A further peculiarity in _Parmelia acetabulum_ attracted Baur’s[592] attention. Carpogonia with trichogynes are extremely numerous in that species as are the spermogonia, the open pores of which are to be found everywhere between the trichogynes, and yet fertilization can occur but rarely, as disintegrating carpogonia are abundant and very few apothecia are formed. Baur makes the suggestion that possibly cross-fertilization may be necessary, or that the spermatia, in this instance, do not fertilize and that development must therefore be apogamous, in which case the small number of fruits formed is due to some unknown cause. Fünfstück[594] thought that degeneration of the carpogonia had not gone so far, but that a few had acquired the power to develop apogamously. In _Parmelia saxatilis_ only a small percentage of carpogonia attain to apothecia, although spermogonia are abundant and in close proximity, but in that species, unlike _P. acetabulum_, a large number reach the earlier stages of fruit formation; the more vigorous apothecia seem to inhibit the growth of those that lag behind. _bb._ _PERTUSARIAE._ In _Pertusaria_, the apothecial primordium is situated immediately below the gonidial zone; the cells have a somewhat larger lumen and thinner walls than those of the vegetative hyphae. In the ascogonium there are more cells than in _Parmelia acetabulum_; the trichogynes are short-lived, and several carpogonia probably enter into the formation of each apothecium; the paraphyses arise from the covering hyphae. So far the course of development presents nothing unusual. The peculiar pertusarian feature as described by Krabbe[595], and after him by Baur[596], does not appear till a later stage. By continual growth in thickness of the overlying thallus, the apothecia gradually become submerged and tend to degenerate; meanwhile, however, a branch from the ascogonial hyphae at the base of the hymenium pushes up along one side and forms a secondary ascogonial cell-plexus over the top of the first-formed disc. A new apothecium thus arises and remains sporiferous until it also comes to lie in too deep a position, when the process is repeated. Sometimes the regenerating hypha travels to the right or left away from the original apothecium, it may be to a distance of 2 mm. or according to Fünfstück even considerably farther. Fünfstück[597] has gathered indeed from his own investigations that such cases of regeneration are by no means rare: ascogenous hyphae, several centimetres long, destined to give rise to new apothecia are not unusual, and their activity can be recognized macroscopically by the linear arrangement of the apothecia in such lichens as _Rhizocarpon_ (_petraeum_) _concentricum_ (Fig. 97). [Illustration: Fig. 97. _Rhizocarpon petraeum_ Massal. Concentrically arranged apothecia, reduced (J. Adams, _Photo._).]