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 ; microscopic inspection shows a loose weft of fungal threads housing orange-tinged filaments of the green alga
Trentepohlia, which supplies
photosynthetic energy.
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 wall is thicker in the upper half and commonly bears a well-defined cap (the ). 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 that branch sparingly and fuse together, while the short hairs () found in many flask lichens are absent. Each spore sac (
ascus) is cylindrical to club-shaped, splits lengthwise when mature (), 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. 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. ==Species==