. Lichens. Lichens. 2i8 PHYSIOLOGY thallus oi Sarcogyne {Biatorelld) pruinosa Lang- estimated the gonidial zone as 175-200/4 in thickness, while the colourless hyphae penetrated the rock to a depth of quite 15 mm. b. Oil-cells of Epilithic Lichens. The general arrangement of the tissues and the occurrence and form of the oil-cells vary in the different species according to the nature of the substratum. This has been clearly demonstrated by Bachmann^ in Aspicilia {Lecanord) calcared, an almost exclusively calcareous lichen as the name implies. On limestone, he found sphaeroid cells formed in gr
. Lichens. Lichens. 2i8 PHYSIOLOGY thallus oi Sarcogyne {Biatorelld) pruinosa Lang- estimated the gonidial zone as 175-200/4 in thickness, while the colourless hyphae penetrated the rock to a depth of quite 15 mm. b. Oil-cells of Epilithic Lichens. The general arrangement of the tissues and the occurrence and form of the oil-cells vary in the different species according to the nature of the substratum. This has been clearly demonstrated by Bachmann^ in Aspicilia {Lecanord) calcared, an almost exclusively calcareous lichen as the name implies. On limestone, he found sphaeroid cells formed in great abundance on the "deeply penetrating rhi- zoidal hyphae (Fig. 120). On a non-calcareous brick substratum*, a specimen had grown which of necessity was epilithic. The cortex and gonidial Fig. i2o. Lccanora {Aspi- zone together were 40/* thick; immediately below cilia) calcarea Sommerf^ there were hyphae with irregular cells free from oil; Early stage of sphaeroid â ' ^ ° cell formation X175 (after lower Still there was formed a compact tissue of ac mann). globose fat-cells. In this case the calcareous lichen still retained the capacity to form oil-cells on the non-calcareous impene- trable substance. Very little oil is formed, as a rule, in the cells of siliceous crustaceous lichens which are almost wholly epilithic, but Bachmann found a tissue of oil-cells in the thallus of Lecanora caesiocinerea; from Labrador, on a granite composed of quartz, orthoclase and traces of mica. A thallus of the same species collected in the Tyrol, though of a thicker texture, contained no oil. Bachmann* suggests no explanation of the variation. On granite, rhizoidal hyphae penetrate the rock to a slight extent between the different crystals, but only in connection with the mica'' are typical sphaeroid cells formed. More or less specialized oil-cells have been demonstrated by FUnfstiick' in several superficial (epilithic) lichens which grow on a calcareous sub- stratum, as for instance
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