460 BOTANICAL GAZETTE [june 



Why is it that we find this great variation in the moss flora 

 within such a relatively small area as that included within this 

 dune complex? There seem to be at least three causal factors 

 which are worthy of special consideration. First is the constant 

 transportation of sand; second, the exposure to high evaporation; 

 third, and in this case of least importance, competition with other 

 plants. Mosses, because of their low growing habits, are not able 

 to endure covering. Even with such a genus as Sphagnum, 

 which i$ able to continue upward growth year after year, and which 

 has tall erect stems, it is not unlikely that a deposit of sand or 

 sediment would entirely destroy its power of regeneration. There 

 is much less probability that other species which do not have 

 this advantageous habit could contend successfully against cover- 

 ing. Numerous places occur within this region where, through 

 rejuvenation of some dune area, the sand is being carried over 

 more or less mesophytic regions. North of the Grand Calumet 



are dunes which have reached the pine stage and 



Miller 



many of the species of moss found in the pme 



south or east of the river. 



man 



) 



dunes to the 



The mosses are now in many places early destroyed by smother- 

 ing, because of the fine sand accumulating about them, and the 

 whole slope, once mesophytic, is undergoing a retrograde succession. 

 Thus it seems quite certain that any dynamic condition which 

 will lead to covering will also bring about the death of anv mosses 



species. Contrary 



growth 



dry 



The water table is 



necessary only to remove 



to find moisture, even during dry weather. The exposure to 



may be great, and this without 



m 



rather than non-availability of the water supply (6). The work 

 of Fuller gives data upon evaporation in the dune associations, 

 secured in this same region north of Miller. The results regarding 

 the difference in the evaporation rate verify in a marked degree 

 the conclusions to be drawn from the location of the xerophytic 



