Rosendahl[783] has recently made observations as to the presence of the oxalate in the thallus of the brown Parmeliae. Of the fourteen species examined by him, eleven contained calcium oxalate as octahedral crystals or as small prisms, often piled up in thick irregular masses. Usually the crystals were located in the medullary part of the thallus, but in two species, Parmelia verruculifera and P. papulosa, they were abundant on the surface cells of the upper cortex.
c. Importance of Calcium Oxalate to the Lichen Plant. It is natural to conclude that a substance of frequent occurrence in any group of plants is of some biological significance, and suggestions have not been lacking as to the value of oxalic acid or of calcium oxalate in the economy of the lichen thallus. Oxalic acid is known to be one of the most efficient solvents of argillaceous earth and of iron oxides likely to be in the soil. These materials are also conveyed to the thallus as air-borne dust, and would thus, with the aid of the acid, be easily dissolved and absorbed. As a direct proof of this, Knop[784] has stated that lichen-ash always contains argillaceous earth. According to Kratzmann[785], aluminium, a product of clay, is stored up in various lichens. He proved the amount in the ash of Umbilicaria pustulata to be 4·46 per cent., in Usnea barbata 1·79 per cent., in U. longissima considerable quantities while in Roccella tinctoria it occurred in great abundance. It was also abundant in Diploschistes scruposus, 28·17 per cent.; it declined in Variolaria (Pertusaria) dealbata to 7·77 per cent., in Cladonia rangiferina to 1·76-2·12 per cent. and in Ramalina fraxinea to 1·8 per cent.
Calcium oxalate is directly advantageous to the thallus by virtue of the capacity of the crystals to reduce or prevent evaporation, as has been pointed out by Zukal[786]. A like service afforded by crystals to the leaves of the higher plants in desert lands has been described by Kerner[787]. These are frequently encrusted with lime crystals which allow the copious night dews to soak underneath them to the underlying cells, while during the day they impede, if they do not altogether check, evaporation.
Calcium oxalate crystals are insoluble in acetic acid, soluble in hydrochloric acid without evolution of gas; they deposit gypsum crystals in a solution of sulphuric acid.
C. Oil-Cells
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]).
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.
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.