In _R. evernioides_, a plant with very wide flat almost decumbent fronds of soft texture, in _R. ceruchis_ and in _R. homalea_ there is a somewhat compact medulla which gives a slight stiffness to the thallus. The other species of the genus are provided with strengthening mechanical tissue within the cortex formed of closely united sclerotic hyphae that run parallel to the surface (Fig. 62). In a transverse section of the thallus, this tissue appears sometimes as a continuous ring which may project irregularly into the pith (_R. calicaris_); more frequently it is in the form of strands or bundles which alternate with the groups of gonidia (_R. siliquosa_, _R. Curnowii_, etc.). In _R. fraxinea_ these strands may be scarcely discernible in young fronds, though sometimes already well developed near the tips. Occasionally isolated strands of fibres appear in the pith (_R. Curnowii_), or the sclerotic projections may even stretch across the pith to the other side (_R. strepsilis_) (Fig. 75 B). In the _Cladoniae_ support along with flexibility is secured to the upright podetium by the parallel closely packed hyphae that form round the hollow cylinder a band called the “chondroid” layer from its cartilage-like consistency. _b._ CHONDROID AXIS. The central medullary tissue in _Ramalina_ is, with few exceptions, a loose arachnoid structure; often the fronds are almost hollow. In one species of _Usnea_, _U. Taylori_, found in polar regions, there is a similar loose though very circumscribed medullary and gonidial tissue in the centre of the somewhat cylindrical thallus, and a wide band of sclerotic fibres towards the cortex. [Illustration: Fig. 63 A. A, _Usnea barbata_ Web. Longitudinal section of filament with young adventitious branch. _a_, chondroid axis; _b_, gonidial tissue; _c_, cortex. × 100 (after Schwendener). B, _U. longissima_ Ach. Hyphae from central axis × 525 (after Schulte).] In all other species of _Usnea_ the medulla itself is transformed into a strong central strand of long-celled thick-walled hyphae closely knit together by frequent anastomoses (Fig. 63 A). This central strand of the Usneas is known as the “chondroid axis.” A narrow band of loose air-containing hyphae and a gonidial zone lie round the central axis between it and the outer cortex (Fig. 63 A, _b_). At the extreme apex, the external cortical hyphae grow in a direction parallel with the long axis of the plant, but further back, they branch out at right angles and become swollen and mostly “decomposed” as in the cortex of _Ramalina_. In _Letharia_ (_L. vulpina_, etc.) the structure is midway between _Ramalina_ and _Usnea_: the central axis is either a solid strand of chondroid hyphae or several separate strands. [Illustration: Fig. 63 B. _Usnea longissima_ Ach. A, transverse section of fibril × 85. B, _a_, chondroid axis; _b_, gonidial tissue; _c_, cortex × 525 (after Schulte).]