Koelreuter[534], a Professor of Natural History in Carlsruhe, who published a work on The discovered Secret of Cryptogams, next hazarded the opinion that the seeds of lichens originated from the substance of the pith, and that the overlying cortical layer supplied the fertilizing sap. Hoffmann[535] devoted a great deal of attention to lichen fructification and he also thought that fertilization must take place within the tissue of the lichens. He regarded the soredia as the true seeds, while allowing that a second series of seeds might be contained in the scutellae (apothecia).
A distinct advance was made by Hedwig[536], a Professor of Botany in Leipzig, towards the end of the eighteenth century. He followed Tournefort in selecting Physcia ciliaris for research, and in that plant he describes and figures not only the apothecia with the dark-coloured septate spores, but also the pycnidia or spermogonia which he regarded as male organs. The soredia, typically represented and figured by him on Parmelia physodes, he judged to be “male flowers of a different type.”
Acharius[537] did not add much to the knowledge of reproduction in lichens, though he takes ample note of the various fruiting structures for which he invented the terms apothecia, perithecia and soredia. Under still another term gongyli he included not only spores, but the spore guttulae as well as the gonidia or cells forming the soredia.
Hornschuch[538] of Greifswald described the propagation of the lower lichens as being solely by means of a germinating “powder”; the more highly organized forms were provided with receptacles or apothecia containing spores which he considered as analogous to flowers rather than to fruits. The important contributions to Lichenology of Wallroth[539] and Meyer[540] close this period of uncertainty: the former deals almost exclusively with the form and character of the vegetative thallus and the function of the “reproductive gonidia.” Meyer, a less prolix writer, very clearly states that the method of reproduction is twofold: by spores produced in fruits, or by the germinating granules of the soredia.
B. Forms of Reproductive Organs
From the time of Tournefort, considerable attention had been given to the various forms of scutellae, tuberculae, etc., as characters of diagnostic importance. Sprengel[541] grouped these bodies finally into nine different types with appropriate names which have now been mostly superseded by the comprehensive terms, apothecia and perithecia. A general classification on the lines of fruit development was established by Luyken[542], who, following Persoon’s[543] classification of fungi, and thus recognizing their affinity, summed up all known lichens as Gymnocarpeae with open fruits, and Angiocarpeae with closed fruits.
a. Apothecia. As in discomycetous fungi, the lichen apothecium is in the form of an open concave or convex disc, but generally of rather small size, rarely more than 1 cm. in diameter ([Fig. 88]); there is no development in lichen fruits equal to the cup-like ascomata of the larger Pezizae. In most cases the lichen apothecium retains its vitality as a spore-bearing organ for a considerable period, sometimes for several years, and it is strengthened and protected by one or more external margins of sterile tissue. Immediately surrounding the fertile disc there is a compact wall of interwoven hyphae. In some of the shorter-lived soft fruits, as in Biatora, this hyphal margin may be thin, and may gradually be pushed aside as the disc develops and becomes convex, but generally it forms a prominent rim round the disc and may be tough or even horny, and often hard and carbonaceous. This wall, which is present, to some extent, in nearly all lichens, is described as the “proper margin.” A second “thalline margin” containing gonidia is present in many genera[544]: it is a structure peculiar to the lichen apothecium and forms the amphithecium.
Fig. 88. Lecanora subfusca Ach. A, thallus and apothecia × 3; B, vertical section of apothecium. a, hymenium; b, hypothecium; c, thalline margin or amphithecium; d, gonidia. × 60 (after Reinke).
At the base of the apothecium there is a weft of light- or dark-coloured hyphae called the hypothecium, which is continued up and round the sides as the parathecium merging into the “proper margin.” It forms the lining of a cup-shaped hollow which is filled by the paraphyses, which are upright closely packed thread-like hyphae, and by the spore-containing asci or thecae, these together constituting the thecium or hymenium. The paraphyses are very numerous as compared with the asci; they are simple or branched, frequently septate, especially towards the apex, and mostly slender, varying in width from 1-4µ, though Hue describes paraphyses in Aspicilia atroviolacea as 8-12µ thick. They may be thread-like throughout their length, or they may widen towards the tips which are not infrequently coloured. Small apical cells are often abstricted and lie loose on the epithecium, giving at times a pruinose or powdered character to the disc. In some genera there is a profuse branching of the paraphyses to form a dense protective epithecium over the surface of the hymenium as in the genus Arthonia.