478 BOTANICAL GAZETTE [june 



The moss successions on rock surfaces may be summarized 

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

 are of special importance in accounting for the better moss develop- 

 ment 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, 

 as soil formers, to form a habitat for herbaceous plants, to initiate 

 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 

 to allow penetration of the sun's rays, even during the summer. 

 Because of grazing there is no shrubby undergrowth. Here are 

 such mosses as Catharinea undulata, Leucobryum glaucum, Cera- 

 todon purpureas, Funaria hygrometrica, Polytrichum 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 usually found only in the mesophytic 

 forest, is probably a relic from a previous period of greater meso- 

 phytism. Polytrichum, while often found in rather dry places, 

 seems usually to originate in mesophytic or even swampy habitats, 

 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- 



