Lichen and Fungi

Lichens are dual organisms composed of an alga and a fungus. The description of each fungi can see on my other blog of Fungi. The body, or thalus, of the lichen consists of alga cells mixed among the thread like hyphae of the fungus; there are no true leaves, stems, or roots. The alga and the fungus live associated in a dependent symbiotic, or mutualistic, relationship; that is, they typically required each other to survive.

Lychens are classified on the basis of the fungal component. The fungi are either Ascomycetes, Basidiomycetes (bracket fungi), or Deuteromycetes (fungi imperfecti). The algae are either green (Chlorophyta), blue green (cyanophyta), or yellow green (Xanthophyta). Most lichen are composed of green algae and ascomycete fungi.

Lichen appear in a wide variety of habitats, including dry deserts, moist woods, mountaintops, under the soil and in the ocean. They may be crustose (firmly attached to and encrusted on some surface); fruticose (projected out in upright, branched or hanging stalks); or foliose (assuming leaflike shapes). Lichen are slow growing, some enlarging as little as 1 mm (0.04 in) per year; and are generally long lived, with some Arctic lichens claimed to be 4000 years old. Lichens play a role in the establishment of plant colonies on bare areas by accumulating soil debris beneath them and by limiting rock disintegration which aid in soil formation.

The algae manufactures and provides itself and the fungus with a carbohydrate, either a simple sugar, such as glucose, or a sugar alcohol, such as sorbitol; the fungus converts the carbohydrate into a sugar alcohol; manitol, which may serve as a storage food. The algae also provides such vitamins as biotin and thiamine. Blue green algae, in addition, are able with certain, or fix, nitrogen from fungus provides the alga with certain physical protection and obtains water vapor from the air, providing moisture for the alga. Few of the lichenized fungi or algae can survive alone; the green alga Trebouxia, found in about half of all lichen species, has never been found in the free living state.

Reproduction of alga can occurs in several ways. The alga or fungi may reproduce separately, in a manner identical to free living forms. The alga commonly does this by reproducing asexually within the lichen thallus. The lichen as a whole may reproduce vegetation by fragmentation or by various special algal-fungal combinations, such as soredia and isidia. A soredium is a package of one or more algal cells and some fungal hyphae, which are commonly released in great numbers through openings in the surface of the thallus containing algal cells; it may break off and reproduce vegetatively.

Lichen of genus Cladonia, known as reindeer moss, are a staple of caribou diet in North America. Old man's beard, Usnea, grows in long, gray green streamers from branches in moist northern woods. Dyes and medicinal substances are extracted from lichens, but only the dye or chil, which is used as a food colorant and the source of litmus (a pH indicator), is commercially important.