i 9 2o] TAYLOR— SUCCESSION OF MOSSES 469 



plants is no doubt an important factor on many such slopes, as 

 they offer conditions increasingly favorable to other ground flora. 

 Erosion decreases in importance as a determining factor in pro- 

 portion as the mesophytism increases. When the ravine reaches 

 its second denudation period, accompanied by greater sunlight 

 and evaporation, the mesophytic mosses are eliminated along 

 with the other mesophytic undergrowth; but these may reappear 

 when the slope has once more attained a relatively permanent 

 condition, and continue on until the climax association is reached, 

 or may even persist into this association if logs and stumps are 

 present. 



In the open oak forests the moisture supply in air and soil 

 probably is again largely the controlling condition, as in the oak- 

 forests on dune sand. Other plants do not occupy the ground to 

 so great an extent as to exclude mosses because of lack of space 

 alone, and there is little probability that the mosses would be- 

 come shaded to a sufficient degree to shut out the light and prevent 

 the necessary photosynthetic work. Just why there is so great 

 a scarcity of mosses in the more mesophytic oak or oak-hickory 

 forests, as well as in the beech-maple climax, both of which pro- 

 vide relatively high humidity and low evaporation rate (6), has 

 not been fully determined. Competition with other plants may 

 be accountable to a great extent, but even this does not seem 

 sufficient to cause the almost complete elimination of mosses from 

 these forests. In some places there is a continuous succession of 

 dense ground vegetation during most of the growing season, which 

 might be able to prevent the development of mosses ; but in other 

 places the vernal flora does not seem to be followed by a con- 

 spicuous aestival flora, yet mosses are not present. Perhaps the 

 competition with the vernal flora in its prime, when most mosses 

 attain their greatest growth, may be sufficient to prevent both 

 spore germination and vegetative growth at this time, so that 

 presence or absence of ground vegetation later in the year is of 

 little consequence. The fact that when old logs are present, 

 mosses are common upon them when not found on the ground, 

 would indicate that they had not been able to hold their own 

 against the herbaceous plants. Another factor which may have a 



