The apothecia may be sessile and closely adnate to or even sunk in the thallus, or they may be shortly stalked. The thalline margin shares generally the characters of the thallus; the disc is mostly of a firm consistency and is light or dark in colour according to genus or species; most frequently it is some shade of brown. Marginate apothecia, _i.e._ those with a thalline margin, are often referred to as “lecanorine,” that being a distinctive feature of the genus _Lecanora_. In the immarginate series, with a proper margin only, the texture may be soft and waxy, termed “biatorine” as in _Biatora_; or hard and carbonaceous as in the genus _Lecidea_, and is then described as “lecideine.” In the subseries Graphidineae, the apothecium has the form of a very flat, roundish or irregular body entirely without a margin, called an “ardella” as in _Arthonia_; or more generally it is an elongate narrow “lirella,” in which the disc is a mere slit between two dark-coloured proper margins. The hypothecium of the lirellae is sometimes much reduced and in that case the hymenium rests directly on a thin layer above the thalline tissue as in _Graphis elegans_ (Fig. 89). [Illustration: Fig. 89. _Graphis elegans_ Ach. A, thallus and lirellae; B, vertical section of furrowed lirella. × ca. 50.] Lichen fruits require abundant light, and plants growing in the shade are mostly sterile. Naturally, therefore, the reproductive bodies are to be found on the best illuminated parts of the thallus. In crustaceous and in most foliose forms, they are variously situated on the upper surface, wherever the light falls most directly. In the genera _Nephromium_ and _Nephromopsis_, on the contrary, they arise on the under surface, though at the extreme margin, but as the fertile lobes eventually turn upwards the apothecia as they mature become fully exposed. In shrubby or fruticose lichens their position is lateral on the fronds, or more frequently at or near the tips. _b._ PERITHECIA. The small closed perithecium is characteristic of the Pyrenocarpeae which correspond with the Pyrenomycetes among fungi. It is partially or entirely immersed in the thallus or in the substratum on which the lichen grows, and is either a globose or conical body wholly surrounded by a hyphal wall, when it is described as “entire” (Fig. 90), or it is somewhat hemispherical in form and the outer wall is developed only on the upper exposed part: a type of perithecium usually designated by the term “dimidiate.” As the perithecial wall gives sufficient protection to the asci, the paraphyses are of less importance and are frequently very sparingly produced, or they may even be dissolved and used up at an early stage. The thallus of the Pyrenocarpeae is often extremely reduced, and the perithecia are then the only visible portion of the lichen.