In the early stage of this change the aspect is strongly 

 that of the marsh series but later, as the water-table recedes beneath 

 the drying substratum, a strong tendency toward the meadow series 

 appears. Instances of such a bizarre mixture of species and asso- 

 ciations are found in nearly all of the lake centers of the region and 

 they are always puzzling to one who has not worked out their 

 significance. 



THE STREAMSIDE MARSH ASSOCIATION 



Still another expression of the marsh formation is seen along 

 such streams as the Dismal and Loup rivers. This modification is 



