24 



L., green ash, and finally the soft maple, Acer saccliariiinm L. 

 It will be noticed that each of these species is more moisture- 

 loving than its predecessors, until as a climax there appears the 

 soft maple, a characteristic tree of the river-bottom swamps. 



These changes in the arborescent vegetation are by no means 

 the only ones which occur. Even before the scarlet oak appears, 

 various species of lianas invade the association. The Vir- 

 ginia creeper is one of the most abundant, but several other 

 species may be found with it. These climb up the trunks of 

 the oak trees or, more generally, trail along the ground. 



There is also a profound change in the herbaceous vegeta- 

 tion corresponding with the gradual increase in moisture. Horse- 

 mint, Monarda nioUis L., starry campion, Silcnc stcllata (L.) 

 Ait.f., and columbine, AquUcgia canadcns'is L., are among the 

 early additions. At a later stage alum root, Hcuchera hispida 

 Pursh, comes in, the ground has some dense carpets of moss 

 and foliaceous lichens, and at the last a fern, IVoodsia obtusa 

 (Spreng.) Torr., and anemone. Anemone canadensis L., ap- 

 pear. Both of these species appear singularly out-of-place when 

 growing in sand on a hillside, possibly at the base of a tree of 

 blackjack oak. The anemone among the herbaceous plants 

 corresponds to the maple among the trees ; like the latter it 

 grows normally in river-bottoms subject to overflow. Thp 

 presence of these two semi-hydrophytes indicates how great 

 has been the environmental change from the original xerophyt- 

 ic oak forest. 



It must not l)c assumed that tliis development could continue 

 in the same direction indcnfinitely, leading to the ultimate es- 

 tablishment of a hydrophytic plant association. The climax 

 or final stage of ecological successions in this region seems 

 to be a mesophytic forest characterized by sugar maple, Acer 

 sacchariim Marsh., basswood, Tilia americana L., and other 

 species. There is no indication in this latest stage of the river 

 dune of the appearance of this climax association, yet on the 

 other hand there is no reason to doubt that it would eventually 

 come in. 



The frequent changes in the channel of tlie Mississippi have 



