Fünfstück[794] has re-examined the thallus of _Petractis exanthematica_, an almost wholly immersed lichen with a gelatinous gonidium, a species of _Scytonema_. The thallus is homoiomerous: the alga forms no special zone, it intermingles with the hyphae down to the very base of the thallus; the hyphae are extremely slender and at the base they measure only about 1µ in width. Oil-cells are abundant in the form of intercalary cells about 3-5µ in thickness. Nearer the surface sphaeroid cells are formed on short lateral outgrowths; they measure 14-16µ in diameter and occur in groups of 15 to 20. The superficial part of the thallus is a mere film; the hyphae composing it are slightly stouter and more thickly interwoven. 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. [Illustration: 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).] [Illustration: 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).]