_b._ =The fibrous cortex.= This type is found in a number of lichens with long filamentous hanging fronds. It consists of parallel hyphae, rarely septate and rarely branched, but frequently anastomosing and with strongly thickened “sclerotic” walls. Such a cortex is the only strengthening element in _Alectoria_, and it affords great toughness and flexibility to the thong-like thallus. It is also present in _Ramalina_ (_Alectoria_) _thrausta_, a species with slender fronds (Fig. 60). [Illustration: Fig. 60. _Alectoria thrausta_ Ach. A, transverse section of frond; _a_, cortex; _b_, gonidia; _c_, arachnoid medulla × 37. B, fibrous hyphae from longitudinal section of cortex. × 430 (after Brandt).] In _Usnea longissima_ the cortex both of the fibrillose branchlets and of the main axis is fibrous, and is composed of narrow thick-walled hyphae which grow in a long spiral round the central strand. The hyphae become more frequently septate further back from the apex (Fig. 61). Such a type of cortex provides an exceedingly elastic and efficient protection for the long slender thallus. [Illustration: Fig. 61. _Usnea longissima_ Ach. Longitudinal sections of outer cortex. A, near the apex; B, the middle portion of a fibril. × 525 (after Schulte).] The same type of cortex forms the strengthening element in the fruticose or partly fruticose members of the family Physciaceae. One of these, _Teloschistes flavicans_, is a bright yellow filamentous lichen with a somewhat straggling habit. The fronds are very slender and are either cylindrical or slightly flattened. The hyphae of the outer cortex are compactly fibrous; added toughness is given by the presence of some longitudinal strands of hyphae in the central pith. Another still more familiar grey lichen, _Physcia ciliaris_, has long flat branching fronds which, though dorsiventral in structure, are partly upright in habit. Strength is secured as in _Teloschistes_ by the fibrous upper cortex. Other species of _Physciae_ are somewhat similar in habit and in structure. In _Dendrographa leucophaea_, a slender strap-shaped rock lichen, Darbishire[376] has described the outer cortex as composed of closely compacted parallel hyphae resembling the strengthening cortex of _Alectoria_ and very different from the fastigiate cortex of the _Roccellae_ with which it is usually classified. B. SPECIAL STRENGTHENING STRUCTURES _a._ SCLEROTIC STRANDS. This form of strengthening tissue is characteristic of _Ramalina_. With the exception of _R. thrausta_ (more truly an _Alectoria_) all the species have a rather weak cortical layer of branching intricate thick-walled hyphae, regarded by Brandt[377] as plectenchymatous, but more correctly by Hue[378] as “decomposed” on account of the gelatinous walls and diminishing lumen of the irregularly arranged cells. [Illustration: Fig. 62. _Ramalina minuscula_ Nyl. A, transverse section of frond × 37; B, longitudinal strengthening hyphae of inner cortex × 430 (after Brandt).]