Genus
Psora comprises lichens recognised for their relatively large, thick , which are the leaf-like structures making up the lichen's body. These squamules, ranging in colour from yellow to red-brown and typically measuring between 2 and 6 mm in diameter, are notable for their absence of a
prothallus, which is an initial growth phase seen in some lichens. The upper cortex of these lichens is thick and well-developed, while the lower cortex may be poorly developed or entirely absent. Instead of a lower cortex,
Psora species often anchor themselves with a well-developed network of fungal
hyphae, which can either lack
calcium oxalate crystals and have a brown pigmentation or be colourless, shorter, and densely covered in these crystals. The symbiotic partner of
Psora, the , is , meaning it consists of spherical
green algae, forming a continuous layer. The
medulla, the innermost layer of the lichen, is well-developed but does not contain
iodine-reactive substances (I–). Reproductive structures, known as
apothecia, are (attached directly by their base), and can be found along the margins or on the surface () of the squamules. These structures are typically brown-black, starting flat or slightly convex in their youth, and may become highly convex or semi-spherical as they mature, occasionally with a white or yellow powdery coating (). Unlike some lichens,
Psora does not have a around its apothecia. The , a ring of hyphae surrounding the apothecial , is colourless to pale brown and often becomes obscured as the apothecia mature. The , the topmost layer inside the apothecia, is red-brown or brown and contains
anthraquinones compounds, such as
parietin, which react red or purple-red when treated with a potassium hydroxide solution (K+), but do not react with a
nitric acid solution (N–). The
hymenium, another internal layer, also does not react to iodine (I–). The beneath it is pale brown to colourless and filled with calcium oxalate crystals.
Psoras supporting filamentous structures, or
paraphyses, are unbranched or sparingly branched and tightly bound together, with slightly swollen tips. The
asci, the spore-bearing cells, are (club-shaped) with eight spores each and have a distinctive blue reaction when stained with
iodine, indicative of the
Porpidia-type asci without an ocular chamber. Ascospores are colourless, smooth,
ellipsoidal, and lack a
septum or a thick outer layer (). The genus also produces
pycnidia, a type of
asexual reproductive structure, which are immersed within the squamules and release elongate- (rod-shaped), colourless
conidia. Chemically,
Psora has a range of compounds including
depsides,
depsidones,
anthraquinones, various acids such as
pulvinic and
usnic acids, and
triterpenes. ==Species==