476 BOTANICAL GAZETTE [june 



rocks near the Des Plaines River, thrown out in straightening the 

 channel, have also been exposed for only a short time. It would 

 seem therefore that the recent exposure in some cases and the 

 distance from bodies of water sufficiently large to locally affect the 

 humidity may be two of the reasons for the poor development of 

 rupicole species. Another probably greater factor, at least for 

 Stony Island and Thornton, is the large amount of dust which 

 accumulates on vegetation, very effectually hindering photo- 

 synthetic work. At Stony Island there is much fine coal dust 

 from smokestacks and trains, as well as dust from factories. 

 At Thornton a large quantity of fine white dust thrown off from 

 the cement factory accumulates in a thin layer and forms almost 

 a crust, after light rains, on the foliage of all plants. There is less 

 dust at Lemont, where there is a somewhat better development of 

 mosses, but still much more than along Carroll Creek, which is 

 bordered only by forests and farm lands, and is far from any 

 factories. The later stages of succession on the rock outcropping 

 near Chicago, as stated before, have been greatly interfered with 

 by man. Evidently the change from pioneer conditions is ex- 

 tremely slow, and there is no development of true forest, so that 

 all moss stages beyond the pioneer are so far wanting. 



Returning once more to the Carroll Creek ravine, in great 

 contrast to the Chicago region there is a narrow valley flanked 

 by steep rock walls upon which direct sunlight falls for only a short 

 number of hours each day. That this has much to do with the 

 lower evaporation and higher humidity is indicated by the more 

 mesophytic undergrowth and the greater luxuriance of mosses on 

 all undisturbed north facing slopes. Whatever moisture enters 

 the air through evaporation from the stream will be carried away 

 slowly, since such a valley is well protected from winds. Another 

 condition which also points to the moisture from the water as 

 an important factor is that the greater growth of mesophytic 

 mosses is found at places where the stream in its meanderings 

 comes close to the rock wall, either on the north or south side of 

 the ravine, and that the mosses are more luxuriant than in other 

 places with a similar exposure but farther from the water. An 

 additional cause may be found in the length of time in which snow 



