The relegation of the carpogonium to a position far removed from the base or primordium of the apothecium need not necessarily interfere with the conception of the primordial tissue as homologous with the conceptacle; but more research is needed, as Baur dealt only with one species, Cl. pyxidata, and Gertrude Wolff confined her attention to the carpogonial stages at the edge of the scyphus.
The Cladoniae require light, and inhabit by preference open moorlands, naked clay walls, borders of ditches, exposed sand-dunes, etc. Those with large and persistent squamules can live in arid situations, probably because the primary thallus is able to retain moisture for a long time[420]. When the primary thallus is small and feeble the podetia are generally much branched and live in close colonies which retain moisture. Sterile podetia are long-lived and grow indefinitely at the apex though the base as continually perishes and changes into humus. Wainio[421] cites an instance in which the bases of a tuft of Cl. alpestris had formed a gelatinous mass more than a decimetre in thickness.
I. Pilophorus and Stereocaulon
These two genera are usually included in Cladoniaceae on account of their twofold thallus and their somewhat similar fruit formation. They differ from Cladonia in the development of the podetia which are not endogenous in origin as in that genus, but are formed by the growth upwards of a primary granule or squamule and correspond more nearly to Tulasne’s conception of the podetium as a process from the horizontal thallus. In Pilophorus the primary granular thallus persists during the life of the plants; the short podetium is unbranched, and consists of a somewhat compact medulla of parallel hyphae surrounded by a looser cortical tissue, such as that of the basal granule, in which are embedded the algal cells. The black colour of the apothecium is due to the thick dark hypothecium.
Stereocaulon is also a direct growth from a short-lived primary squamule[422]. The podetia, called “pseudopodetia” by Wainio, are usually very much branched. They possess a central strand of hyphae not entirely solid, and an outer layer of loose felted hyphae in which the gonidia find place. A coating of mucilage on the outside gives a glabrous shiny surface, or, if that is absent, the surface is tomentose as in St. tomentosum. In all the species the podetia are more or less thickly beset with small variously divided squamules similar in form to the primary evanescent thallus. Gall-like cephalodia are associated with most of the species and aid in the work of assimilation.
Stereocaulon cannot depend on the evanescent primary thallus for attachment to the soil. The podetia of the different species have developed various rooting bases: in St. ramulosum there is a basal sheath formed, in St. coralloides a well-developed system of rhizoids[423].