This series includes the lichens of one family only, the Cladoniaceae, the genera of which are characterized by the twofold thallus, one portion being primary, horizontal and stratose, the other secondary and radiate, the latter an upright simple or branching structure termed a “podetium” which narrows above, or widens to form a trumpet-shaped cup or “scyphus” (Fig. 66). The apothecia are terminal on the podetium or on the margins of the scyphi; in a few species they are developed on the primary thallus. Some degree of primary thallus-formation has been demonstrated in all the genera, if not in all the species of the family. The genus _Cladina_ was established to include those species of _Cladonia_ in which, it was believed, only a secondary podetial thallus was present, but Wainio[389] found in _Cladonia sylvatica_ a granular basal crust and, in _Cladonia uncialis_, minute round scales with crenate margins measuring from ·5 to 1 mm. in width. In some species (subgenus _Cladina_) the primary thallus is quickly evanescent, in others it is granular or squamulose and persistent. Where the basal thallus is so much reduced as to be practically non-existent, apothecia are rarely developed and soredia are absent. Renewal of growth in these lichens is secured by the dispersal of fragments of the podetial thallus; they are torn off and scattered by the wind or by animals, and, if suitable conditions are met, a new plant arises. _Cladonia_ squamules vary in size from very small scales as in _Cl. uncialis_ to the fairly large foliose fronds of _Cl. foliacea_ which extend to 5 cm. in length and about 1 cm. or more in width. It is interesting to note that when the primary thallus is well developed, the podetia are relatively unimportant and frequently are not formed. As a rule the squamules are rounded or somewhat elongate in form with entire or variously cut and crenate margins. They may be very insignificant and sparsely scattered over the substratum, or massed in crowded swards of leaflets which are frequently almost upright. In colour they are bluish-grey, yellowish or brownish above, and white beneath (red in _Cl. miniata_), frequently becoming very dark-coloured towards the rooting base. These several characteristics are specific and are often of considerable value in diagnosis. In certain conditions of shade or moisture, squamules are formed on the podetium; they repeat the characters of the basal squamules of the species. B. TISSUES OF THE PRIMARY THALLUS The stratose layers of tissue in the squamules of _Cladonia_ are arranged as in other horizontal thalli.