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.
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.
Fig. 62. Ramalina minuscula Nyl. A, transverse section of frond × 37; B, longitudinal strengthening hyphae of inner cortex × 430 (after Brandt).