Lichen growth forms
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. They are regularly grouped by their external appearance – a characteristic known as their growth form. 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. Lichenologists (those who study lichens) have described a dozen of these forms: areolate, byssoid, calicioid, cladoniform, crustose, filamentous, foliose, fruticose, gelatinous, leprose, placoidioid and squamulose. Traditionally, crustose (flat), foliose (leafy) and fruticose (shrubby) are considered to be the three main forms. In addition to these more formalised, traditional growth types, there are a handful of informal types named for their resemblence to the lichens of specific genera. These include alectorioid, catapyrenioid, cetrarioid, hypogymnioid, parmelioid and usneoid.
Lichens are composite organisms made up of multiple species: a fungal partner, one or more photosynthetic partners (also known as photobionts), and sometimes a yeast.[1] It is a symbiotic relationship, to which each partner contributes. In most cases, the fungal partner provides the structure in which the various partners live; this structure helps to protect the photobiont from environmental pressures.[2] The photosynthetic partner(s) provide the nutrients which the various partners need to survive.[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.[3][4]
With the exception of calicioid lichens, lichen growth forms are based on the appearance of the thallus, which is the vegetative (non-reproductive) part of the lichen.[5] In most species, this form is determined by the lichen's fungal partner, though in a small number, it is instead the alga or cyanobacteria (the lichen's photosynthetic partner) that determines the organism's overall shape.[6] Growth form groupings are not always consistent with lichen taxonomy; lichens with similar growth forms are not necessarily related, and some of those which are related do not have similar growth forms.[7] Not every species can be easily categorized. Some show characteristics of two growth forms, and different authors may place such species in different groups.[8] In general, a particular species shows same overall growth form wherever it is found, but this is not always the case.[8] Traditionally, crustose, foliose and fruticose are considered to be the three main forms.[9]
An areolate lichen is the most common form of crustose lichen.[10][11] As with all crustose lichens, it has a paint-like appearance, and is inseparable from the substrate on which it grows. However, its thallus is broken into regular polygonal sections, which can look a bit like cracked mud, flaking paint or little islands.[6][10] These sections, known as areolae, are surrounded by a thin layer of fungal hyphae called a hypothallus. This layer, which is usually dark, generally grows faster than the thallus which rides above it.[11] This growth form is an adaptation which allows the lichen to cope with alternating periods of wet and dry. During wet periods, the lichen can absorb water, its tissues can swell, and the cracks close.[10] The term "areolate" is derived from the Latin word areolatus, meaning "with areolae" (the plural of a diminutive form of area, meaning "halo" or "open space") combined with the Latin suffix -atus, meaning "provided with" or "likeness".[12]
