The Role of the Terrestrial Alga in Nature 211 



confined to those points at which a slow trickle of water normally 

 descends the rock face, whereas the endolithic community ap- 

 pears to be of two types. The type described by Diels inhabits 

 very narrow, outwardly not recognizable clefts in the rocks 

 which apparently owe their origin to atmospheric agencies. 

 These aerial Algae may be described as chasmolithophytes." 

 They differ in general from those found on die exposed surface 

 and are no doubt shade forms. On the contrary, the community 

 described by Bachmann and others" consists of Chroococcales 

 occupying small pits in the rock surface which are believed to 

 result from a direct solvent action of acid excreted by the Alga 

 on the calcareous substratum (true endolithophytes) . When 

 such growths are numerous, the surface layer of the rock to a 

 depth of 1.5 mm. becomes quite soft and porous, so that a direct 

 erosive action of the alga on the substratum is evident. But, even 

 in respect to the chasmolithophytes, one cannot doubt that the 

 presence of the Algae will lead to a gradual widening of the 

 clefts and progressive erosion of the surface.^ 



On the whole, the chemical nature of the underlying rock 

 does not appear to have much effect on the aerial algal commu- 

 nity, which is apparendy more influenced by the nature of the 

 surface, the amount of available moisture, and the intensity of 

 insolation. Fremy" has made a study of the composition of the 

 aerial algal growth on siliceous and calcareous rocks in Nor- 

 mandy, and mentions as the only striking difiference the absence 

 of Stigonema minutum on the calcareous as opposed to its fre- 

 quent occurrence on the siliceous rocks (cf. *°). He points out 

 that the hardness of the rock definitely influences the abundance 

 and nature of the algal growth. Hard, dry rocks bear no growth 

 at all when exposed to full insolation (cf.also"), but when shaded 

 they support Pleurococcus and Trentepohlia aurea, forms which 



