In the Ascomycetes, certain cells or cell-processes associated with the ascogonium have been described as trichogynes or receptive cells. In one of the simpler types, Monascus[631], the “trichogyne” is a cell cut off from the ascogonial cell. When fertilization takes place, the wall between the two cells breaks down to allow the passage of the male nucleus, but closes up when the process is effected. In Pyronema confluens[632] it is represented by a process from the ascogonial cell which fuses directly with the male cell. A more elaborate “trichogyne” has been evolved in Lachnea stercorea[633], another Discomycete: in that fungus it takes the form of a 3-5-septate hypha with a longer terminal cell; it rises from some part of the ascogonial cell but has no connection with any process of fertilization, so that the greater elaboration of form is in this case concomitant with loss of function.
In the Laboulbeniaceae, a numerous and very peculiar series of Ascomycetes that live on insects, there are, in nearly all of the reproductive bodies, a carpogonial cell, a trichophoric cell and a trichogyne. The last-named organ is in some genera a simple continuous cell, in others it is septate and branched, occasionally it is absent[634]. The male cells are spermatia of two kinds, exogenous or endogenous, and the plants are monoecious or dioecious. Laboulbeniaceae have no connection with lichens. Faull[635], a recent worker on the group, states that though he observed spermatia attached to the trichogynes, he was not able to demonstrate copulation (possibly owing to over-staining), nor could he trace any migration of the nucleus through the trichophoric cell down to the carpogonial cell. In two species of Laboulbenia that he examined there were no antheridia, and the egg-cell acquired its second nucleus from the neighbouring trichophoric cell. These conjugate nuclei divided simultaneously and the two daughter nuclei passed on to the ascus and fused, as in other Ascomycetes, to form the definitive nucleus.
Convincing evidence as to the importance of the trichogyne in fungi was supposed, until lately, to be afforded by the presence and functional activity of that organ associated with spermogonia in a few Pyrenomycetes—in Poronia, Gnomonia and Polystigma. Poronia was examined by M. Dawson[636] who found that a trichogyne-like filament distinct from the vegetative hyphae rose from the neighbourhood of the ascogonial cells. It took an upward course towards the exterior, but there was no indication that it was ever receptive. In Gnomonia erythrostoma and in Polystigma rubrum spermogonia with spermatia—presumably male organs—are produced in abundance shortly before the ascosporous fruit is developed. The spermatia in both cases exhibit the characters of male cells, i.e. very little cytoplasm and a comparatively large nucleus that occupies most of the cell cavity, along with complete incapacity to germinate. Brooks[637] found in Gnomonia that tufts of the so-called trichogynes originated near the ascogonial cells, but they were “mere continuations of ordinary vegetative hyphae belonging to the coil.” They are septate and reach the surface, and the tip-cell is longer than the others as in the lichen trichogyne.
A somewhat similar arrangement is present in Polystigma, in which Blackman and Welsford[638] have proved that the filaments, considered as trichogynes by previous workers, are merely vegetative hyphae. A trichogyne-like structure is also present in Capnodium, one of the more primitive Pyrenomycetes, but it has no sexual significance.
Lindau[639] in his paper on Gyrophora suggested that the trichogyne in lichens acted as a “terebrator” or boring apparatus, of service to the deeply immersed carpogonium in enabling it to reach the surface. Van Tieghem[640] explained its presence on physiological grounds as necessary for respiration, a view also favoured by Zukal[641], while Wainio[642] and Steiner[643] see in it only an “end-hypha,” the vigorous growth of which is due to its connection with the well-nourished cells of the ascogonium.
Lindau’s view has been rejected by succeeding writers: as has been already stated, it is the paraphyses that usually open the way outward for the apothecium. Van Tieghem’s theory has been considered more worthy of attention and both Dawson and Brooks incline to think that the projecting filaments described above may perform some service in respiration, even though primarily they may have functioned as sexual receptive organs.
There is very little support to be drawn from fungi for the theory that the presence of a trichogyne necessarily entails fertilization by spermatia. Lichens in this connection must be judged as a class apart.
It has perhaps been too lightly assumed that the trichogyne in lichens indicates some relationship with the Florideae[644]. Such a view might be possible if we could regard lichens and Florideae as derived from some common remote ancestor, though even then the difference in spore production—in one case exogenous, and in the other in asci and therefore endogenous—would be a strong argument against their affinity. But all the evidence goes to prove that lichens are late derivatives of fungi and have originated from them at different points. Fungi are interposed between lichens and any other ancestors, and inherited characters must have been transmitted through them. F. Bachmann’s suggestion[645] that Collema pulposum should be regarded “as a link between aquatic red algae and terrestrial ascomycetes such as Pyronema and the mildews” cannot therefore be accepted. It seems more probable that the lichen trichogyne is a new structure evolved in response to some physiological requirement—either sexual or metabolic—of the deeply embedded fruit primordium.
b. The Ascogonium. In fungi there is usually one cell forming the ascogonium, a coenogamete, which after fertilization produces ascogenous hyphae. There are exceptions, such as Cutting[646] found in Ascophanus carneus, in which it is composed of several cells in open contact by the formation of wide secondary pores in the cell-walls. In lichens the ascogonium is divided into a varying number of uninucleate cells. Darbishire[647] (in Physcia) and Baur[648] (in Anaptychia) have described an opening between the different cells, after presumed fertilization, that might perhaps constitute a coenogamete. Ascogenous hyphae arise from all, or nearly all the cells, whether fertilized by spermatia or not, and asci continue to be formed over a long period of time. There may even be regeneration of the entire fruiting body as described in Graphis elegans and in Pertusaria, apparently without renewed fertilization.
Spermogonia (or pycnidia) and the ascosporous fruits generally grow on the same thallus, though not unfrequently only one of the two kinds is present. As the spermogonia appear first, while the apothecia or perithecia are still in the initial stages, that sequence of development seems to add support to the view that their function is primarily sexual; but it is equally valid as a proof of their pycnidial nature since the corresponding bodies in fungi precede the more perfect ascosporous fruits in the life-cycle.