1. Structural classification
In 1984, scientists Harsworth and Hill divided lichens into five groups based on structure.
(i) Leprose lichen : This is the simplest lichen. A single hyphae of the fungus envelops the alga. The fungus looks like a molecule or powder in the container. For example – Lapraria incana.
(ii) Crustose lichen : Those lichens which are intimately attached to the habitat and form a thin, flat and hard covering are called crustose lichens. They are also called lichens. Like- Graphis scripta, Lecanara muralis, Strigula angulul, Cryptothecia rubrocinta, Diploicia canescens, Calicium indicum, Pyrenula micheneri, Arthonia radiata, Trypethelium virens, Xylographa vitiligo etc.
(iii) Foliose lichen : The lichens which are broad like leaves with irregular surface, flat, broad, divided edges and many grooves are called foliose lichen. They are also called foliar lichens. Rhizine is produced from its lower surface. Such as Xanthoria calcicola, Peltigera canina, Parmelia Cetraria, Parmotrema tincotorum, Collema tenax, Flavoparmelia caperata, Zanthoria peltigera, Physcia caesia etc.
(iv) Fruticose lichen : Cylindrical, filamentous and branched hanging lichens with attractive and complex structures are called fruticose lichens. They are also called flowering lichens. Such as Cladonia leporina, Usnea barbata, Alectonia sarmentosa, Letharia columbiana, Evernia prunastri etc.
(v) Filamentous lichen : The lichen in which the algal part is filamentous is called filamentous lichen. Eg Ephebe lanata, Racoleus trichophorus etc.
2. Habitat classification
(i) Corticolous : This lichen grows on the bark or stems of trees in tropical regions. Like- Graphis scripta, Evernia prunastri, Parmelia sulcata, Usnea barbata etc.
(ii) Terricolous : They grow in moist and upland soils. For example – Collema tenax, Cladonia leporina, Lecidea atrobrunnea, Cora pavonia etc.
(iii) Sexicolous : They grow on rocks or rocks in temperate regions. Such as Xanthoria calcicola, Aspicilia contorta, Dermatocarpon miniatum etc.
(iv) Lignicolous: They grow on wet wood. Such as Calicium lenticulare, Chaenotheca trichialis, Cyphelium notarisii, Piptoporus betulinus etc.
(v) Foliicolous : They grow on the leaves of ferns or succulent plants. Like Porina epiphylla, Strigula angulul etc.
(vi) Omnicolous : They grow on leather, wood, iron etc. For example, Lecanora dispersa.
3. Classification based on lichen forming fungi
(i) Ascolichen : If the lichen forming fungus belongs to the class of Ascomycetes then it is called ascolichen. E.g. Rhizocarpon macrosporum.
(ii) Basidiolichen : If the lichen forming fungus belongs to the class Basidiomycetes then it is called Basidiolichen. For example – Clavaria mucida.
(iii) Deuterolichen : If the lichen-forming fungi belong to the class of Deuteromycetes, they are called deuterolichens. Such as Racodium rupestre, Leparia incana etc.
4. Structural classification
(i) Homoiomerous : Lichens which contain algae and fungi equally are called homoiomeric. For example – Collema tenax.
(ii) Heteromerous : Those lichens which have algae and fungi unequally are called heteromerous lichens. For example- Parmelia sulcata.
