. Lichens. Lichens. RADIATE THALLUS 103 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, Darbi- shire' has described the outer corte
. Lichens. Lichens. RADIATE THALLUS 103 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, Darbi- shire' 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 charac- teristic 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^ as plectenchymatous, but more correctly by Hue' as "decomposed" on account of the gelatinous walls and diminishing lumen of the irregularly arranged cells. In R. evermoides, 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Smith, Annie Lorrain. Cambridge, At the University Press
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