I920J 



TAYLOR—SUCCESSION OF MOSSES 



461 



and mesophytic types of moss. Stations for the location of the 

 atmometers were selected in the cottonwood, pine, and oak associ- 

 ations near Miller, and for the beech-maple association at Otis, 

 Indiana. The last, however, is upon morainal clay and not on 

 dune sand. It is not necessary to enter into a detailed account of 

 these results. Fig. 1, taken from Fuller's work, shows the 

 average of the mean daily evaporation rates in these associations 

 for the three seasons 1910, 191 1, and 191 2. Fig. 2 indicates the 

 curves for the average of the mean daily evaporation rates in the 

 four associations for the growing seasons of these years. 



The absence of mosses on the beach and the foredune is due 

 to the continual change in the surface material and the exposure 



Cottonwood dune 

 Pine dune 

 Oak dune 

 Beech-maple forest 



Fig. 1. — Average of mean daily evaporation rates for the 4 associations for 

 seasons 1910, 1911, 1912. 



to evaporation. Competition with other plants does not enter 

 into the question. There is not the struggle with wave action on 

 the foredune as on the beach, but there is still constant movement 

 of sand by winds. The plants forming the nucleus of the foredune 

 cast little shade, so that both desiccation by sun and wind and the 

 probability of being covered by sand are as great as on the beach 

 below. The cottonwood dune is higher, the trees afford much 

 more shade, humus begins to accumulate, and as the dune tends 

 toward stabilization there may be much greater protection from 

 wind on the leeward side. However, even on a moderately windy 

 day fine sand is deposited over the ground vegetation so that 

 there is still the struggle to overcome the tendency to covering, 

 and for opportunity for photosynthetic work on which the life of 

 the mosses depends. Evaporation by exposure to bright sunlight 

 and strong winds, while still high, may be somewhat less than on 



