NOSTOC - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION

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A. CLASSIFICATION:

Division – Cyanophyta
     Class – Cyanophyceae
          Order – Nostocales
                Family – Nostocaceae
                     Genus – Nostoc
The genus Nostoc comprises about 23 species in India and found to grow both on terrestrial and aquatic habitats. Terrestrial species grow on damp soils, especially in paddy fields and forms lathery or rubbery sheaths (Nostoc commune). Aquatic species are often attached to the substratum or they may be free floating on stagnant water bodies. Some species of Nostoc are also found to live in association with other plant species.
Nostoc thalli occur in colony forming closely packed trichomes. They are enclosed by a common envelop and forms a ball-like structure called ‘Nostoc ball’.
Some of the common Indian Nostoc species are, – N. commune, N. endophytum, N. muscorum, N. hatei, N. punctiforme, N. calcicola, etc.
B. STRUCTURE OF THE VEGETATIVE BODY:
The plant body is un-branched filamentous trichome. The filaments are aggregated together within a mucilaginous envelop forming a ball-like gelatinous colony, known as ‘Nostoc ball’. Nostoc balls are bluish-green or greenish in colour. Each ball or colony contains numerous trichomes which remain inter-winged with each other.
Each trichome is enveloped by a thin mucilaginous, hyaline or coloured sheath forming a filament. Each filament consists of a row of rounded oval cells, which seems to be arranged like beads on a string. Such a habit is called moniliform.
Some cells of the filament are quite larger, rounded and thick walled cells having transparent contents. These are called heterocysts. Besides the formation of heterocysts, the filament occasionally forms akinetes.

C. REPRODUCTION: Nostoc  reproduces entirely asexually by the following methods –
1. Colony Fragmentation:- The Nostoc colony as it gets larger frequently breaks up into flat expanses as a result of storms and other disturbances. Each of these grows into the size of the parent colony.
2. Hormogonia:- Hormogone formation is very common in Nostoc. In such case the trichome ruptures at places where heterocyst and the vegetative cells adjoin. In this way short, short segments of living cells are formed called the hormogonia.
The hormogonia slip out of the gelatinous matrix and establish new colonies by division. The terminal cells of the hormogonia differentiate into heterocysts. The intercalary cells then divide in the plane parallel to the axis of the trichome forming a packet of cells. This is called aseriate stage.
3. Akinetes:- Under certain conditions any cell or some vegetative cells of the trichome become enlarged and secretes a thick, highly resistant wall around it. They get filled with the reserved food materials. Such specially modified vegetative cells are called akinetes or resting spores. These are well adapted to survive the unfavourable conditions like water shortage and unsuitable temperature.
The resting spores survives during the unfavourable conditions and with the return of the favourable conditions, they germinate into new plants.
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