242 



Salix nigra Xantliium canadense 



salix longifolia Rudbeckia liirta 



Populus deltoides Bidens frondosa 



Salix cordata Polygonum virginiauiim 



Ambrosia trifida Helianthus strumosiis 



Impatiens biflora Apocynum caunibinum 



Impatieus pallida Lobelia syphilitica 



Rumex crispus Cicnta maculata 



Later stages of the succession occur at other points. The succession 

 is rapid, the ilood-plains of this kind quickly become mesophytic. The 

 large flood-plain at the bend of the river is thoroughly mesophytic, as 

 shown by the vegetation of the undisturbed portions of it. It is to be 

 noted, however, that the very narrow flood-plains of this type do not 

 become mesophytic quickly, because of their submergence at every flood. 

 By reason of the high gradient of the stream, the current is swift at 

 flood time, and very little material is deposited. In this respect, these 

 areas differ from the usual hydrophytic flood-plains. 



A third feature of the gorge floor is the presence of numerous and 

 well-defined flood-plains of a distinctly xerophytic nature. These occur 

 commonlj^ just below the fall lines, in distinction from those of the sides 

 of the ponded stretches. The manner of their formation is easily seen. 

 As already mentioned, the gradient of the stream is high, and at flood 

 times the swift current carries considerable masses of rock and coarse 

 gravel over the lower land immediately below the various fall lines. As 

 a result, these flood-plains below the fall lines and at the curves of the 

 stream are composed of gravel and rock fragments. The islands at the 

 fall line near the lower end of the gorge are subject to the same con- 

 ditions. 



The vegetation of these areas is quite distinctive. On the upstream 

 side, no plants occur for a considerable distance from the water's edge. 

 This corresponds roughly to the bare lower beach of a lake or the ocean, 

 and is the region that is covered by the slight rises of the river during 

 the summer. The pioneer plants are usually Xantliium canadense and 

 Bidens connata. Back of these occur Salix longifolia, Platanus occidentalis 

 and Populus deltoides. Of these, Platanus is the typical species, and 

 from the bluffs the xerophytic flood-plains may be picked out by the 



