218 PHYSIOLOGY 



thallus of Sarcogyne (Biatorella) pruinosa Lang 1 estimated the gonidial zone 

 as I75-2OO//, 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 2 in Aspicilia (Lecanora) calcarea, 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 8 , a specimen had grown which of 

 necessity was epilithic. The cortex and gonidial 

 Fig. 120. Lecanora (Aspi- zone together were 40 fjL thick; immediately below 

 cilia) cah-arca Sommerf. there were hyphae with irregular cells free from oil ; 



Early stage of sphaeroid 



cell formation x 175 (after lower still there was formed a compact tissue of 

 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 8 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 4 are 

 typical sphaeroid cells formed. 



More or less specialized oil-cells have been demonstrated by Funfstiick 8 

 in several superficial (epilithic) lichens which grow on a calcareous sub- 

 stratum, as for instance Lecanora (Placodium) decipiens, Lecanora crassa and 

 other similar species. The oil in these lichens is usually restricted to more or 

 less swollen or globose cells; but it may also be present in the ordinary 

 hyphae as globules. Zukal 6 found that the smooth little round granules 

 sprinkled over the thallus of the soil-lichens, Baeomyces roseus and B. rufus, 

 contained in the hyphae typical sphaeroid oil-cells and that they -were 

 specially well developed in specimens from Alpine situations. In still another 

 soil-lichen, Lecidea granulosa, shimmering green oil was found in short-celled 

 torulose hyphae. 



Rosendahl's 7 researches on the brown Parmeliae resulted in the unex- 



1 Lang 1906, p. 171. 3 Bachmann 1892. 3 Bachmann I9O4 1 . * Bachmann 1904". 



6 FUnfstiick 1895. 6 Zukal 1895. p. 1372. 7 Rosendahl 1907. 



