374 ECOLOGY 



calcareous rocks. Collemopsidium is the only Xanthocapsa associate that is 

 silicicolous. 



d. SILICICOLOUS. There is greater variety in the mineral composition 

 and in the nature of the surface in siliceous than in calcareous rocks ; they are 

 also more durable and give support to a large number of slow-growing forms. 



Silicon enters into the composition of many different types, from the 

 oldest volcanic to the most recent of sedimentary rocks. Some of these are 

 of hard unyielding surface on which only a few lichens are able to attach 

 themselves. Such a rock is instanced by Servit 1 as occurring in Bohemia, 

 and is known as Lydite or Lydian stone, a black flinty jasper. The associa- 

 tion of lichens on this smooth rock was almost entirely Acarospora chloro- 

 phana and Rinodina oreina, which as we shall see occur again as a "desert" 

 association in Nevada; these two lichens grow equally well in sun or shade, 

 and either sheltered or exposed as regards wind and rain. Acarospora chloro- 

 pliana, according to Malinowski 2 , arrives among the first on rocks newly 

 laid bare, and forms large societies, though in time it gives place to Lecanora 

 glaucoma (L. sordida}, a common silicicolous lichen. 



A difference has been pointed out by Bachmann 3 between the lichens 

 of acid and of basic rocks. The acid series, such as quartz- and granite- 

 porphyry, contain 70 per cent, and more of oxide of silica ; the basic diabase 

 and basalt not nearly 50 per cent. He observed that Rhizocarpon geographi- 

 cum was the most frequent lichen of the acid porphyry, while on basalt there 

 were only small scattered patches. Pertusaria corallina was abundant only 

 on granitic rocks. On the other hand Pertusaria lactea f. cinerascens, Diplo- 

 scliistes scruposus, D. bryophilus and Buellia leptocline preferred the basic sub- 

 stratum of diabase and basalt. In this case it is the chemical rather than the 

 physical character of the rocks that affects the lichen flora, as porphyry and 

 basalt are both close-grained, and are outwardly alike except in colouration. 



Other rocks, such as granite, in which the different crystals, quartz, mica 

 and felspar are of varying hardness, are favourite habitats as affording not 

 only durability but a certain openness to the rhizoidal hyphae, though in 

 Shetland, West* found the granitic rocks bare owing to their too rapid 

 weathering. In these rocks the softer basic constituents such as the mica are 

 colonized first; the quartz remains a long time naked, though in time it 

 also is covered. Wheldon and Wilson 5 point out that the sandstone near to 

 intrusive igneous rocks has become close-grained and indurated and bears 

 Lecanora squamulosa, L. picea, Lecidea rivulosa and Rhizocarpon petraeum, 

 which were not seen on the unaltered sandstone. It was also observed by 

 Stahlecker 6 , that, in layered rocks, the lichen chose the surface at right 

 angles to the layering as the hyphae thus gain an easier entrance. 



1 Servit 1910. 2 Malinowski 1911. 3 Bachmann 1914. 4 West 1912. 



5 Wheldon and Wilson 1913. * Stahlecker 1906. 



