329 



Invading Species 



Solidago graniinifolia Juncus canadensis 



Iris versicolor Hypericum virginicum 



Aspidium thelypteris Salix longifolia 



Steironema quadriflorum Asclepias incarnata 

 Ly thrum alatinn 



This association is the last of the strictly aquatic associations, 

 whose dominant species compose from 85 to 100 per cent, of their 

 area. The following are land associations in which the dominant 

 species are usually more numerous and more openly distributed in the 

 area. Secondary species are much more numerous, and lead in de- 

 termining the different seasonal aspects of the associations. The 

 marsh group of associations is transitional to either prairie or for- 

 est. 



THK CIvADIUM MARISCOIDES ASSOCIATION 



Developing on mucky soil just back of the Scirpus ainericanns, a 

 little above the hight of standing water but not sufficiently high for 

 the surface to become dry, is the Cladium association, about 98 per 

 cent, of whose plants are the sedge, Cladium niariscoides. In the 

 youngest swales the Cladium and Scirpus americanus are adjacent, 

 while in the middle-aged swales they are often separated by the de- 

 velopment of the Calamagrostis canadensis association on the ten- 

 sion line bet\\een them. In the oldest swales the Cladium is entirely 

 absent. There is seldom any mingling of these associations, even on 

 their border lines. The vegetation is so dense in the main part of 

 this association that secondary species can obtain a foothold only on 

 the tension line between this and other associations. In level places 

 this association will spread out to a width of 15 or 20 meters, with 

 a uniform structure throughout. More usually, however, it occurs 

 as a zone around ponds or as a belt along swales, seldom attaining a 

 width of one meter, but exhibiting the same uniformity of vegeta- 

 tional structure. During the growing season the color tone of this 

 association is dark green — the color of the stems and leaves. About 

 the first of August the plants come into bloom and the color tone is 

 changed to brown, which makes the association stand out very 

 sharply from the surrounding ones. This is especially the case dur- 

 ing years of drought, as 1908 and 1910, when every plant of Cladium 

 blooms. During normal seasons, as 1909, when Figure 2, Plate LIV, 

 was taken, scarcely half of the plants bloom. Usually Cladium per- 

 sists from season to season by the growth of the root stalks. The 



