Description
Xylographa produces an inconspicuous crust that lies flush with, or just beneath, the surface of decaying wood. Where visible the thallus is a thin, diffuse film and may carry scattered brown goniocysts —microscopic granules in which a few spherical green algal cells are wrapped by fungal hyphae. Because the crust is either immersed or extremely thin, it often blends with the substrate and is easiest to detect once its fruit bodies appear. Chemical analyses reveal either the stictic acid complex, unnamed secondary metabolites, or no detectable lichen products. [4]
The lichen's ascomata take the form of narrow lirellae that emerge partly embedded and mature into elongate, often one-sided slits aligned with the grain of the wood. Individual lirellae range from nearly round to distinctly linear but share a characteristic brown to dark-brown pigmentation and a low, flat disc . They lack a thalline margin ; instead, a thin true exciple —pale to mid-brown rather than black—outlines each slit. Under the microscope the hymenium is colourless yet stains blue with iodine, while the hypothecium beneath remains clear. Delicate paraphyses thread the hymenium; these filaments branch only sparingly, widen gradually toward their brown-tipped apices and sometimes fuse with neighbouring threads. Club- to cylinder-shaped asci each contain eight smooth, single-celled ascospores that are initially colourless and only rarely become grey-brown in very old material. The ascus apex shows a diagnostic light-blue lateral zone surrounding a colourless plug in iodine preparations, matching the Trapelia structural type. Immersed, brown-black pycnidia frequently accompany the lirellae and release slender, slightly curved conidia. The combination of pale-brown exciple and less intensely pigmented lirellae separates Xylographa from the look-alike lichen Ptychographa (which has black, slit-domed discs), as well as from the non-lichenised wood-dwelling fungus Agyrium , whose apothecia are convex and dull orange. [4]
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