Bachmann[795] and Lang[796] have further described the anatomy of endolithic thalli especially with reference to oil-cells, and have supplemented the researches of previous workers. New methods of cutting the rock in thin slices and of dissolving away the lime enabled them to see the tissues in their relative positions. In these immersed lichens, as described by them and by previous writers, and more especially in calcicolous species, the gonidial zone of Protococcaceous algae lies near the surface of the rock, and is mingled with delicate, thin-walled hyphae which usually do not contain oil. The more deeply immersed layer is formed of a weft of equally thin-walled hyphae, some of the cells of which are swollen and filled with fat globules. These oil-cells may occur at intervals along the hyphae or they may form an almost continuous row. In addition, strands or bundles of hyphae ([Fig. 118]) containing few or many oil globules traverse the tissue, and true sphaeroid cells are generally present. These latter arise in great numbers on short lateral branchlets, usually near the tip of a filament and the groups of cells are not unlike bunches of grapes. Sometimes the oil-cells are massed together into a complex tissue. Hyphae from this layer pierce still deeper into the rock and constitute the rhizoidal portion of the thallus. They also produce sphaeroid oil-cells in great abundance ([Fig. 119]). In the immersed thallus of Sarcogyne (Biatorella) pruinosa Lang[797] estimated the gonidial zone as 175-200µ in thickness, while the colourless hyphae penetrated the rock to a depth of quite 15 mm.
Fig. 118. Biatorella (Sarcogyne) simplex Br. and Rostr. a, sphaeroid oil-cells; b, strand of oil-hyphae from 10-15 mm. below the surface. × 585 (after Lang).
Fig. 119. Biatorella pruinosa Mudd. a, complex of sphaeroid oil-cells from 10 mm. below the surface; b, hypha of sphaeroid cells also from inner part of the thallus. × 585 (after Lang).
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.
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.