8. Pilophorus, Stereocaulon and Argopsis. These closely related genera are classified with Cladonia as they share with it the twofold thallus and the lecideine apothecia. The origin of the podetium being different they may be held to constitute a phylum apart, which has however taken origin also from some Biatora form.
The primary thallus is crustaceous or minutely squamulose and the podetia of Pilophorus, which are short and unbranched (or very sparingly branched), are beset with thalline granules. The podetia of Stereocaulon and Argopsis are copiously branched and are more or less thickly covered with minute variously divided leaflets. Cephalodia containing blue-green algae occur on the podetia of these latter genera; in Pilophorus they are intermixed with the primary thallus.
The tissue systems are less advanced in these genera than in Cladonia: there is no cortex present either in Pilophorus or in Argopsis or in some species of Stereocaulon, though in others a gelatinous amorphous layer covers the podetia and also the stalk leaflets. The stalks are filled with loose hyphae in the centre.
BB. Lecanorales. This second group of Cyclocarpineae is distinguished by the marginate apothecium, a thalline layer providing a protecting amphithecium. The lecanorine apothecium is of a more or less soft and waxy consistency, and though the disc is sometimes almost black, neither hypothecium nor parathecium is carbonaceous as in Lecidea. The affinity of Lecanora is with sect. Biatora, and development must have been from a biatorine form with a persistent thallus. The margin or amphithecium varies in thickness: in some species it is but scanty and soon excluded by the over-topping growth of the disc, so that a zone of gonidia underlying the hypothecium is often the only evidence of gonidial intrusion left in fully formed fruits.
The marginate apothecium has appeared once and again as we have seen. It is probable however that its first development was in this group of lichens, and even here there may have been more than one origin as there is certainly more than one phylum.
aa. Course of Development. At the base of the series, the thallus is of the crustaceous type somewhat similar to that of Lecidea, but there are none of the very simple primitive forms. Lecanora must have originated when the crustaceous lecideine thallus was already well established. Its affinity is with Lecidea and not with any fungus: where the thallus is evanescent or scanty, its lack is due to retrogressive rather than to primitive characters.
bb. Lecanoraceae. A number of genera have arisen in this large family, but they are distinguished mainly if not entirely by spore characters, and by some systematists have all been included in the one genus Lecanora, since the changes have taken place within the developing apothecium.
There is one genus, Harpidium, which is based on thalline characters, represented by one species, H. rutilans, common enough on the Continent, but not yet found in our country. It has a thin crustaceous homoiomerous thallus, the component hyphae of which are divided into short cells closely packed together and forming a kind of cellular tissue in which the algae are interspersed. The dorsiventral stratose arrangement prevails however in the other genera and a more or less amorphous “decomposed” cortex is frequently present. The medulla rests on the substratum.
With the stouter thallus, there is slightly more variety of crustaceous form than in Lecideaceae: there occurs occasionally an outgrowth of the thalline granules as in Haematomma ventosum which marks the beginning of fruticulose structure. Of a more advanced structure is the thallus of Lecanora esculenta, a desert lichen which becomes detached and erratic, and which in some of its forms is almost coralline, owing to the apical growth of the original granules or branches: a more or less radiate arrangement of the tissues is thus acquired.