_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