STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION IN 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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