466 BOTAXICAL GAZETTE [tone 



with other plants begins, but is not of great importance; while 

 that with shifting sand has nearly ceased. Whether the moss 

 flora of the transition conifer-deciduous regions resembles more 

 nearly that of the former or of the latter type seems to depend 

 chiefly on local conditions, such as adjacent bodies of water and 

 exposure to winds, greater humidity tending to increase the growth 

 of mosses, and a high evaporation rate bringing about their destruc- 

 tion. In the oak dunes the higher evaporation leads to elimination 

 of the relic species, while it may also lead to the appearance of new 

 xerophytic types. Competition with other plants, especially vernal 

 herbs, becomes a deciding factor, while that of moving sand may be 

 omitted from consideration. 



Morainal clay successions. — The early stages of moss 

 succession on morainic drift were studied near Glencoe, Illinois. 

 On newly eroded bluffs along Lake Michigan mosses are absent, 

 and in fact do not appear until after other vegetation has begun to 

 take possession and the surface is no longer subject to very active 

 erosion or slumping. On slopes partly covered with Juniper us 

 communis, with or without Thuja occidentalism mosses, while con- 

 spicuous, do not form a mat of large extent. The species are almost 

 identical with those on sand at Miller. Anomodon rostratus, Thelia 

 Lescurii, and Thuidium delicatulum are the most common. The 

 same similarity on dune sand and morainic clay bluffs has been 

 noted by Cowles (3) for the higher plants. Neither do mosses 

 appear in the early stages of ravines while vertical erosion is active. 

 In later stages, however, they become important and may take no 

 inconsiderable part in stabilization of the surface. Unfortunately 

 it was not possible to study ravines of all degrees of mesophytism, 

 so that the exact period at which mosses appear was not deter- 

 mined. Most of the work was done in ravines having sides of 

 rather gradual slope covered with a subclimax forest and meso- 

 phytic undergrowth. A vertical succession, not so evident on the 

 dune slopes, is here a noticeable feature. In one such ravine 

 Polytrichum commune is conspicuous among the arbor vitae at 

 the top. Just below this is a good display of Catharinea undulata. 

 About midway down the slope is a mixture of mesophytic species 

 such as Bartramia pomiformis, Dicranella heteromalla, Anomodon 



