44 ECOLOGICAL RELATIONS OF THE FLORA. 



the conditions in the swamp association as last described are very 

 similar to those in the mesophytic woods. Probably the chief difference 

 in factors is that of elevation which has kept the ground in the former 

 water soaked, i. e. not aerated, and acid. Probably the chief difference 

 in the makeup of these two associations was the predominance of 

 Betula alba papyrifera, Salix sp., Cornus circinata, and of Alnus incana 

 in the wet soil, whereas the mesophytic formation was more open. The 

 growth-form of the latter impressed one as a growth of trees, penetrable 

 and clean underneath, not as one of saplings with frequently impene- 

 trable under brush. 



Mesophytes — Forest '^Formation. 



Poplar-Birch-Maple Association : The thicket which fringed the 

 marsh graded into a dense wood of young trees, the latter exhibiting a 

 well defined stratification. Those members of the Alnus association 

 that could endure shade were here, and where the shrubs were domi- 

 nant before, the sapling was now the prevaihng growth-form. 



This formation is mesophytic. The great increase in the number 

 of shade plants, the presence of trees such as Ulmus americana, Betula 

 alba papyrifera, Populus tremuloides , etc., and the loose rich humus 

 covering the ground, showed that here the various factors of light soil 

 and moisture were in a moderate amount. 



Detail of the Poplar-Birch-Maple Association. 

 Dominant Species: 



Populus tremuloides. 



Betula alba papyrifera. 



Acer rubrum. 

 Sub-Dominant Species : 



Equisetum sylvaticum. 



Thuja occidentalis. 



Agrostis alba. 



Car ex tribuloides. 



Alnus incana. 



Ulmus americana. 



Boehmeria cylindrica. 



Dicentra cucullaria. 



Rubus idaeus var. aculeatissimus. 



Rubus hispidus. 



Rubus allegheniensis. 



Viola incognita. 



Trientalis americana. 



Rhus toxicodendron. 



Aralia nudicaulis. 



