200 THE PLANT WORLD 



to more than a hundred years. They frequently appear in considerable 

 numbers and make for themselves a distinctive zone. So, finally, we 

 have a willow-cottonwood-elm society with its attendant flora, which 

 near the shore is hydrophytic and approaches meso-hydrophytic con- 

 ditions towards the interior. 



The island, like everything else in nature, cannot keep still. The 

 checked currents below the island drop sediments continually and 

 additions are constantly made which run the same course in vegetative 

 life as did the previous portions of the island. The contraction of the 

 channel of the river by the lateral growth of the island causes a more 

 rapid current in the river, and the result is the island ceases growing 

 above and laterally on the upper portions, and erosion begins, 

 more pronounced in the upper stages of the water, but gradually the 

 shore trees are undermined, and falling in, are carried away. All the 

 larger islands show that their upper portions are worn inland to the 

 Cottonwood and elm zones. The land, however, grows downward, and as 

 the centuries pass the island keeps up its constant movement seaward. A 

 rapid change in the river channel or unusually high water may carry 

 away completely the island, but under normal conditions the island 

 remains a feature of the river, although moving constantly down the 

 river. 



Associated with the salix-cottonwood-elm society many character- 

 istic plants of the river islands may be found. Acer dasycarpum (the 

 soft maple) prefers the cottonwood zone, though it may be found on 

 either side of this zone. Along with the soft maple are Vernonia fas- 

 cicidafa (the iron weed), Cephalanthus occidentalis (the button bush), 

 Sfeironema ciliatum, Stachys aspera, Sambucus Canadensis, Echinocystis 

 lohata, Helenium autumnale, Aster diffusus, etc. 



If a space occurs where the trees are less numerous and the soil is 

 low, Bidens frondosa appears in great numbers and Nasturtium palustre 

 is also found. The damp low soil gives us Rumex verticillaia, which 

 may also be found in the low sand flats near the river. Its stem runs 

 deep into the soil and the heavy rootstock is strongly attached by its 

 fibrous roots to the soil particles. Other open spaces in the cotton- 

 wood zone have Leersia Virginica in quantity, forming a miniature 

 meadow. 



A little higher ground gives us Laportea Canadensis in strong 

 healthy colonies. The interior dune formation gives a flora peculiar to 

 itself. This flora brought in by the wind, high water, birds, and other 

 agencies, is the culmination of the island flora. Here we may find 

 Smilax liispida, Menispermum Canadense (themoonseed), Viola papilion- 

 acea, Fraxinus Americana, Solanum nigrum, a thick-leaved form, also, 

 Vitis riparia. Sisymbrium officinlae, Xanthium Canadense (cockle bur J, 



