Lichens are symbiotic organisms made up of multiple species: a fungus, one or more photobionts (an alga and/or a cyanobacteria) and sometimes a yeast.
This form, which is based on the appearance of vegetative part of the lichen (its thallus), varies depending on the species and the environmental conditions it faces.
Those who study lichens (lichenologists) have described a dozen of these forms: areolate, byssoid, calicioid, cladoniform, crustose, filamentous, foliose, fruticose, gelatinous, leprose, placodioid and squamulose.
[2] The yeast (where present) appears to help ward off microbes and potential predators through the production of various chemicals.
[1] Thallus types have evolved to provide the lichen's photobiont with optimal levels of light, water, and carbon dioxide, with different environmental conditions favouring different forms.
However, its thallus is broken into regular polygonal sections, which can look a bit like cracked mud, flaking paint or little islands.
[12] A byssoid lichen has a wispy, cottony or teased wool appearance due to the loosely woven hyphae in its thallus.
In one type, the thallus is dominated by fungal hyphae, with a photobiont – typically a coccoid green alga – sprinkled throughout.
[16] Unlike the other growth forms detailed here, a calicioid lichen is distinguished by its fruiting bodies rather than its thallus.
[19] Calicioid lichens are generally restricted to old-growth forest, and can be used as indicators of the age and quality of such ecosystems.
[22] A crustose lichen, as its name suggests, is crust-like and two-dimensional, closely and completely bound at nearly all points to the substrate on which it grows.
[23] Some crustose lichens are thick and lumpy, others thin and smooth, and some are almost completely submerged into the substrate with only apothecia emerging to the surface.
A thin layer of fungal hyphae surrounds an algal chain, resulting in a thread-like or hair-like structure.
[33] A foliose lichen has flat, leaf-like lobes that are generally not firmly bonded to the substrate on which it grows.
[35] Foliose lichens are attached to their substrate either by hyphae extending from the cortex or medulla, or by root-like structures called rhizines.
[40] Instead, a cortex covers its entire surface, and the photobiont layer lies just below this, on all sides of the lichen's branches.
Chains of the photobiont, rather than fungal hyphae, make up the bulk of the thallus, which is unlayered (and undifferentiated) as a result.
[43] Gelatinous lichens are particularly common in areas with erratic rainfall or periodic inundation (such as rock pools).
[44][45] The presence of Nostoc cyanobacteria allows the lichen to absorb significant amounts of moisture, swelling in the process.
Its undifferentiated thallus is an irregular mix of fungal hyphae and scattered photobiont cells, lacking a cortex or any definable layers.
[48] The cell walls of leprose lichens contain chemical compounds which make them hydrophobic, and thus largely water repellent.
[48] They can be completely covered in soredia – small aggregates of fungal hyphae and photobiont cells which can break off to form new lichen colonies.
They are fruticose, typically with a beard-like thallus that is dangling or clustered; members of the genera Bryoria, Oropogon, Pseudephebe, and Sulcaria also have this growth type.
[60] Catapyrenioid lichens were historically members of the genus Catapyrenium; many have now been moved to other genera within the family Verrucariaceae.
[61] Cetrarioid lichens were historically classified in the genus Cetraria; many have now been moved to other genera within the family Parmeliaceae.
[64] Parmelioid lichens were historically classified in the genus Parmelia; many have now been moved to other genera within the family Parmeliaceae.
They are primarily foliose, often closely attached to the substrate upon which they grow, and have apothecia and pycnidia over their entire surface (laminal), rather than only at the margins.