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]. V. STRUCTURES PECULIAR TO LICHENS 1. AERATION STRUCTURES A. CYPHELLAE AND PSEUDOCYPHELLAE