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 ■' ^ ° 



cell formation X 175 (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 Lecanora iPlacodium) 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. ZukaP 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' researches on the brown Parnieliae resulted in the unex- 



^ Lang 1906, p. 171. 2 Bachmann 1892. * Bachmann 1904'. ^ Bachmann 1904''. 



s Fiinfstuck 1895. " Zukal 1895, p. 1372. ' Rosendahl 1907. 



