1920] 



TAYLOR— SUCCESSION OF MOSSES 



461 



moss 



Stations for the location of the 



1 



atmometers 



Miller, and for the beech-m 



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



dune sand. 



necessary 



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 191 o, 1911, 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 



» 



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

 seasons 1910, 191 1, 191 2. 



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 



• # 



01 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 



