Fig. 100. Physcia ciliaris DC. Vertical section of apothecium still covered by the cortex. a, paraphyses; b, hypothecium; c, gonidia of thallus and amphithecium. × 150 (after Baur).
b. Emergence of the Ascocarp. Hue[658] has taken up this subject in recent years and has described the process by which the vegetative hyphae surrounding the fruit primordium, excited to active growth by contact with the generative system, take part in the later stages of fruit formation. The primordium generally occupies a position near to, or just within, the upper medulla, and the hyphae in contact with it soon begin to branch freely in a vertical direction, surrounding the developing fruit and carrying it upwards generally to a superficial position. The different methods of the final emergence give two very distinct types of mature apothecium: the lecideine in which the gonidial zone takes no part in the upward growth, and the lecanorine into which the gonidia enter as an integral part.
In the lecideine series ([Fig. 101]) the encircling hyphae from the upper medulla rise as a compact column through the gonidial zone to the surface of the thallus; they then spread radially before curving up to form the outer wall or “proper margin” round the spore-bearing disc. The branching of the hyphae is fastigiate with compact shorter branches at the exterior. In such an apothecium gonidia are absent both below the hypothecium and in the margins.
Fig. 101. Lecidea parasema Ach. Vertical section of thallus and apothecium with proper margin only × ca. 50.
In lecanorine development the ascending hyphae from the medulla, in some cases, carry with them algal cells which multiply and spread as a second gonidial layer under the hypothecium ([Fig. 102]). These hyphae may also spread in a radial direction while still within the thallus and give rise to an “immersed” apothecium which is lecanorine as it encloses gonidia within its special tissues, for example, in Acarospora and Solorina. But in most cases the lecanorine fruit is superficial and not unfrequently it is raised on a short stalk (Usnea, etc.); both the primary gonidial zone of the thallus and the outer cortex are associated with the medullary column of hyphae from the first and grow up along with it, thus providing the outer part of the apothecium, an additional “thalline margin” continuous with the thallus itself. It is an advanced type of development peculiar to lichens, and it provides for fertility of long continuance which is in striking contrast with the fugitive ascocarps of the Discomycetes.
Fig. 102. Lecanora tartarea Ach. Vertical section of apothecium. a, hymenium; b, proper margin or parathecium; c, thalline margin or amphithecium. × 30 (after Reinke).
The distinction between lecideine and lecanorine apothecia is of great value in classification, but it is not always easily demonstrable; it is occasionally necessary to examine the early stages, as in the more advanced the thalline margin may be pushed aside by the turgid disc and become practically obliterated.