Parmeliaceae

Parmeliaceae thalli are most often foliose, fruticose or subfruticose, but can be umblicate, peltate, caespitose, crustose, or subcrustose. One genus, Nesolechia, is even a lichenicolous fungus. They can be a variety of colours, from whitish to grey, green to yellow, or brown to blackish (or any combination therein). Many genera are lobe forming, and nearly all are heteromerous (which are corticate on both sides). Species are usually rhizinate on the lower surface, occasionally with holdfasts, rhizohyphae, or a hypothallus. Only a few genera have a naked lower surface (for example Usnea, Hypogymnia and Menegazzia). The upper surface has a pored or non-pored epicortex. Medulla is solid, but often loosely woven.[8]

Apothecia are lecanorine, produced along the lamina or margin, and sessile to pedicellate (or less often sunken). Thalline exciple is concolorous with the thallus. Asci are amyloid, and the vast majority of species have eight spores per ascus, though a few species are many-spored, and several Menegazzia species have two spores per ascus.[8]

Ascospores are simple, hyaline, and often small. Conidia generally arise laterally from the joints of conidiogenous hyphae (Parmelia-type), but arise terminally from these joints in a small number of species (Psora-type). The conidia can have a broad range of shapes: cylindrical to bacilliform, bifusiform, fusiform, sublageniform, unciform, filiform, or curved. Pycnidia are immersed or rarely emergent from the upper cortex, are produced along the lamina or margins, pyriform in shape, and dark-brown to black in colour.[8]

Members of the Parmeliaceae exhibit a diverse chemistry, with several types of lichenan (Xanthoparmelia-type, Cetraria-type, intermediate-type), isolichenan and/or other polysaccharides being known from the cell walls of many species.[8] The wide diversity in the types of chemical compounds includes depsides, depsidones, aliphatic acids, triterpenes, anthraquinones, secalonic acids, pulvinic acid derivatives, and xanthones. The compounds usnic acid and atranorin, which are found exclusively in the Parmeliaceae, are of great importance in the systematics of the family, and the presence or absence of these chemicals have been used in several instances to help define genera. Parmelia and Usnea are the best chemically characterized genera, while the species Cetraria islandica and Evernia prunastri have attracted considerable research attention for their bioactive compounds.[9]

The main photobiont genus that associates with Parmeliaceae species is the Chlorophyte Trebouxia. In particular, the species Trebouxia jamesii appears to be especially prominent. Some Parmeliaceae genera are also known to associate with Asterochloris,[4] but the frequency of this association is not yet known. In general, photobiont diversity within the Parmeliaceae is a little studied subject, and much is left to discover here.