If it be conceded that fully developed carpogonia with emergent trichogynes, accompanied by spermogonia and spermatia, represent fertilization, or the probability of fertilization, then the process may be assumed to take place in a fairly large and widely distributed series of lichens. Copulation between the spermatium and the trichogyne has been seen by Stahl[650], Baur[651] and by F. Bachmann[652] in _Collema_. In _Physcia pulverulenta_ Darbishire[653] could not prove copulation in the earlier stages, but he found what he took to be the remains of emptied spermatia adhering to the tips of old trichogynes. Changes in the trichogyne following on presumed copulation have been demonstrated by several workers in the Collemaceae, and open communication as a result of fertilization between the cells of the ascogonium has been described in two species. This coenocytic condition of the ascogonium (or archicarp), considered by Darbishire and others as an evidence of fertilization, has been demonstrated by Fitzpatrick[654] in the fungus _Rhizina undulata_. The walls between the cells of the archicarp in that Ascomycete became more or less open, so that the ascogenous hyphae growing from the central cells were able easily to draw nutrition from the whole coenocyte, but no process of fertilization in _Rhizina_ preceded the breaking down of the septa and no fusion of nuclei was observed until the stage of ascus formation. The real distinction between fertile and vegetative hyphae lies, according to Harper[655], in the relative size of the nuclei. F. Bachmann speaks of one large nucleus in the spermatium of _Collema pulposum_ which would indicate sexual function. There is however very little nuclear history of lichens known at any stage until the beginning of ascus formation, when fusion of two nuclei certainly take place as in fungi to form the definitive nucleus of the ascus. The whole matter may be summed up in Fünfstück’s[656] statement that: “though research has proved as very probable that fertilization takes place, it is an undoubted fact that no one has observed any such process.” F. FINAL STAGES OF APOTHECIAL DEVELOPMENT The emergence of the lichen apothecium from the thallus, and the form it takes, are of special interest, as, though it is essentially fungal in structure, it is subject to various modifications entailed by symbiosis.