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87 In short, the lichens make a very successful living on even


inhospitable areas, both the partners must have been helpful to each other in their co-existence. On the otherhand, artifical creation of lichen is impossible. Scientists isolated the algal and fungal components and studied well. But they failed to create a new type in the laboratory. The work is not yet a success. Classification: The classification is based solely on the fungal partner because the thalli and fruiting bodies of them are largely fungal in structure. Miller (1984) assigned lichen to a subdivision in true fungi (Eumycophyta) and divided into two classes. 1. Class : Ascolichens : Fungal partner is an Ascomycotina. 2. Class : Basidiolichens : Fungal partner is a Basidiomycotina. Alexopoulos and Mims (1979) divided lichen into three groups.


1. Basidiolichens : Fungal component is a Basidiomycotina. 2. Deuterolichens : Sterile lichens that donot produce spores. 3. Ascolichens : Fungal component is a Ascomycotina. These groups are further divided into orders, suborders, families etc. Ascolichens form an important group and vary considerably in their size and form.


USNEA


Systematic Position: Subdivision Class Type


Genus


: : : :


Eumycophyta Ascolichens Fruticose Usnea


Usnea belongs to the class Ascolichens of fruticose type growing on


tree trunks of temporate forests. The thalli is shrub-like in nature and frequently branched. They are seen attached to the substratum by a conspicuous basal part and may be seen drooping from it. The main branch produces a number of lateral branches. Its colour is often whitish grey or greenish yellow. The basal part of the main branch is attached to the trunks by rhizines which are rhizoid – like structures. Since Usnea is a lichen, the thallus is formed of fungal and an algal


partners. They are so interconnected and behave as a single thallus. There exists a symbiotic relationship between them. The fungus shares the food produced by the algal cells while in return it protects the algal cells and also absorbs water.


Fig. 2.15 A- Usnea (Fruticose Lichen) B-Thallus T.S. C. Soredia


Internal structure Since Usnea is a fructicose lichen, the thallus is more or less circular


in transverse section. There are four distinct regions in the thallus. 1. Peripheral zone: It is the outermost region of the thallus formed of closely packed fungal hyphae. The spaces in between the hyphae are filled with a mucilagenous substance.


2. Cortex: It is comparatively broad region in which the hyphae are placed at right angles to the longitudinal axis of the thallus. The hyphae are often loosely packed with much interspaces. The cortex is divisible into two, an outer and inner region.


3. Medulla: It is the centre region of the thallus in which the hyphae are closely packed to give an almost pseudoparanchymatous appearance.


4. Algal zone: Just inner to the outer cortex, the algal partners are distributed in the form a layer called algal layer or gonidial layer (The algal cells are also called as gonidia). The algae are often unicellular. At certain regions the hyphae are loosely interwoven forming


localized openings called the breathing pores, which are the organs of aeration. Reproduction: Usnea reproduces by vegetative, asexual and sexual means


Vegetative method: It reproduces by the breaking off the fragments of the thallus containing both fungal hyphae and photosynthetic cells. New indi- viduals can also be formed by the capture of an appropriate alga or cyanobacterium by a lichen fungus in its free living hyphal stage. Sometimes


88


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