456 BOTANICAL GAZETTE [june 



a precarious existence. Mosses are entirely absent, no evidence 

 having been found even of early germination stages. In addition 

 to the continual change in the surface there is exposure to high 

 evaporation, another factor very unfavorable to plant life. 



The foredunes are a result of the obstruction offered to the sand 

 laden winds by plants or other obstacles. Among the plants which 

 may act as windbreaks are Populus deltoides, Prunus pumila, Salix 

 glaucophylltt, and Salix syrticola; or grasses, as Ammophila arenaria 

 and Calamovilfa longijolia. There is no indication that mosses 

 ever form a part of the flora. Exposure to evaporation and danger 

 of smothering by sand are probably nearly or quite as great here 

 as on the beach itself. As we enter the cottonwood dune, which is 

 the first of the dune series characterized by trees, we still find 

 constant shifting of sand. Evaporation, however, because of the 

 shade cast by the trees, is somewhat less than in the earlier associa- 

 tion. Gradually the sand increases in height about the trees, which 

 continue to grow by adventitious roots (3). In time deposit of 

 refuse from the cottonwoods and growth of ground flora add to 

 the humus content as well as lead to stabilization of the sand. 

 Occasionally under the larger trees or on the more protected lee- 

 ward side of the dune a few mosses may win out in the competition 

 and live. The first species to appear are such xerophytic forms as 

 Ceratodon purpureas, Bryum ventricosum, and B. caespiticium. If well 

 sheltered, these mosses may continue on into the Pinus Banksiana 

 association; or if exposed by change in direction of wind, may be 

 entirely killed out before the cottonwoods are replaced by pines. 

 In no place on the cottonwood dune does there seem to be any 

 considerable growth of mosses. The species meijtioned form only 

 scattered tufts or cushions, although in most cases sporophytes 

 are borne freely. Either germination of spores does not often 

 occur, or the young plants do not survive the unfavorable environ- 

 ment. These species probably do not spread so readily by vege- 

 tative growth as do many others. 



From the cottonwood to the pine dune we usually find a 

 gradual transition, in which Pinus Banksiana begins to appear 

 more and more abundantly until the cottonwoods have been 

 eliminated. At about this time Pinus Strobus becomes mixed 



