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. Pertusaria communisTlC. Ver- T?n<!PndahI» has rerentlv made nhspr- 



ticai section of thallus. a, cortex; b, Kosenoani nas recently maae oDser 



gonidia; c, medulla; d, crystal of cal- vations as to the presence of the Oxalate 



ciuni oxalate, xca. lOO. • ,i ., 11 /■ ^i i_ r> /• 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 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" has stated that lichen-ash always contains argillaceous 

 earth. According to Kratzmann', aluminium, a product of clay, is stored 

 up in various lichens. He proved the amount in the ash of Umbilicaria 



' Braconnot 1825. ^ Zopf 1907. ^ grrera 1893. * Euler 1908, p- 7. 



5 Rosendahl 1907. ^ Knop 1872. ' Kratzmann 1913. 



