472 BOTAXICAL GAZETTE [juxe 



the rocks have been long exposed, only very depauperate specimens 

 of these same species occur. The later stages of the rock suc- 

 cession are absent. All of these places, with the exception of 

 Stony Island, are surrounded by agricultural lands, and whatever 

 has been the natural fate of this series has been too nearly obliter- 

 ated by man to allow of its determination. At Stony Island the 

 top of the rock is covered with prairie vegetation. The presence 

 of a few oak trees seems to indicate that without the intervention 

 of man the grasses would have been followed by an oak forest. 

 The conditions at Lemont may have been much the same. In the 

 ravines themselves the mosses belong almost without exception 

 to the Hypnaceae and are without sporophytes, and hence are 

 difficult to determine. Brachythecium digastrum is a rather com- 

 mon species. 



The Carroll Creek ravine, where humidity is much greater 

 and there is considerable seepage of moisture over the rock sur- 

 face, is a much more favorable habitat for mosses than are the 

 rock outcrops in the Chicago region. The number of species is 

 not large, but those which do occur are plentiful and they form a 

 thick covering over the rocks. Wherever the stream comes in 

 contact with the rocks, and in other very moist places, liverworts 

 are the first plants. Above the liverwort zone, or on rocks less 

 closely in contact with the water, is the zone of crustose lichens. 

 These are usually followed by foliose lichens, although quite 

 often the pioneer mosses may succeed the crustose and contend for 

 possession with the foliose lichens. The first moss is Grimmia 

 apocarpa. On rocks in the open, exposed to strong insolation the 

 greater part of the day, this species is abundant both on horizontal 

 and vertical surfaces. Accompanying this is Bryum argenteum, 

 which may occur almost if not quite as early, and in even greater 

 quantity, particularly on horizontal surfaces. 



This region offers the best illustration of a very definite suc- 

 cession of mosses on mosses. Here a second or even third moss 

 stage is common and may occur on rocks in the open as well as on 

 those in mesophytic shaded places in the ravine. The species 

 which constitute the later stages differ in the two situations. In 

 sunny places Bryum argenteum frequently forms the second stage. 



