i 9 2o] TAYLOR— SUCCESSION OF MOSSES 477 



remains upon these north facing slopes. In places sheltered from 

 the warm spring sunlight the snows melt slowly, and the moisture 

 soaks into the humus instead of running or! rapidly, as it must do 

 on such an incline when the snow melts more quickly. It is well 

 known that in general the moss flora becomes more conspicuous 

 as we go north into the cold temperate regions. This condition 

 is comparable to that of the northern habitats where much of the 

 snow disappears under the action of sunlight and not of rains. 

 Since these slopes are exposed to a lower degree of insolation even 

 during the summer, the mosses are never subject to extreme desic- 

 cation. This cannot be true of the rock habitats which lie within 

 the Chicago region. 



The great economic importance of such a moss covering is 

 demonstrated by the growth of seedlings of higher plants upon 

 the moss mat, which leads to the initiation of the tree associ- 

 ations. Herbaceous plants grow to maturity and produce seed 

 on moss covered rocks, with the roots obtaining nutriment only 

 from the decayed moss material. The slower growing tree seed- 

 lings can exist in a like manner for several years, by which time 

 their roots may be able to penetrate through the crevices or 

 between the rocks to the soil below. Mosses are very hygro- 

 scopic and quickly absorb water during rains, but give it up 

 slowly. Several days after rains water can be pressed from these 

 mosses even though seepage is not an important factor. In 

 addition to this is the immense value of a moss covering on rock 

 slopes to conserve the water supply and prevent flooding of the 

 adjacent land along the lower course of the streams. The great 

 value of mosses in relation to the conservation of moisture and 

 their effect upon the soil was observed by Oltmanns (8) . He says : 



A moss carpet acts as a sponge. A dense low carpet with countless capil- 

 lary spaces between leaves and rhizoids absorbs capillary and superficial water, 

 but obtains little or none by suction from soil and internal conduction. Con- 

 sequently living and dead carpets of moss imbibe and evaporate approximately 

 the same amount of water. A carpet of moss does not desiccate the soil .... 

 they dry it to a less degree than does other vegetation, and they protect dry 

 easily heated soil from desiccation. 



Evans and Nichols (5) also discuss the economic value of 

 mosses in such situations. 



