47 8 BOTANICAL GAZETTE [june 



ment 



Le moss successions on rock surfaces may be summarized 

 two inain heads: (i) There are at least four factors which 

 special importance in accounting for the better moss develop- 

 on rocks along Carroll Creek than in the Chicago region: 

 the greater humidity in the former place because of nearness 

 to a stream and lessened exposure; a lower evaporation rate 

 due largely to the fact that the rocks are sheltered from direct 

 rays of the sun for a greater number of hours each day; the slow 

 evaporation of the large quantity of water taken up by the moss 

 mat during the gradual melting of the snow, and consequent 

 lack of desiccation; and the freedom from atmospheric dust, 

 common about any large city, which tends to retard photo- 

 synthesis. (2) Mosses are of special value on a rock substratum, 



formers, to form 



• . » 



the early tree associations, to conserve water supply and to prevent 

 floods by too rapid run-off, and to add to the aesthetic beauty of 

 the landscape. 



River bluff succession. — Another somewhat xerophytic 

 habitat is that of a high river bluff as seen at Thornton, Illinois. 

 In this region Thorn Creek, a comparatively small stream, has 

 cut down much below its former level, resulting in drainage of 

 the adjacent land and a consequent lowering of the water table. 

 The trees along the bluff are deciduous and sufficiently scattered 



grazing 



sun's rays, even during the summer 



nn shrnhhv nnderprowth. Here an 



such mosses as Catharinea undulata, Leucobryum glaucum, Cera- 

 todon purpureus, Funaria hy gr ometrica , Poly trie hum commune, 

 and Physcomitrium turbinatum, all of which are quite abundant. 

 All of these, except the last, are found in the neighboring swamp 



forest. Catharinea. which is nmiallv fminH nnlv in the 



mes 



from a Drevious Deriod of greater meso- 



phytism. Polytrichum, while often found in rather dry 



) 



swampy habitais, 



so that it also is likely a relic. Leucobryum and Funaria have a 

 wide range of habitat, and may be either relics from a more moist 

 condition, or pioneers on soil constantly becoming more xerophytic 

 at the surface. Ceratodon and Physcomitrium are doubtless sub- 



