Lichens are a small group of plants of composite nature, consisting of two dissimilar organisms, an alga-phycobiont (mykos- fungus, bios- life) and a fungus-mycobiont (mykes-fungus, bios-life), living in a symbiotic association. Generally, fungal partners cover mostly portion and developed their own reproductive structures. Algal partner produced food through photosynthesis.
The plant body is a thalloid with different shapes and colours.
Hawksworth and Hill (1984) categorised the lichen into five main types:
Lichens reproduce by all three means, vegetative, asexual and sexual.
1.Vegetative reproduction:
2. Asexual reproduction:
3. Sexual reproduction:
The fungal partner of the lichen reproduces sexually and forms fruit bodies. Sexual reproduction in Ascolichen is the members of Ascomycota, whereas in Basidiolichen is like that of Basidiomycotina members.
In Ascolichen, the female sex organ named as carpogonium and the male sex organ , as spermogonium. Multicellular carpogonium is differentiated into basal coiled ascogonium and upper elongated multicellular trichogyne. The ascogonium remains embedded in the algal zone.
The spermatogonium is flask-shaped and develops spermatia from the inner layer. The spermatia behave as male gametes. The spermatium, after liberating from spermatogonia, gets attached with the trichogyne at the sticky projected part. The nucleus of spermatium migrates into the carpogonium and fuses with the egg when the common wall dissolved.
In Basidiolichen, the result of sexual reproduction is the formation of basidiospores that develop on basidium as in typical Basidiomycotina. The fungal member along with the blue-green alga, as an algal partner, forms the thalloid plant body. The thallus grown over soil produces hypothallus without rhizines, but on tree trunks, it grows like bracket fungi and differentiated internally into the upper cortex, algal layer, medulla and lower fertile region with basidium bearing basidiospores.
Many ascogenous hyphae develop from fertilised ascogonium. The binucleate penultimate cell develops into an ascus. Both the nuclei fuse and form a diploid nucleus (2n), which undergoes first meiotic and then mitotic division – results in eight haploid daughter nuclei. Each haploid nucleus with some cytoplasm metamorphoses.
The asci remain intermingled with paraphyses (sterile hyphae). Then asci and paraphyses become surrounded by vegetative mycelium and form a fruit body. This type may be asco hymenial type or apothecium as in Parmelia, Anaptychia or perithecium as in Verrucaria. Apothecium consists of three distinct parts, the middle thecium, consisting of asci and paraphyses, is the fertile zone covered by two sterile zones- the upper epitheca and lower hypotheca. This region differentiated like the vegetative thallus into the outer cortex, algal zone and central medulla.
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Lichens are a small group of plants of composite nature, consisting of two dissimilar organisms, an alga-phycobiont (mykos- fungus, bios- life) and a fungus-mycobiont (mykes-fungus, bios-life), living in a symbiotic association.
They reproduce by all three means, vegetative, asexual and sexual.
These are small greyish-white, bud-like outgrowths developed on the upper cortex of the thallus.
some lichen developsevelop pycniospores or spermatium inside the flask-shaped pycnidium. They usually behave as gametes. Under the favourable condition, they germinate and develop fungal hyphae.
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