c. Cellulose. Several workers claim to have found true cellulose in the cell-walls of the hyphal tissues of a few lichens; but the more careful analyses of Escombe[764], Wisselingh[762] and Wester[765] have disproved their results. The cell-walls of all the gonidia, however, are formed of cellulose, or according to Escombe of glauco-cellulose, except those of Peltigera in which Wester found neither cellulose nor chitin. Czapek[766] suggests that the blue reaction with iodine characteristic of the cell-walls in some apothecia, of the asci and of the hyphae in cortex or medulla in a few instances, may be due to the presence of carbohydrates of the nature of galactose. Moreau[767] in a recent paper terms the substance that gives a blue reaction with iodine at the tips of the asci “amyloid.” In Peltigera the ascus tip is occupied by such a plug of amyloid which at maturity is projected like a cork from the ascus and may be found on the surface of the hymenium.
B. Contents and Products of the Fungal Cells
a. Cell-substances. The cells of lichen hyphae contain protoplasm and nucleus with glucoses. It is doubtful if starch has been found in fungal hyphae; it is replaced, in some of the tissues at least, by glycogen, a carbohydrate (C₆H₁₀O₅) very close to, if not identical with, animal glycogen, a substance which is soluble in water and colours reddish-brown (wine-red) with iodine. Errera[768] first detected its presence in Ascomycetes where it is associated with the epiplasm of the cells, more especially of the asci, and he considered it to be physiologically homologous with starch. He included lichens, as Ascomycetes, in his survey of fungi and quotes, in support of his view that lichen hyphae also contain glycogen, a statement made by Schwendener[769] that “the contents of the ascogenous hyphae of Coenogonium Linkii stain a deep-brown with iodine.” Errera also instances the red-brown reaction with iodine, described by de Bary[770], as characteristic of the large spores of Ochrolechia (Lecanora) pallescens, while the germinating tubes of these spores become yellow with iodine like ordinary protoplasm. Glycogen has been, so far, found only in the cells of the reproductive system.
Iodine was found by Gautier[771] in the gonidia of Parmelia and Peltigera, i.e. both in bright-green and blue-green algae. The amount was scarcely calculable.
Herissey[772] claims to have established the presence of emulsin in a large series of lichens belonging to such widely separated genera as Cladonia, Cetraria, Evernia, Peltigera, Pertusaria, Parmelia, Ramalina, and Usnea. It is a ferment which acts upon amygdalin, though its presence has been proved in plants such as lichens where no amygdalin has been found[773]. Diastase was demonstrated in the cells of Roccella tinctoria, R. Montagnei and of Dendrographa leucophaea by Ronceray[774] who states that, in conjunction with air and ammonia, it forms orchil, the well-known colouring substance of these lichens. Diastatic ferments have also been determined[775] in Usnea florida, Physcia parietina, Parmelia perlata and Peltigera canina.
b. Calcium Oxalate. Oxalic acid (C₂H₂O₄) is an oxidation product of alcohol and of most carbohydrates and in combination is a frequent constituent of plant cells. Knop[776] held that it was formed in lichens by the reduction and splitting of lichen acids, though, as Zopf[777] has pointed out, these are generally insoluble. Hamlet and Plowright[778] demonstrated the presence of free oxalic acid in many families of fungi including Pezizae and Sphaeriae. The acid combines with calcium to form the oxalate (CaC₂0₄), which in the crystalline form is very common in lichens. In the higher 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[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.
Fig. 116. Pertusaria communis DC. Vertical section of thallus. a, cortex; b, gonidia; c, medulla; d, crystal of calcium oxalate. × ca. 100.