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...
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