Jennings : A Botanical Survey of Presque Isle. 351 



of sand was mainly from the westward so that the formation as a whole 

 is considerably narrower on the east side of the lagoon (see Plate 

 XXVI). 



The habitat may be said to be dissophytic. During the most xero- 

 phytic periods the sand even at its surface is generally distinctly dark- 

 colored on account of the water it contains. The water-table is so 

 near the surface that even in the loose sand capillarity suffices to keep 

 the water-content of even the surface very high. However, with the 

 drifting in of sand the zone is gradually elevated and becomes dryer at 

 the surface, while a zone of wet sand is simultaneously formed inside 

 the first zone, thus providing a habitat for a succeeding formation. 

 During the year 1906 no little seedling cottonwoods or willows could 

 be found in the innermost zone of wet sand around Lagoon Aa, and 

 thus the Populus SalLx formation has here reached its territorial limits 

 for this lagoon. 



In the manner indicated, there is brought about a remarkable reg- 

 ularity in age and size among the plants constituting the Popuhis- 

 Salix zone, so that the formation often appears like a planted hedge 

 surrounding the lagoon. All over the sand-plain there are long reg- 

 ular lines of cottonwoods indicating the shore of a former lagoon, 

 long since filled up with sand. These vegetational structures are to be 

 regarded, not as component parts of the Paniciim-Artemisia forma- 

 tion, but rather as "relicts" of a Popuhis-Salix formation, although 

 they may figure quite prominently in the general landscape. 



The typical structure of the Populiis-Salix formation is : 



Fades. — 



Populus deltoides, Salix syrticola. 



Secondary Species. — 



Artemisia caudata, Artemisia canadensis, 



Onagra biennis, Aster ericoides, 



Panicum virgatiim. 



During the first few years of this formation there are practically no 

 species present other than the facies, but, as the surface -level becomes 

 more elevated, approaching more closely the conditions of the sand- 

 plain, there are a few invaders from the latter habitat, ranking, how- 

 ever, merely as secondary species. 



