21 



Chamaechrista L., and button-weed, Diodia teres Walt., or 

 someimes also of deep-rooted perennials as bush clover, Les- 

 pedeza capitata Michx., puccoon, Lithospcnnum Gmelini 

 (Michx.) Hitchc., and spurge, Euphorbia corollata L. The den- 

 sity of the plant-covering- depends primarily upon the rapidity of 

 movement of the sand, and partly upon the portion of the slope 

 occupied. The area below the general level, which is being 

 freshly exposed, is much more thinly covered than the upper 

 portion, composed of deposited sand. The same type of vege- 

 tation is also found on the lee side of the dune. 



The upper margin of the lower part of the slope, which indi- 

 cates the former level of the region before the formation of 

 the dune, is usually marked by a thin layer of dark-colored 

 sand. This is caused by organic matter deposited by past 

 generations of plants which occupied this surface before the 

 dune was formed, or at least before it had migrated so far in- 

 land. The outcrop of this old soil layer along the river front 

 affords better conditions for plant life then the sterile sand 

 above and below it, and is usually marked by a line of wild 

 rye. Elymus canadensis L. 



The next group of plants to appear may be termed the 

 thicket association, and is composed of several species of 

 plants aggregated into dense thickets. Green ash, 

 Fraximis pcnnsylvanica Marsh, var. lanceolata (Borkh.) Sarg., 

 is the first of the group to appear, its seeds being blown in by 

 the wind from the neighboring bottom-land forests. Honey- 

 locust, Gleditsia triacanthos L., appears at an early stage, al- 

 though the way in which its pods are scattered is not known. 

 These young trees attract a bird population, which in turn are 

 instrumental in the dissemination of various plants with edible 

 fruits. The mature thicket is composed of an impenetrable tan- 

 gle of green ash, honey-locust, crab, Pyrus ioensis (Wood) Bai- 

 ley, plum, Pniniis sp., and choke cherry. Primus virginiana L., 

 with some admixture of other species. It is covered with a 

 luxuriant growth of tangled vines of wild grape. Vitis vulpina 

 L., moonseed, Menispcrninni canadcnse L., poison ivy. Rhus 

 Toxicodendron L., Virginia creeper, Psedera quinqiiefolia (L.) 



