Lichen or Mychlophycophyta is group of plants whose plant body is made up of alga as well fungus both, which live together in an intimate relationship which is called symbiosis. Fungi in lichens are slightly parasitic upon algal cells from which they obtain food and in turn they help algae in absorption and retention of water.
Algal components of lichens are members of cyanophyta (blue green algae) and chlorophyta (green algae), the fungi are chiefly ascomycetes rarely basidiomycetes. Science of lichens is called lichenology. Lichens lie on shady sides of tree trunks, rocks, log of wood. Plant body in lichens is a thallus. It is grey or greyish green irregularly shaped structure. Some species are yellow, orange, brown or red in colour. Lichen thalli are classified under three types:
(1) Crustose lichens: These lie on rocks and bork which adhere closely to the substratum.
(2) Foliose lichens: They are flat, broad, much lobed leaf like. It grows more or less free of substratum but close to it. Foliose thallus is attached to rocks and twigs by rhizoid like out growths called rhizines e.g.: xanthoria, Physcia, Peltigera and Parmelia.
(3) Fructicose lichens: Thallus of these lichens in slender and much branched. Branches may be cylindrical to ribbon like in form. Thallus grows erect or lauging from rocky areas, foliage and branches of tree. It is attached to substratum by basal disc only.
e.g.: Usnea cladonta.
Internal structure:
Thallus is partly made of alga and partly of fungus such Thallus is called consortium. In the centre is medulla made up of closely twisted hyphae. Space between outer find and medulla is occupied by cortex which is again made up of interwoven fungal hyphae. On the upper surface are alga cells forming gonidial layer.
Reproduction:
(1) Vegetative reproduction: It takes place by fragmentation or by soredia. Soridia are spherical structure formed by mycelia. These contain a sort of cortex inside them. The cortex is made up of loose hypahe and algal ells. Each soredium is capable of germinating into a new plant.
(2) Asexual reproduction: In some lichens asexual reproduction takes place by black coloured spores which are formed in phycmidia. On germination each spore forms many fungi hyphae when these hypahe come in contact with algal cells lichen thallus is formed.
Sexual reproduction: This is similar to Ascomycetes as most lichens are asoclichens and hence fungi body represents the ascomycetes nature. Thus fruitification like apothecia and perithecia occur in lichens as end products of sexual reproduction. Only fungi partner of lichens takes part in sexual reproduction. Female reproductive organ is called carpogonium which is coiled and multi-cellular male organ is a flask shaped structure and is called as spermogonium. It produces motile spermatia. The male nucleus fuses with female nucleus. Fused cell produces ascogenous hypahe within which develop 8 ascospores and asci. The hynenium is made up of Asci and paraphysis. The fruiting body may be either apothericia or perithericia.
Economic importance of Lichens or uses of lichens:
(1) Formation of soil or ecological importance: Most crustose lichens growing on rocks dissolve and disintegrate them into soil particles. Limestone rocks are dissolved by the action of certain chemical secreted by lichens and the disintegration of rocks is due to stress and strain included by contraction and expression of gelatinous thalli. When the lichens die and decay, they form humus soil together with rock particles, mosses, forms and other plants grow on it. Thus they prepare ground for succession of vegetation.
(2) Food value: Many lichens are consumed as food for insects and their larvae. Some lichens like cladomia rangifera commonly called Reinderargrass and is used as food by reindeers of arctics. Some lichens like cetraria icelandica, umbillcasila, lecnora use as food by human beings.
(3) Medicinal value: Lichen mass produces mucilaginous substance which is used as laxative. It is also ingredient of salves, pudding and cultural media for bacteria. Many lichen produce medicines.
(4) For dyes: Many lichens like Pocelia tinctoria gives purple dyes which is used for centuries.
(5) Sweet scented thalt of some lichens are used in perfumeries.
.jpg)