Bacidia


Bacidia is a genus of lichen-forming fungi in the family Ramalinaceae. These lichens are characterised by their crustose growth form, distinctive club-shaped reproductive structures called apothecia that contain long, needle-like spores, and their symbiotic partnership with green algae. Bacidia species typically grow on various substrates including tree bark, rock surfaces, and occasionally soil, and are distinguished from related genera by their specific spore characteristics and internal structures.

Taxonomy

The genus was circumscribed by Giuseppe De Notaris in 1846. In his original description, De Notaris distinguished Bacidia from the closely related genus Biatora based on several key characteristics. He noted that Bacidia species have apothecia that are typically pink or reddish, or sometimes nearly cinnamon-coloured, with margins that persist throughout all developmental stages and are surrounded by a distinct border. The spores are rod-shaped with eight cells separated by cross-walls, and are hyaline and two-celled. De Notaris described the thallus as crustose,, and often effuse, ranging from whitish to greyish-brown in colour. He included two species in his original circumscription: Bacidia rosella and Bacidia carneola.

Description

Bacidia is characterised by its crustose lichen [growth forms|growth form]. The main body of these lichens typically appears as a thin layer that can be smooth, cracked, warty, or in texture. The thallus may sometimes develop specialised structures such as soredia, isidia, or tiny scale-like features. Its colour usually ranges from whitish to pale green, greenish-grey, pale grey, or fawn.
Like all lichens, Bacidia species represent a symbiotic partnership with algae. Their belongs to the group, featuring spherical or broadly oval-shaped cells. The fungal component produces distinctive reproductive structures called apothecia, which are disc-shaped and typically measure up to 1 mm across. These apothecia sit directly on the thallus surface and can vary from flat to strongly convex in shape and come in various colours.
The internal structure of the apothecia is diagnostic for the genus. They lack a but possess a – a protective layer composed of tightly packed, thick-walled fungal threads that have narrow internal channels and become gel-like. When stained with iodine, the spore-producing layer turns blue. Within this layer are specialised cells called paraphyses, which are rarely branched and often have swollen tips.
The spore-producing cells are club-shaped to cylindrical-club-shaped and each contains eight. The spores themselves are colourless and typically long and narrow or S-shaped, though some species produce more cylindrical to spindle-shaped spores. These spores usually have three or more cross-walls when mature.
The genus also produces asexual reproductive structures called pycnidia, which can be sunken into or sitting on the thallus surface. These structures produce colourless spores of various shapes, which may sometimes have cross-walls. When tested with common chemical spot tests used in lichen identification, the thallus shows no reaction, though the reproductive structures may contain various pigments.

Species