Anisomeridium forms an inconspicuous crust that lies almost completely within its substrate—usually the bark of broad-leaved trees, but sometimes decaying wood, mosses, or shaded rock. Because the thallus is immersed it appears only as a faint pale-grey film or scatter of whitish patches, and it lacks a separate protective cortex; microscopic inspection shows a loose weft of fungal threads housing orange-tinged filaments of the green alga Trentepohlia, which supplies photosynthetic energy.[5]
Sexual reproduction takes place in tiny flask-shaped fruit bodies (perithecia) that are half-buried to almost fully exposed. Individual perithecia are hemispherical to nearly spherical and can merge into small compound clusters. Their carbonised wall is thicker in the upper half and commonly bears a well-defined cap (the involucrellum). A quick field test with potassium hydroxide solution turns this wall a greenish tint, helping to separate Anisomeridium from similar genera. Internally, the cavity is filled with a clear gel threaded by slim, long-celled pseudoparaphyses that branch sparingly and fuse together, while the short hairs (periphyses) found in many flask lichens are absent. Each spore sac (ascus) is cylindrical to club-shaped, splits lengthwise when mature (fissitunicate), and possesses a small lens-like ocular chamber at the tip. Eight colourless ascospores are arranged in one or two rows; they are egg- to spindle-shaped, carry one to three cross-walls, and typically develop their first septum nearer the basal end of the spore.[5]
Asexual propagules are just as common. Immersed or slightly protruding pycnidia produce two sizes of conidia in separate structures: larger macroconidia that are ellipsoid rods and smaller microconidia that are almost spherical. Both types are simple (non-septate) and may be expelled en masse as a slimy white tendril. No secondary lichen substances have been detected in European representatives of the genus, so identification relies on the combination of an almost invisible thallus, greenish K-reaction of the perithecial wall, and thin-walled, multi-septate spores with a distinctive low-end septum.[5]
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