Solorina, also a member of Peltigeraceae, was added to the list of apogamous genera by Metzger[618] and his work was confirmed and amplified by Baur[619]: certain hyphae of the gonidial zone branch out into larger ascogonial cells which increase by active intercalary growth, by division and by branching, and so gradually give rise to the ascogenous hyphae and finally to the asci. Baur looked on this and other similar formations as instances of degeneration from the normal carpogonial type of development. Moreau[620] (Fernand and Mme) have also examined Solorina with much the same results: the paraphyses rise first from cells that have been produced by the gonidial hyphae; later, ascogenous hyphae are formed and spread horizontally at the base of the paraphyses, finally giving rise at their tips to the asci. Metzger[618] had further discovered that spermogonia were absent and trichogynes undeveloped in two very different crustaceous lichens, Acarospora (Lecanora) glaucocarpa and Verrucaria calciseda, the latter a pyrenocarpous species and, as the name implies, found only on limestone.
Krabbe[621] had noted the absence of any fertilization process in Gyrophora vellea. At a later date, Gyrophora cylindrica was made the subject of exact research by Lindau[622]. In that species the spermogonia (or pycnidia) are situated on the outer edge of the thallus lobes; a few millimetres nearer the centre appear the primordia of the apothecia, at first without any external indication of their presence. The initial coil which arises on the lower side of the gonidial zone consists of thickly wefted hyphae with short cells, slightly thicker than those of the thallus. It was difficult to establish their connection with the underlying medullary hyphae since these very soon change to a brown plectenchyma. From about the middle of the ascogonial coil there rises a bundle of parallel stoutish hyphae which traverse the gonidial zone and the cortex and slightly overtop the surface. They are genetically connected at the base with the more or less spirally coiled hyphae, and are similar to the trichogynes described in other lichens. Lindau did not find that they had any sexual significance, and ascribed to them the mechanical function of terebrators or borers. The correctness of his deductions has been disputed by various workers: Baur[619] looks on these “trichogynes” as the first paraphyses. The reproductive organs in Stereocaulon were examined by G. Wolff[623], and the absence of trichogynes was proved, though spermogonia were not wanting. She also failed to find any evidence of fertilization in Xanthoria parietina, in which lichen the ascogenous hyphae branch out from an ascogonium that does not form a trichogyne.
Rosendahl[624], as already stated, could find no trichogynes in Parmelia glabra. In Parmelia obscurata, on the contrary, Bitter[625] found that carpogonia with trichogynes were abundant and spermogonia very rare. In other species of the subgenus, Hypogymnia, he has pointed out that apothecia are either absent or occur but seldom, while spermogonia are numerous, and he concludes that the spermatia must function as spores or conidia. Baur[626] however does not accept that conclusion; he suggests as probable that the male organs persist longer in a functionless condition than do the apothecia.
Still more recently Nienburg[627] has described the ascogonium of Baeomyces sp. and also of Sphyridium byssoides (Baeomyces rufus) as reduced and probably degenerate. His results do not disprove those obtained by Krabbe[628] on the same lichen (Sphyridium fungiforme). The apothecia are terminal on short stalks in that species. When the stalk is about ·5mm. in height, sections through the tip show numerous primordia (12 to 15) ranged below the outer cortex, though only one, or at most three, develop further. These ascogonial groups are connected with each other by delicate filaments, and Nienburg concluded that they were secondary products from a primary group lower down in the tissue. Spirals were occasionally seen in what he considered to be the secondary ascogonia, but usually the fertile cells lie in loose uncoiled masses; isolated hyphae were observed to travel upwards from these cells, but they never emerged above the surface.
Usnea macrocarpa—if Schulte’s[629] work may be accepted—is also apogamous, though in Usnea barbata Nienburg[627] found trichogynes ([Fig. 95]) and the various developments that are taken as evidence of fertilization. Wainio[630] had demonstrated emergent straight trichogynes in Usnea laevis but without any sign of fertilization.
E. Discussion of Lichen Reproduction
In Ascolichens fertilization by the fusion of nuclei in the ascogonium is still a debated question. The female organ or carpogonium, as outlined above, comprises a twisted or spirally coiled multi-septate hypha, with a terminal branch regarded as a trichogyne which is also multi-septate, and through which the nucleus of the spermatium must travel to reach the female cell. It is instructive to compare the lichen carpogonium with that of other plants.
a. The Trichogyne. In the Florideae, or red seaweeds, in which the trichogyne was first described, that organ is merely a hair-like prolongation of the egg-cell and acts as a receptive tube. It contains granular protoplasm but no nucleus and terminates in a shiny tip covered with mucilage. The spermatium, unlike that of lichens, is a naked cell, and being non-motile is conveyed by water to the tip of the trichogyne to which it adheres; the intervening wall then breaks down and the male nucleus passes over. After this process of fertilization a plug of mucilage cuts off the trichogyne, and it withers away.
In Coleochaete, a genus of small freshwater green algae, a trichogyne is also present in some of the species: it is again a prolongation of an oogonial cell.