_a._ OIL-CELLS OF ENDOLITHIC LICHENS. Calcicolous immersed lichens are able to dissolve the lime of the substratum, and their hyphae penetrate more or less deeply into the rock. In some forms the entire thallus may thus be immersed, the fruits alone being visible on the surface of the stone. In two such species, _Verrucaria calciseda_ and _Petractis_ (_Gyalecta_) _exanthematica_, Steiner[788] detected peculiar sphaeroid or barrel-shaped cells that differed from the other hyphal cells of the thallus, not only in their form, but in their greenish-coloured contents. Similar cells were found by Zukal[789] in another immersed (endolithic) lichen, _Verrucaria rupestris_ f. _rosea_. He describes them as roundish organs crowded on the hyphae and filled with a greenish shimmering protoplasm. He[790] found the same types of sphaeroid and other swollen cells in the immersed thallus of several calcicolous lichens and he finally determined the contents as fat in the form of oil. He found also that these fat-cells, though very frequent, were not constantly present even in the same species. His observations were confirmed by Hulth[791] for a number of allied crustaceous lichens that grow not only on limestone but on volcanic rocks. In them he found a like variety of fat-cells—intercalary or torulose cells, terminal sphaeroid cells and hyphae containing scattered oil-drops. Bachmann[792] followed with a study of the thallus of purely calcicolous lichens. The specialized oil-cells were fairly constant in the species he examined, and, as a rule, they were formed either in the tissues immediately below, or at some distance from, the gonidial zone. Fünfstück[793] has also published an account of various oil-cells in a large series of calcicolous lichens (Fig. 117). [Illustration: Fig. 117. _Lecidea immersa_ Ach. A, sphaeroid fat-cells from about 8 mm. below the surface × 550. B, oil-hyphae in process of emptying: _a_, sphaeroid cells containing oil; _b_, cells with oil-globules × 600 (after Fünfstück).] The occurrence of oil-(or fat-)cells is not dependent on the presence of any particular alga as the gonidium of the lichen. Fünfstück[794] has described the immersed thallus of _Opegrapha saxicola_ as one of those richest in fat-cells. The gonidia belong to the filamentous alga _Trentepohlia umbrina_ and form a comparatively thin layer about 160µ thick near the upper surface; isolated algal branches may grow down to 350µ into the rock, while the fungal elements descend to 11·5 mm., and though the very lowest hyphae were without oil—as were those immediately beneath the gonidia—the interlying filaments, he found, were crowded with oil-cells. Sphaeroid terminal cells were not present.