Pyrenula nitida has also been studied by Baur[609]. It is a very common species on smooth bark, with a thin crustaceous thallus immersed among the outer periderm cells. Unlike most other lichens, it forms carpogonia in spring only, from February to April. A primordial coil of hyphae lies at the base of the gonidial layer, and, before there is any appearance of carpogonia, a thick strand of hyphae is seen to be directed upwards, so that a definite form and direction is given to the perithecium at a very early stage. The ascogonial cells which are differentiated are extremely small, and, like those of all other species examined, are uninucleate. There are five to ten carpogonia in each primordium; the trichogynes grow up through the hyphal strand and emerge 5-10 µ above the surface. After their disappearance, a weft of ascogenous tissue is formed at the base, and, at the same time, the surrounding vegetative tissue takes part in the building up of a plectenchymatous wall of minute dark-coloured cells. Further development is rapid and occupies probably only a few weeks.
In many of the pyrenocarpous lichens—Verrucariae and others—the walls of the paraphyses dissolve in mucilage as the spores become mature, a character associated with spore ejection and dispersal. In some genera and species, as in Pyrenula, they remain intact.
D. Apogamous Reproduction
Though fertilization by an externally produced male nucleus has not been definitely proved there is probability that, in some instances, the fruit may be the product of sexual fusion. There are however a number of genera and species in which the development is apogamous so far as any external copulation is possible and the sporiferous tissue seems to be a purely vegetative product up to the stage of ascus formation.
In Phlyctis agelaea Krabbe[610] found abundant apothecia developing normally and not accompanied by spermogonia; in Phialopsis rubra studied also by him the primordium arises among the cells of the periderm on which the lichen grows, and he failed to find any trace of a sexual act. In his elaborate study of Gloeolichens Forssell[611] established the presence of carpogonia with trichogynes in two species—Pyrenopsis phaeococca and P. impolita, but without any appearance of fertilization; in all the others examined, the origin of the fruit was vegetative. Wainio[612] records a similar observation in a species of Pyrenopsis in which there was formed a spiral ascogonium and a trichogyne, but the latter never reached the surface.
Neubner[613] claimed to have proved a vegetative origin for the asci in the Caliciaceae; but he overlooked the presence of spermogonia and his conclusions are doubtful.
Fünfstück[614] found apogamous development in Peltigera (including Peltidea) and his results have never been disputed. The ascogonial cells are surrounded at an early stage by a weft of vegetative hyphae. No trichogynes are formed and spermogonia are absent or very rare in the genus, though pycnidia with macrospores occur occasionally.
Some recent work by Darbishire[615] on the genus supplies additional details. The apothecial primordium always originated near the growing margin of the thallus, where certain medullary hyphae were seen to swell up and stain more deeply than others. These at first were uninucleate, but the nuclei increased by division as the cells became larger, and in time there was formed a mass of closely interwoven cells full of cytoplasm. “No coiled carpogonia can be made out, but these darkly stained cells form part of a connected system of branching hyphae coming from the medulla further back.” Long unbranched multi-septate hyphae—evidently functionless trichogynes—travelled towards the cortex but gradually died off. Certain of the larger cells—the “ascogonia”—grew out as ascogenous hyphae into which the nuclei passed in pairs and finally gave rise to the asci.
These results tally well with those obtained by M. and Mme Moreau[616], though they make no mention of any trichogyne. They found that the terminal cells of the ascogenous hyphae were transformed into asci, and the two nuclei in these cells fused—the only fusion that took place. In Nephromium, one of the same family, the case for apogamy is not so clear; but Fünfstück found no trichogynes, and though spermogonia were present on the thallus, they were always somewhat imperfectly developed.
Sturgis[617] supplemented these results in his study of other lichens containing blue-green algae. In species of Heppia, Pannaria, Hydrothyria, Stictina and Ricasolia, he failed to find any evidence of fertilization by spermatia.