_a._ APOTHECIA. As in discomycetous fungi, the lichen apothecium is in the form of an open concave or convex disc, but generally of rather small size, rarely more than 1 cm. in diameter (Fig. 88); there is no development in lichen fruits equal to the cup-like ascomata of the larger _Pezizae_. In most cases the lichen apothecium retains its vitality as a spore-bearing organ for a considerable period, sometimes for several years, and it is strengthened and protected by one or more external margins of sterile tissue. Immediately surrounding the fertile disc there is a compact wall of interwoven hyphae. In some of the shorter-lived soft fruits, as in _Biatora_, this hyphal margin may be thin, and may gradually be pushed aside as the disc develops and becomes convex, but generally it forms a prominent rim round the disc and may be tough or even horny, and often hard and carbonaceous. This wall, which is present, to some extent, in nearly all lichens, is described as the “proper margin.” A second “thalline margin” containing gonidia is present in many genera[544]: it is a structure peculiar to the lichen apothecium and forms the _amphithecium_. [Illustration: Fig. 88. _Lecanora subfusca_ Ach. A, thallus and apothecia × 3; B, vertical section of apothecium. _a_, hymenium; _b_, hypothecium; _c_, thalline margin or amphithecium; _d_, gonidia. × 60 (after Reinke).] At the base of the apothecium there is a weft of light- or dark-coloured hyphae called the _hypothecium_, which is continued up and round the sides as the _parathecium_ merging into the “proper margin.” It forms the lining of a cup-shaped hollow which is filled by the paraphyses, which are upright closely packed thread-like hyphae, and by the spore-containing asci or _thecae_, these together constituting the thecium or hymenium. The paraphyses are very numerous as compared with the asci; they are simple or branched, frequently septate, especially towards the apex, and mostly slender, varying in width from 1-4µ, though Hue describes paraphyses in _Aspicilia atroviolacea_ as 8-12µ thick. They may be thread-like throughout their length, or they may widen towards the tips which are not infrequently coloured. Small apical cells are often abstricted and lie loose on the epithecium, giving at times a pruinose or powdered character to the disc. In some genera there is a profuse branching of the paraphyses to form a dense protective epithecium over the surface of the hymenium as in the genus _Arthonia_.