212 E E. Eritsch 



are also found on rather softer dry rocks subjected to full insola- 

 tion. When shaded, these softer dry rocks bear a blue-green 

 growth (Gloeocapsa spp., Tolypothrix byssoidea f. saxicola, Stig- 

 oneina minutum) which, with the difference noted above, is 

 found alike on siliceous and calcareous rocks of a certain degree 

 of hardness. Ercegovic^^ regards Scytonema myochrous and 

 Calothrix parietina as photolithophytes because they always 

 occur on places exposed to strong sun. Diatoms rarely form con- 

 spicuous growths on relatively dry rock surfaces. Schade," how- 

 ever, records yellowish brown coverings caused by Eragilaria 

 capucina Desm. (cf. also ^). 



Apart from obvious algal coverings, Falger" has shown that 

 Algae may also be found on rock surfaces that afford no macro- 

 scopically visible evidence of growth. He suggests that these 

 primary colonizers prepare the way for the more obvious 

 growths. Again Myxophyceae are dominant, although some 

 Chlorophyceae and Diatoms are also found. Sandstone is much 

 richer than other kinds of rocks and here, as in Fremy's observa- 

 tions, a direct effect of the degree of porosity of the substratum, 

 implying a varied retention of moisture, is evident. Adequate 

 data on the moisture content of exposed rock surfaces are, how- 

 ever, at present scarcely available, although Schade" has calcu- 

 lated that in sandstone the water content may amount to 12 per 

 cent of the weight of the rock. 



The forms inhabiting trees are probably of less importance in 

 the economy of nature, but physiologically are perhaps the most 

 highly adapted terrestrial Algae. The faculty that Pleurococcus 

 possesses of withstanding extreme and prolonged drought and 

 other unfavorable conditions is almost proverbial. According to 

 Schmid,*^ this Alga does not stand continuous immersion in 

 water and normally satisfies all its moisture requirements by 



