280 



In beaches which are being" destroyed, such as the region between 

 Winthrop Harbor and Kenosha, a narrow tension association of Po- 

 tentilla anserina separates the grassy plain (Poa compressa associa- 

 tion) from the very low ridge of a very dense growth of Juncus bal- 

 ticits littoralis. In the course of the destruction of the shore, as has 

 been mentioned above, there is exposed an area of open sand between 

 the sand-plain and the relic dunes. (See Fig. i, Plate XLVI.) For 

 the most part, this area is devoid of plants but in slightly sheltered 

 places, Potcntilla comes in and spreads out radially, forming mats a 

 few meters in width and several meters in length. The leaves are 

 usually half buried and the runners can scarcely keep above the sand. 

 It may be for this reason that here the internodes of the runners are 

 so short. With it are seldom any secondary species. At the edge of 

 the grass on the sand-plain (Fig. i, Plate XLVI) is a well-developed 

 association of Potcntilla, and mixed with it are Sforobolus cryptan- 

 drus and sandbur {Ccuchnis caroliiiianus). This makes a denser 

 vegetation during the growing season than the grassy sand-plain it- 

 self shows, and effectually prevents any blowing during that period, 

 thus protecting the grassy plain. During the winter, when the sand 

 is rendered mobile with the drying of the Potentilla, a general south- 

 ward movement of the sand takes place in sufficient quantities to be 

 noticed from year to year. 



The Dune Formation 



Landward from the beach formation occurs the dune formation. 

 This has been so frequently and so well described, (e.g., Cowles, 

 1899), that only a brief summary of the characteristics need be given 

 before dealing with the associations. The essential conditions for 

 dunes are wind, dry mobile sand, and a nucleus to allow the sand to 

 accumulate (cf. Warming, 1909:263). 



Ecological Characteristics. — (Cf. Cowles, 1899:106-111). The 

 sand-dune is a very xerophytic habitat because of the agencies that in- 

 crease transpiration and at the same time keep down the water sup- 

 ply, such as intense light and heat and strong winds. The water 

 supply for sand-dune plants is deficient because water passes through 

 sand very readily and but a small amount is retained in it. To this 

 may be added the low nutritive value of the sand. On account of 

 the insolubility of the sand grains and the easy access of air, organic 

 matter which otherwise would form humus is rapidly oxidized. Wa- 

 ter continually passing through the sand washes away even the less 

 soluble food constituents (Livingston, 1903:14). A sand-dune, how- 



