. Lichens. Lichens. STRATOSE THALLUS 85 gelatinous lichens in which the tips of the hyphae are cut off at the surface by one or more septa. The resulting cells are wider than the hyphae and they cohere together to form, in some species, disconnected patches of cells; in others, a continuous cortical covering one or more cells thick, while in the margin of the apothecium they form a deep cellular layer. The cellular type of cortex is found also, as already stated, in some crustaceous Pertu- sariae, and in a few squamulose genera or species. It forms the uppermost layer of the Peltigera thallus
. Lichens. Lichens. STRATOSE THALLUS 85 gelatinous lichens in which the tips of the hyphae are cut off at the surface by one or more septa. The resulting cells are wider than the hyphae and they cohere together to form, in some species, disconnected patches of cells; in others, a continuous cortical covering one or more cells thick, while in the margin of the apothecium they form a deep cellular layer. The cellular type of cortex is found also, as already stated, in some crustaceous Pertu- sariae, and in a few squamulose genera or species. It forms the uppermost layer of the Peltigera thallus and both cortices of many of the larger foliose lichens such as Sticta, Parmelia, etc. 5. The "fibrous" cortex must be added to this series, as was pointed out by Heber Howe' who gave the less appropriate designation of "simple" to the type. It consists of long rather sparingly branched slender hyphae that grow in a direction parallel with the surface of the thallus (Fig. 48). It is characteristic of sfeveral fruticose and foliose lichens with more or less upright growth, such as we find in several of the Pkysciae, and in the allied genus Teloschistes, in Alectoria, several genera of Roccellaceae, in Usnea longissima and in Parmelia pubescens, etc. Zukal would have included all the Usneae as the tips are Fig. 48. Physcia ciliaris DC. Vertical section of thallus. a, cortex; b, gonidial zone; c, medulla, x 100. More than one type of cortex, as already stated, may appear in a genus; a striking instance of variability occurs in Solorina where, as Hue^ has pointed out, the cortex of S. octospora is fastigiate, that of all the other species being plectenchymatous. Cortical development is a specific rather than a generic characteristic. b. Origin of Variation in Cortical Structure. The immediate causes making for differentiation in cortical development are: the prevailing direction of growth of the hyphae as they rise from the gonidial zone; the amount of
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