280 MINNESOTA BOTANICAL STUDIES 



drier and more compact. The continuation of these processes by 

 aquatic plants finally results after many years, as Cowles (32) and 

 others have emphasized, in preparing the way for their own elim- 

 ination and the establishment of such plants as pond lilies and others 

 with slender stems and floating- leaves. Through the agency of 

 further humus accumulation a suitable foothold is eventually afford- 

 ed for the development of the bulrush and other members of the 

 marsh formation. From this time the latter, very different stage in 

 the vegetative cycle dominates the positions once held by the pond- 

 weeds until this type also is replaced by a still later phase of the 

 succession. 



THE WATER LILY ASSOCIATION 



The usual position of this association, dominated by Nymphaea 

 advena, in the vegetative cycle appears to be immediately behind or 

 following the former association. This relation is sometimes seen 

 in the sandhill lakes where the water lily occupies a belt lying be- 

 tween the pondweeds and the marsh species. Quite frequently, 

 however, when the marsh formation is young and comparatively 

 open the lilies are seen in considerable abundance among the rushes. 

 In the latter case the lily is probably to be regarded as a relict. 

 Under these conditions the "mixed association" is characterized by 

 a more diffuse stand of rushes and the production of the condition 

 that Tansley (66) has called "the open reed or rush swamp associa- 

 tion." Probably because of the generally shallow water of these 

 lakes the lily association does not always hold its normal position, 

 but is more commonly found in the more open portions between the 

 beds of submerged aquatics. The floating leaves almost completely 

 cover the water in such places over areas many square yards in 

 extent. 



Additional floating-leaf aquatics are occasionally seen in mix- 

 ture with the lily. The commonest of these secondary species are : 

 Potamogeton natans in the deeper water and, where the depth is not 

 so great, P. lonchites and Polygonum amphibium are common. 

 With continued humus addition and elevation of the lake bottom the 

 lily colonies become invaded more and more completely by Scirpus, 

 Typha or Phragmites until finally the "mixed association" gives 

 way completely to the dense aggregation of the marsh formation 

 when Nymphaea disappears completely. 



