1914] FULLER— EVAPORATION AND SOIL MOISTURE 197 



material 



This 



forms 



at first quite bare, but with forests soon advancing upon them. 

 Grasses and shrubs constitute the first vegetation, but in the moist 

 depressions the seeds of the Cottonwood (Populus deltoides) germi- 

 nate, and some of the seedlings are able to maintain themselves. 

 A more detailed account of their establishment has been given 

 elsewhere by the writer (7), and it is sufficient here to note that they 

 surmount the moving dunes and at a distance of 100-200 meters 

 from the shore establish the pioneer tree association that may be 



more 



a 



cottonwood dune." This persists upon more or less actively 



movin 



meters or more in height, until from various causes, among which 

 the vegetation is the most important, movement is checked and 

 the dunes become fixed. It is an association of a single tree species 

 and a paucity of shrubs and herbs, all having strongly xerophytic 

 structures. Among the prominent species are Salix glaucophylla, 



folia, Ammophila 

 herbaceous plants. 



pumila, Cornus stolonifera, Calamovilf< 



become almost com 



fixed (fig. 1) 

 e atmometer 



meters from 



the lake shore, some 100 meters apart, and about 12 meters above 

 the level of the waters of Lake Michigan. Three of these stations 

 were upon comparatively level areas, surrounded by the usual open 

 stand of cottonwoods and willows (fig- 2). At all stations the 

 atmometers received a small amount of shade for a few hours 

 of the day, and on account of the open nature of the association 



wind 



sharp 



The differences of exposure 



some 



the different stations, but affected very slightly the average rate 

 for the season. Station 4 was upon the leeward slope of a very 

 slowly advancing dune, and was further sheltered by a good stand 

 of Comus stolonifera. It was about 3 meters below the crest of 



