A manual of British lichens : containing descriptions of all the species and varieties / by William Mudd.

  • Mudd, William.
Date:
1861
Catalogue details

Licence: Public Domain Mark

Credit: A manual of British lichens : containing descriptions of all the species and varieties / by William Mudd. Source: Wellcome Collection.

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    both the upper and under of several foliaeeous and squamulose, but only the upper of crustaceous ones. It generally consists of an uncoloured cellular tissue, which varies in the thickness and size of the cellules, according to the nature of the species. The most superficial part is ■ amorphous and coloured ; which is called, by some, epidermis, and by others, epithallus. In some cases, it consists of an expansion of angular cells altogether distinct from the underlying elements, as in Leptogium, &c.; in others, of narrow tubular cavities, arranged in an irregular manner longitudinally, as in Ramalina, &c.; and, at times, is altogether wanting the upper surface of the thallus then piesenting a pulverulent appearance, as in several crustaceous species. The gonidiac stratiun is found immediately below the cortical stratum, or between it and the medullary, and consists chiefly of green (when living) cells or granules, either disposed in a tolerably even layer, or in dissociated masses. It is, in general, very easy to distinguish the limits of this layer from the others^ by its peculiar green or yellowish-green colour, and, in fact, the green aspect which most Lichens present is owing to the presence of this stratum. The elements which constitute the medullary stratum are of a more variable nature than in the preceding, but from its being always un- coloured, there is no difficulty in distinguishing it. Three forms seem to be worth noticing, viz.: 1st. The Jilamentose, which consists of simple or branched hyaline filaments, in foliaeeous species, loosely intersected and entangled; but more or less conglutinated together in the fruticu- lose, and assuming a longitudinal direction. In Usnea, these filaments are very strongly agglutinated together, constituting a kind of axil for sustaining the thallus, and are easily separated from the other strata. In Stereocaulon, SphceropJioron, &c., they are more entangled, and much less strongly agglutinated together, yet usually can be separated without much difficulty from the other elements. 2nd. The cretaceous, a modi- fication, which consists for the most part, of white molecular granules, amongst which may at times be found eight-sided crystals of oxalate of lime. Nearly all the crustaceous Lichens, possessing a tartareous thallus, have this form of the medullary. 3rd. The celhdose, which consists of an uncoloured tissue of angular, rounded, or oblong cells, amongst which are sometimes seen scattered gonidiac granules. In some species the cells have a tendency to reunite into filaments, then to separate into rows, and offering some resemblance to the filamentose medullary—as Endocarpon, some Pannarice and Verrucarice. The hypotlialline stratum is the inferior stratum of the thallus, and that over which the others stratify themselves. When present it is readily distinguished by its colour, which is mostly black, dark brown, or pale brown, and sometimes, though rarely, white; but in a great many species it is either invisible or altogether wanting. It usually presents itself under a two-fold aspect, viz. : Rhizines, that is to say, the black or blown fibres, which occur on the under side of most foliaeeous Lichens. These fibres are often branched, or someAvhat tufted at their extremities, and apparently answer the purpose of roots for holding the thallus to the different bodies on which it lives ; but it is doubtful Avhc- ther they contri))ute anything in the way of nutrition. They either consist of simple or articulated hyaline filaments, cemented together, and coated over with a coloured or partly coloured cellulai* substance; or of a great number of minute coloured and uncoloured cells, agglu-