ECOLOGICAL CROUPS. 269 



of fine, brown silt overlying a stiff, bluish ehiy, contains orj^^anic mat- 

 ter in considerable quantity and is therefore capable of supporting a 

 denser plant growth than is found upon the sands. It is of course 

 saturated with salt or brackish water. 



There is no outcrop of any kind of rock on tlic island. 



THE PLANT FORMATIONS, THEIR COMPOSITION AND PHYSIOG- 

 NOMY. 



The various assemblages of species and individuals which make up 

 the plant covering of Ocracoke Island may be classified as follows:* 



L Sand-strand vegetation. 

 1. Treeless (open). 



(a) Beach formation: Croton-Physalis association. 



(b) Dune formation: * Uniola-Yucca association. 

 3. Evergreen trees and shrubs. 



(a) Tree formation: Quercus virginiana association. 

 (6) Thicket formation: Ilex vomitoria association. 



II. Salt-marsh vegetation. 



1. Creek-marsh (closed) formation. 



(a) Spartina stricta association. 



(b) Juncus roemerianus association. 



3. Dune-marsh formation; Lippia-Monniera association. 

 3. Tidal flat (open) formation: Sesuvium-Tissa association. 



III. Pastures and ruderal plants. 



IV. Cultivated plants. 



It is not to be supposed that the severial groups are always or even 

 commonly sharply defined. On the contrary the transition from one 

 to another is almost always gradual, so that iDortions of the plant 

 covering are difficult to classify. Nevertheless, the formations and 

 associations are distinct features of the landscape, easily recognizable 

 by any observer. 



' It has seemed best to use the word ''formation *' in the same sense as employed 

 by the German and most other plant geo-raphers— i. e., to designate ihe 

 larger assemblages. For more restricted groups, whether composed of one or 

 many species, the term ''association'' is to be preferred. The nearly ejuivalent 

 German word "Verein"' is used as a translation of the Danish "Sanifuud" in 

 one of the most important works on the subject (Warming, Lehrbuch) for the 

 larger assemblages or formations; but, in the want of a better English word it has 

 been thought expedient to employ '' association *' for the more restricted assem- 

 blages, which are peculiar to each biogeographical area. While the ft^rmations are 

 purely ecological elements which recur in the strand vegetation of other regions, 

 being for the most part closely dependent upon topographical features, the asso- 

 ciations are often quite local; and owe much of their character to the particular 

 groups of species which compose them. 



' It is not possible to distinguish here several dune formations, such as occur, 

 for example, on the coast of Virginia. 



