470 BOTANICAL GAZETTE [jcne 



decided influence is that of the chemical change in the soil due 

 to increase of humus. Just what the difference is which seems 

 favorable to the germination of the seedlings of the climax trees 

 and not to those of the former association, and how much of this 

 difference is chemical and how much physical and related to 

 light, are questions for future solution. Whatever it is, it would 

 probably affect mosses as well as other plants. That an acid 

 condition of the substratum alone is not detrimental is indicated 

 by the luxuriant growth of many species on decaying wood and 

 upon needles of conifers. 



The great abundance of mosses in the upland oak forests along 

 the Des Plaines River seems to be related to the slightly greater 

 humidity of the atmosphere and larger supply of available soil 

 moisture. There are indications that much of this region has 

 been and still is at certain seasons somewhat swampy, so that 

 there may be some question whether it belongs in the xerarch 

 succession proper or should be placed in the hydrarch swamp 

 series. While the final outcome would be the same in the two 

 series, the intermediate successions would differ to a very large 

 degree. The presence of the relic species in the grazed woods 

 or partially cut-over land seems to be explainable by the fact 

 that they are mosses of wide extremes of habitat, and are highly 

 light tolerant. The change in environment appears to have taken 

 place so gradually that the mosses have been able to become 

 adapted to the greater xerophytism without themselves being 

 materially altered. 



The successions on morainic drift may be summed up in a 

 few points. Mosses are entirely absent on the newly eroded 

 bluffs and in the early stages of the ravines. They do not become 

 conspicuous in the ravines until a rather advanced state of 

 mesophytism has been reached, but they probably play an impor- 

 tant part in the stabilization of the clay surface and addition of 

 humus, which hasten the advance of the seed plants. Mosses 

 appear in the conifer stage on the bluffs, forming part of the heath 

 mat under the juniper. They are most abundant in the middle 

 aged ravines, before the second xerophytic stage is initiated by the 

 widening of the ravine and decrease of the angle of the slope. 



