. Lichens. Lichens. 214 PHYSIOLOGY plants the crystals are formed within the cell, but in lichens they are always deposited on the outer surface of the hyphal membranes, mainly of the medulla and the cortex. Calcium oxalate was first detected in lichens by Henri Braconnot\ who extracted it by treating the powdered thallus of a number of species {Pertu- saria communis, Diploschistes scruposus, etc.) with different reagents. The quantity present varies greatly in lichens: ZopP found that it was abundant in all the species inhabiting limestone, and states that in such plants the more purely liche
. Lichens. Lichens. 214 PHYSIOLOGY plants the crystals are formed within the cell, but in lichens they are always deposited on the outer surface of the hyphal membranes, mainly of the medulla and the cortex. Calcium oxalate was first detected in lichens by Henri Braconnot\ who extracted it by treating the powdered thallus of a number of species {Pertu- saria communis, Diploschistes scruposus, etc.) with different reagents. The quantity present varies greatly in lichens: ZopP 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' 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' 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, Haema- tomma coccineum., H. ventosum, Lecanora saxicola, Lecanora tdrtarea, etc., contain large amounts either in the form of octa- hedral crystals or as small granules. Fig. 116. PertusariacommunisTlC. Ver- T?n /• r\c m the thallus of the brown Farmeiiae. 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 o
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