210 E E. Fritsch 



Such dense and extensive mats o£ aerial Algae are usually 

 supposed to play an active part in rock erosion, and there is every 

 reason to believe that this is correct, but at present there is litde 

 positive evidence of it. The peeling off of the growth v^^hich 

 Welwitsch, Bews, and others describe as occurring regularly in 

 dry weather may well act on the rock surface in a way similar 

 to that recorded by Frye^ for certain Lichens, but exact investi- 

 gation is required. It is not known whether in all such occur- 

 rences a direct corroding eflFect may be exerted on the under- 

 lying rock with the help of substances excreted from the Alga, 

 as has been established for Myxophyceae inhabiting calcareous 

 rocks (cf. below). On horizontal rock surfaces the dense wefts 

 or sheets (Phormidium) of Blue-green Algae are bound sooner 

 or later to provide a sufficient thickness of humus for the growth 

 of Bryophyta, Ferns, and small Phanerogams, and sometimes this 

 no doubt also occurs in connection with more or less vertical 

 substrata. When adequate moisture is available for a consider- 

 able length of time, successive phases of development of these 

 mats may be traced ;^^'^^ a sufficiently thick growth is established 

 in a short space of time to afford a starting point for the develop- 

 ment of higher types of vegetation. In short, although no doubt 

 the rigors of the habitat often prevent development beyond the 

 algal mat, there is every reason to suppose that a similar growth 

 preceded the establishment of Phanerogamic vegetation in the 

 more sheltered valleys and on the lower slopes of the cliffs where 

 such growths are now observable on the upper parts. 



On calcareous rock surfaces in the Alps and the Dolomites, 

 Bachmann^ and Diels" have recorded both an epilithic and an 

 endolithic community of Algae, composed in large part of 

 Chroococcales with occasional filamentous Blue-green forms and 

 species of Trentepohlia (cf. also^*). The epilithic community is 



