Types of Lichenes

Types of Lichenes

1. Foliose lichens

These are some of the largest and perhaps most complex lichens. The thallus generally forms flat, leaf-like lobes, with differentiated layers of tissue, the upper and lower cortices, forming the upper and lower surfaces. The lobes are commonly, but not always, appressed to the substrate surface, but can be lifted away. The lower cortex is often differently coloured, frequently brown or black and usually bears rhizines. In Peltigera the lower surface is ecorticate. 

In foliose lichens with multiple branches of the thallus that may stand away from the substrate, the differentiated lower cortex distinguishes them from fruticose lichens, e.g. Evernia prunastri, in which the thallus lobes are white beneath, and Pseudevernia furfuracea, in which the undersides are black when mature. 

2. Fruticose lichens 

The thallus is extended up into a tufted or pendant branched structure, the branches beingcovered by a single cortex. In fruticose lichens with flattened branches, e.g. Ramalina spp., the cortex extends round both surfaces of the branch. Consequently, they differ from foliose lichens with branched, aerial lobes such as Evernia. 

3. Squamulose lichens 

In squamulose lichens, the thallus is composed of usually small, flat, usually massed, often overlapping scales – ‘squamules’. They differ anatomically from the smaller foliose lichens in that the squamules do not have a lower cortex, or at most it is weakly differentiated, though the underside may be differently coloured from the rest of the medulla and sometimes (as in Catapyrenium and Placidium), rhizoidal hyphae may be produced. 

In some lichens, e.g. Cladonia species such as C. digitata or C. foliacea, the squamules are relatively large, several millimetres in length, and may be lobed and/or held away from the substrate; they nevertheless remain limited in size. Extension of the thallus is typically by growth of the underlying prothallus. In some genera, such as some Dermatocarpon species with massed, single lobes, the distinction between "foliose" and "squamulose" is arguable. 

4. Crustose and Placodioid lichens 

 a. Crustose and Placodioid lichens 

The thallus forms a crust over the substrate and is firmly attached to it. There is an upper cortex, at least in early development, but no lower cortex and the medulla is in direct contact with the substrate and commonly grows into it to some extent. Consequently, the lichen normally cannot be collected intact without collecting a portion of the substrate along with it. 

There are various terms to describe the nature of the thallus surface (quite apart from sexual reproductive structures such as apothecia or asexual reproductive structures such as soralia or isidia). The surface may be smooth or lumpy ('warted') and frequently the surface is areolate, i.e. composed of small, separate islands of thallus seated on an underlying prothallus or hypothallus. These areoles may be clearly separate ('dispersed') or be closely contiguous and often represent break-up of the thallus surface ('cracked-areolate'). If the surface is cracked but not broken up into discrete areoles, it is said to be 'rimose'. Sometimes the surface is pruinose. 

It follows that close examination of the surface, often with a hand-lens, is essential for identification. Any photographs must be in close view (macro mode or using a dedicated macro-lens) and avoiding any movement to ensure the result is pin-sharp. Snapshots of crustose lichens posted in Internet forums in the hope of identification are often completely useless for the purpose. In any case, only with considerable field experience (if then) can many crustose lichens be identified without microscopic and/or chemical confirmation. In American usage these are 'microlichens', liable to be excluded from identification guides. 

4. b. Placodioid lichens (incl. 'crustose-placodioid', 'squamulose-placodioid') 

The thallus is generally crustose, but the margin extends into distinct, radiating lobes, which still lack a lower cortex but which may not always be so firmly attached to the substrate. This is really a variant of "crustose" and many individual species may be simply crustose or may alternatively become placodioid. However, when well developed, the placodioid morphology may be striking – hence the separate treatment here. 

Some placodioid species can be confused with foliose species, e.g. crustose-placodioid species of Caloplaca, especially C. flavescens, can can resemble the foliose Xanthoria elegans, but the latter has true foliose lobes with a lower cortex. There can also be intergradation between placodioid and squamulose morphologies. as in Fulgensia fulgens and Solenopsora holophaea. 

- Basidiolichens are lichenized members of the Basidiomycota, a much smaller group of lichens than the far more common ascolichens in the Ascomycota. In arctic, alpine, and temperate forests, the most common basidiolichens are in the agaric genus Lichenomphalia[1] (including former members of Omphalina or Gerronema) and the clavarioid genus Multiclavula.[2] Several lichenized genera occur in tropical regions, the most common being the foliose Dictyonema. Previously basidiolichens had been classified in their own subclass, Basidiolichenes. Molecular based phylogeny does not support classification of the genera together. 

- Lichens take the external shape of the fungal partner and hence are named based on the fungus. The fungus most commonly forms the majority of a lichen's bulk, though in filamentous and gelatinous lichens this may not always be the case. The lichen fungus is typically a member of the Ascomycota—rarely a member of the Basidiomycota, and then termed basidiolichens to differentiate them from the more common ascolichens