_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[798] 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 rhizoidal hyphae (Fig. 120). On a non-calcareous brick substratum[799], a specimen had grown which of necessity was epilithic. The cortex and gonidial zone together were 40µ thick; immediately below there were hyphae with irregular cells free from oil; 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 impenetrable substance. [Illustration: Fig. 120. _Lecanora_ (_Aspicilia_) _calcarea_ Sommerf. Early stage of sphaeroid cell formation × 175 (after Bachmann).] 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[799] 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[800] are typical sphaeroid cells formed. More or less specialized oil-cells have been demonstrated by Fünfstück[801] in several superficial (epilithic) lichens which grow on a calcareous substratum, 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[802] 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.