The two British species of _Roccella_—_R. fuciformis_ and _R. phycopsis_—grow on maritime rocks, the latter also occasionally on trees. In _R. fuciformis_, the attaching sheath is a flat structure which slopes up a little round the base of the upright frond. It is about 2 mm. thick, the cortex occupying about 40 µ of that space; a few scattered gonidia are present immediately below. The remaining tissue of the sheath is composed of firmly wefted slender filaments. Towards the lower surface, there is a more closely compacted dark brown layer from which pass out the hyphae that penetrate the rock. The sheath of _R. phycopsis_ is a small structure about 3 to 4 mm. in width and 1·5 mm. thick. A few gonidia may be found below the dense cortical layer, but they tend to disappear as the upright fronds become larger and the shade, in consequence, more dense. Lower down the hyphae take an intensely yellow hue; mixed with them are also some brown filaments. A somewhat larger sheath 7 to 8 mm. wide forms the base of _R. tinctoria_. In structure it corresponds—as do those of the other species—with the ones already described. In purely filamentous species such as _Usnea_ there is also primary sheath formation: the medullary hyphae spread out in radiating strands which force their way wherever possible into the underlying substance; on trees they enter into any chink or crevice of the outer bark like wedges; or they ramify between the cork cells which are split up by the mere growth pressure. By the vertical increase of the base, the fronds may be hoisted up and an intercalary basal portion may arise lacking both gonidia and cortical layer. Very frequently several bases are united and the lichen appears to be of tufted habit. A basal sheath provides a similar firm attachment for _Alectoria jubata_ and allied species: these are slender mostly dark brown lichens which hang in tangled filaments from the branches of trees, rocks, etc. These attaching sheaths differ in function as well as in structure from the horizontal thallus of the Cladoniaceae. They may be more truly compared with the primary thallus of the red algae _Dumontia_ and _Phyllophora_ which are similarly affixed to the substratum, while upright fronds of subsequent formation bear the fructifications. IV. STRATOSE-RADIATE THALLUS 1. STRATOSE OR PRIMARY THALLUS A. GENERAL CHARACTERISTICS [Illustration: Fig. 66. _Cladonia pyxidata_ Hoffm. Basal squamule and podetium. _a_, apothecia; _s_, spermogonia (after Krabbe).]