76 MORPHOLOGY 



c. CHEMICAL NATURE OF THE SUBSTRATUM. Lichens growing on 

 calcareous rocks or soils are more or less endolithic, those on siliceous 

 rocks are largely epilithic, but Bachmann 1 found that the mica crystals in 

 granite were penetrated, much in the same way as limestone, by the lichen 

 hyphae. These travel through the mica in all directions, though they tend 

 to follow the line of cleavage, thus taking the direction of least cohesion. 

 He found that oil-hyphae were formed, and also certain peculiar bristle-like 

 terminal branches; in other cases there were thin layers of plectenchyma, and 

 gonidia were also present. If however felspar or quartz crystals, no matter 

 how thin, blocked the way, further growth was arrested, the hyphae being 

 unable to pierce through or even to leave any trace on the quartz 2 . On 

 granite containing no mica constituents the hyphae can only follow the 

 cracks between the different impenetrable crystals. 



Stahlecker 3 has confirmed Bachmann-'s observations, but he considers 

 that the difference in habit and structure between the endolithic and 

 epilithic series of lichens is due rather to the chemical than to the physical 

 nature of the substratum. Thus in a rock of mixed composition such as 

 granite, the more basic constituents are preferred by the hyphae, and are 

 the first to be surrounded: mica, when present, is at once penetrated; 

 particles of hornblende, which contain 40 to 50 per cent, only of silicic 

 acid, are laid hold of by the filaments of the lichen before the felspar, of 

 which the acid content is about 60 per cent.; quartz grains which are pure 

 silica are attacked last of all. though in the course of time they also become 

 corroded. 



The character of the substratum also affects to a great extent the 

 comparative development of the different thalline layers: the hyphal tissues 

 in silicicolous lichens are much thinner than in lichens on limestone, and 

 the gonidial zone is correspondingly wider. In a species of Staurothele on 

 granite, Stahlecker 8 estimated the gonidial zone to be about 600 //, thick, 

 while the lower medullary hyphae, partly burrowing into the rock, measured 

 about 6 mm. Other measurements at different parts of the thallus gave a 

 rhizoidal depth of 3 mm., while on a more finely granular substratum, with 

 a gonidial zone of 350/11, the rhizoidal hyphae measured only imm. On 

 calcareous rocks, on the contrary, with a gonidial zone that is certainly no 

 larger, the hyphal elements penetrate the rock to varying depths down to 

 1 5 mm. or even more. 



Lang 4 has recorded equally interesting measurements for Sarcogyne 

 (Biatorella) latericola-. on slaty rock which contained no mixture of lime, 

 the gonidial zone had a thickness of 80 /it, a considerable proportion of the 

 very thin thallus. Funfstiick 5 has indeed suggested that this lichen on acid 



1 Bachmann 1904. 2 Bachmann 1904. 3 Stahlecker 1906. 



4 Lang 1903. 8 Fiinf stilck 1899. 



