204 WILLIAM S. COOPER 



The increasing mesophytism from the heath mat stage onward 

 is due, more than to any other factor, to the increasing abund- 

 ance of the three mosses just named, with its results, continual 

 additions to the humus accumulations, and effective conserva- 

 tion of moisture. Before they enter, the amount of humus upon 

 the rock surfaces is almost negligible; in the climax forest the 

 depth is sometimes a meter, and it is composed largely of moss 

 remains, together with rotten wood and tree waste of various 

 kinds. 



In discussion of the place of mosses in the Rock Shore Succes- 

 sion a stage must be recognized which in an account of the suc- 

 cession in general would more naturally be included with the 

 climax forest. In certain parts of the forest, particularly upon 

 the slopes of the ridges, boulders and small cliffs, remaining 

 from shore or talus conditions, are common. These are usually 

 covered with mosses, including a large number of species, toge- 

 ther with a few liverworts, lichens,- and higher plants. That 

 this is an intermediate stage in the succession, although within 

 the climax forest, is shown by the tendency of the climax mosses, 

 Calliergon Schreberi, Hylocomium proliferum, and Hypnum crista- 

 castrensis to extend their dominance at the expense of the less 

 mesophytic species. Certain of the rock surfaces, formerly bare, 

 are already covered with a thick carpet of the climax species, 

 and it is evident that upon the disintegration of the boulders and 

 cliffs this carpet will extend itself over all. The composition of 

 the moss population of habitats such as these is of interest. The 

 following list, made up from several localities, includes three 

 groups: (1) climax mosses; (2) relict species remaining from 

 earlier stages of the Rock Shore Succession; (3) forms that are 

 specially characteristic of this habitat, including some found in 

 no other. 



(1) (2) 



Culliergon Schreberi (Willd.) Grout. Hedwigia albicans (Web.) Lindb. 



Hylocomium proliferum (L.) Lindb. Orthotrichum anomalum Hedw. 



Hylocomium triquetrum (L.) B. & S. Swartzia montana (Lamk.) Lindb. 



Hypnum crista-caslrensislj. Thuidium abietinum (L.) B. & S. 



