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.