Calcium oxalate was first detected in lichens by Henri Braconnot[779], who extracted it by treating the powdered thallus of a number of species (_Pertusaria communis_, _Diploschistes scruposus_, etc.) with different reagents. The quantity present varies greatly in lichens: Zopf[780] found that it was abundant in all the species inhabiting limestone, and states that in such plants the more purely lichenic acids are relatively scarce. Errera[781] has calculated the amount of calcium oxalate in _Lecanora esculenta_, a desert lime-loving lichen, to be about 60 per cent. of the whole substance of the thallus. Euler[782] gives for the same lichen even a larger proportion, 66 per cent. of the dry weight. In _Pertusaria communis_, a corticolous species, the oxalate occurs as irregular crystalline masses in the medulla (Fig. 116) and has been calculated as 47 per cent. of the whole substance. Other crustaceous species such as _Diploschistes scruposus_, _Haematomma coccineum_, _H. ventosum_, _Lecanora saxicola_, _Lecanora tartarea_, etc., contain large amounts either in the form of octahedral crystals or as small granules. [Illustration: Fig. 116. _Pertusaria communis_ DC. Vertical section of thallus. _a_, cortex; _b_, gonidia; _c_, medulla; _d_, crystal of calcium oxalate. × ca. 100.] 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.