The plant body of lichens is a thallus and its growth is slow and irregular. They appear in different colours such as grey, green, yellow, orange, red, brown or black on the surfaces of substrata. On the basis of the general habit of growth, form and manner of attachment to the substratum and morphological structure of thalli, they are of the following three types:
1.Crustose lichen ( Crust like ) : Crustose lichens grow on the substrata like bark of trees, ground and surface of rocks. They are flattened thin or thick thalles without any distinct lobes. They remain partly embedded or completely embedded inside the surface on which they grow.. They are strongly attached to the substratum, so, they cannot be isolated easily from the substratum. Thus, they are also encrusting or crustaceous lichens. The surface of the thallus is usually divided into hexagonal areas called the acrolate. The upper surface of the thallus bears several cup-shaped fruiting bodies called the apothecia. The fruiting bodies (ascocarps) are present on the surface for sexual reproduction Examples, Graphis, Lecanora , Locidea, Rhizocarpon , etc
2.Foliose lichen ( leaf like ) :Foliose lichens grow free on the substratum such as rocks/ stones, soil, bark of trees,etc. They have a dorsoventrally flattened and broad thallus with much lobed margins. They are also called foliaceous lichens. The lower surface may be white or sooty. The thalli are attached to the substratum by delicate rhizoid-like outgrowth or branched hyphal system, called the rhizinaes present on the ventral side. The function of rhizinaes is anchorage and absorption. The upper surface of the thallus bears fruiting bodies called apotheica e.g. Parmelia, Physcia and Peltigera, etc.
3.Fruticose lichen ( shrubby ) : Fruticose lichens are very distinct in comparison to other lichens. They have a most complex thallus which is slender and freely branched. The branches may be cylindrical or ribbon-like. The form thread-like or twig-like tufts. The thalli are attached only on the substratum by a flattened mucilaginous disc. Their thalli are not differentiated into upper and lower surfaces. They may be erect eg. Cladonia sp or hanging down or pendent e.g. Usnea sp The tips of branches bear fruiting bodies the apothecia for sexual reproduction. They are also called frutic lichens e.g. Cladonia (reindeer moss), usnea ( old man's beard )