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Subject : Botany Topic : Usnea Descriptive and easily understandable. Important diagrams included. Written in point wise.
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Kingdom : Fungi Division : Mycota Class : Lichens Sub-class : Ascolichens Order : Parmeliales Family : Usneaceae Genus : Usnea Usnea antarctica Usnea longissima Usnea is a fruticose lichen. It has a cylindrical to ribbon-like and much branched thallus. It is attached to the trunks of trees by rhizoid-like outgrowths called rhizines.
The plant grows erect or pendent with a main branch, bearing many lateral branches. The colour of the thallus is grey, green or greenish yellow. The lateral branches terminally bear large, plate-like apothecia. Their margin is fringed with bristle-like outgrowths. Structure of the thallus :- Usnea is a heteromerous lichen in which the algal component is restricted to a specific zone or layer, called ‘algal zone’. A cross section of the thallus reveals four distinct regions, namely peripheral zone , cortex, algal zone and medulla.
The algal cells are sometimes called gonidia and so this layer is often referred to as gonidial layer. It is the photosynthetic region of the lichen thallus. Medulla : It forms the central core. of the thallus. Hyphae are very closely interwoven, giving a pseudoparenchymatous appearance. Structure of the apothecium :- The fruiting body of Usnea is called apothecium. It is produced by sexual reproduction. The surrounding vegetative hyphae also actively take part in the formation of apothecium. Apothecium is a saucer-shaped structure, produced on a long stalk. Each apothecium contains a large number of asci, produced from the ascogenous
hyphae. Each ascus contains eight ascospores. The asci and paraphyses are closely packed to form a continuous fertile layer, called the hymenial layer or hymenium. It is also known as thecium. Beneath the hymenium, is a region consisting of a dense mass of sterile tissue, called subhymenium. It consists of closely packed and interwoven hyphae. Below the subhymenium, is the hypothecium that forms the main body of apothecium. Hypothecium contains loosely arranged hyphae internally, and compactly arranged peridium externally. The outer part of peridium contains several short, stiff, simple or branched bristle-like outgrowths.