3. It is a fruiticose 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 with main branch, bearing many lateral branches.
The colour of thallus is grey, green or greenish yellow.
Several conspicuous bristles or fibrils cover the thallus.
The lateral branches terminally bear large, plate like apothecia.
Their margin is fringled with bristle like outgrowth that may be simple or branched.
4. STRUCTURE OF THALLUS
Usnea is a heteromerous lichen.
A cross section of the thallus reveals four distinct regions namely peripheral zone,
cortex, algal zone and medulla.
Peripheral Zone:
Outermost region of the thallus
Thick and protective.
Hyphae in this region are closely interwoven.
The spaces among these hyphae are filled with gelatinous matrix.
5. Cortex:
It is a broad zone where the hyphae are loosely packed with interspaces.
It is divided into outer cortex and inner cortex.
Algal Zone:
Consist of algal cells and a tangled network of loosely interwoven fungal hyphae.
The phycobiont is unicellular green alga called Protococcus.
The algal cells are sometimes called gonidia and so this layer is ofyen referred to as
gonidial layer.
Photosynthetic region.
6. Medulla:
It forms the central core of the thallus.
Hyphae are very closely interwoven.
Pseudoparenchymatous appearance.
8. VEGETATIVE REPRODUCTION
Takes place by fragmentation and by the formation of structures called soredia.
Fragmentation:
During fragmentation the thallus breaks up into segments which later develop into
individual lichens.
It is accomplished either by ageing or by accidental breakage.
Soredia:
Are small, rounded granules that develop on the surface of the thallus.
Each soredium contain one or more algal cells closely surrounded by fungal hyphae.
Soredia are dispersed by wind.
When a soredium falls on suitable substratum, it germinates to a new lichens.
9. SEXUAL REPRODUCTION
Sexual reproduction is performed by fungal partner.
If the mycobiont is an ascomycetous fungus.
The male and female sex organs would be spermagonium and ascogonium
respectively
10. Spermagonium:
Is a flask shaped structure immersed in the thallus of usnea.
It opens at the surface by a small pore called ostiole.
The cavity of the spermagonium is covered by many fertile and sterile hyphae.
The fertile hyphae produce several minute, rounded cells at their tip called spermatia.
They are non motile and are produced in large numbers in spermagonium.
When liberated they ooze out through the ostiole.
11. Ascogonium:
Is produced on ascogonial filament.
It is a multicellular and specialized filament having two portions namely lower coiled
portion and upper straight portion.
The coiled portion is the ascogonium which is multicellular.
The ascogonium lies deep in the medullary portion of the thallus.
The straight upper portion of ascogonium is the trichogyne which is also multicellular.
All cells of the ascogonial filament are generally uninucleate.
12. FERTILIZATION
The liberated spermatia get attached to the trichogyne.
The interwening walls between the spermatium and trichogyne dissolves
The nucleus of the spermatium passes into the ascogonium.
This is followed by dikaryotization (pairing of two nuclei)
The dikaryon divides by conjugative division and the cells of the ascogonium become
diplodized.
From the ascogonium, ascogenous hyphae develop.
By this time the trichogyne collapses.
Asci are produced either from penultimate cells or directly from the terminal cells.
Each ascus is a club shaped structure with two haploid nuclei, which by fusion from a diploid
nucleus.
13. The diploid nucleus divides twice.
The first division is meiotic and it results in the formation of ascospores.
The ascus is seen intermingled with many sterile hyphae called paraphyses.
14. STRUCTURE OF APOTHECIUM
The fruiting body of Usnea is calked 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.
These asci are found intermingled with paraphyses.
Each ascus contain eight ascospores.
16. The asci and paraphyses are closely packed to form a continuous fertile layer called
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
sub hymenium. It consists of closely packed and interwoven hyphae.
Below the sub hymenium, is the hypothecium that forms the main body of
apothecium. Hypothecium contain loosely arranged hyphae internally and compactly
arranged peridium externally.
The outer part of peridium contains several short, stiff, simple or branched bristle like
outgrowths.
17. When the apothecium matures, the ascospores are liberated.
They are very light and are carried by wind.
They germinate to form new hyphae.
Those hyphae, coming in contact with suitable algal cells, develop to new Usnea
thalli.
