A lichen is an association between a fungus and a photosynthetic symbiotic that results in a stable thallus or body , of specific structure . The photobiont is either an alga or a cyanobacterium . A remarkable feature of a liche n is the transformation that the symbionts , in particular the fungus , undergo during the association . A new entity , the thallus , is formed , and unique chemical compounds are synthesized . The physiological behaviour of the symbionts also changes in symbiosis .

There are about 15 ,000 species of lichens , an indication that this type of symbiosis has been highly successful and has involved many species of fungi . Surprisingly , only about 30 different types of algae and cyanobacteria have been reported as photobionts A lichen thallus usually consists of layers such as an upper and lower cortex , algal layer , and medulla . The layers differ in thickness and are better developed in some species than in others .

Fungal hyphae make up most of a thallus the photobiont cells are only a small percentage (about 7 ) of the There are three main types of thalli : crustose , foliose , and fruticose A crustose thallus lacks a lower cortex and is generally considered to be the most primitive type . Thalli of Lepraria species do not have layers but consist only of powdery granules . There are more species of crustose lichens than other types , and most of them belong to the genera Lecanora and Lecidea .

Many crustose lichens stick tightly to the substratum and appear to be painted on it . Some species grow inside rock crevices and bark and still manage to produce separate layers . Squamules are typical of many species of Cladonia . Squamules are a specialized type of crustose thallus and are at only one end to the substratum A foliose thallus has an upper and lower cortex , an algal layer , and medulla and is usually loosely to the substrate by hairlike structures called rhizines .

The thallus has many different sizes and shapes and is often divided into lobes . Common foliose genera include Anaptychia , Cetraria , Parmelia , Physcia , and Xanthoria . Some foliose lichens , such as Umbilicaria (rock tripe , have thalli that are to the substrate by only one central point Fruticose thalli are upright or hanging , round or flat and often highly branched . Thalli of Usnea are hairlike and can reach a length of 5 m whereas those of Evernia are shorter and strap-shaped .

The layers of a fruticose thallus may surround a central thick cord , as in Usnea , or a hollow space as in some Cladonia species. Lichens grow practically everywhere – on and within rocks , on soil and tree bark , on almost any inanimate object . They grow in deserts and in tropical rainforests , where they occur on living leaves of plants and ferns . They have been found on the shells of tortoises in the Galapagos Islands and on large weevils in New Guinea .

In the dry valleys of Antarctica , endolithic lichens , such as Buellia and Lecidea , grow inside sandstone crevices. The rock moss can then grow and cover the outcrop. Once it covers the outcrop, other organisms build on it. Heathers, ferns, berries, and even trees colonize the once barren rock. It all starts when the lichen settles onto the rock and begins to make a home for itself where no one else will go. The humble lichen is a great pioneering being. For some people, it is an assurance of a safe passage to solid land once again.