The Role of the Terrestrial Alga in Nature 209 



sula. A rich flora of diverse green forms sometimes occurs on trees 

 and woodwork,** but as a rule the growth is very monotonous. 



In respect to their role in nature, the rock-inhabiting forms 

 are of special interest. Blue-green Algae in damp, mountainous 

 tracts in temperate regions give one some idea of their cap- 

 abilities for growth on rock surfaces,"' " but, in order to realize 

 their full possibilities, one must observe them in the damp Trop- 

 ics. The growths there have been familiar since Welwitsch's 

 description,*^ in 1868, of the "Pedras Negras" or black rocks of 

 Pungo Andongo in Angola, where the dark color is caused by 

 the prolific growth of Scytonema myochrous (Dillw.) Ag. var. 

 chorographicum West. Bews,* p. 553, has described similar ex- 

 tensive coverings of Blue-green Algae from the Drakensberg 

 region in Natal, where the communities of diverse Myxophyceae 

 "cover enormous areas of the cliffs, especially the cave-sandstone 

 cliffs and are strikingly apparent from a distance as longitudinal 



black strips During the very dry weather the algal covering 



dries up and peels from the rock in small flakes, but it does not 

 disappear to any great extent or leave the rock again bare" (cf. 

 also ^*), in this respect differing from the growth that Welwitsch 

 has recorded in Angola. I myself have described similar phenom- 

 ena on a smaller scale in Ceylon,^"* and McLean,^"" speaking of 

 the Pedra da Gavea, west of Rio de Janeiro, says, "The larger 

 part of the rock-surface is covered by blue-green Algae which 

 form great dark stripes upon it!' He also records "a species of 

 Trentepohlia (T. aurea) which is extremely abundant on the 

 damper patches'.' The so-called "Tintenstriche" recorded by 

 diverse workers on the rocks and precipices of various mountain- 

 ous tracts in Europe,"' "' " are caused by similar growths of Blue- 

 green Algae along vertical lines down which there is a more or 

 less constant trickle of water. 



