STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION IN VAUCHERIA
Order
– Heterosiphonales
The genus Vaucheria comprises
about 54 species, out of which only 9 species are Indian. Most species are
terrestrial and aquatic. A few species are marine (V. piloboloides).
Terrestrial species grow on moist soil and in ploughed fields where they form
green velvety mats. Aquatic species occur widely in stagnant brackish water and
some are seen in shallow fresh waters of ponds and ditches or near the banks of
slow flowing streams.
The common Indian species of Vaucheria are
– V. sessilis, V. geminata, V. amphibian,
etc.
B. STRUCTURE OF THE VEGETATIVE BODY:
The
plant body i.e., thallus is filamentous. The filament is variously branched,
cylindrical and tubular in structure, long and yellowish green in colour. The
thallus is multi-nucleate and without any transverse wall or septation. Septa
formation takes place during the formation of reproductive structures. Most
terrestrial species are attached to the substratum by means of hapteron like
colourless branched outgrowth called rhizoids. Rhizoids are absent or ill
developed in floating species.
Cell
wall of the filament is thin and weak. It is composed of an outer layer of
pectic substance and inner layer of cellulose. A thick layer of cytoplasm is
present in the periphery just beneath the cell wall. Numerous discoid
chloroplast and nuclei remain embedded in the cytoplasm. Next to the cytoplasm
is the central vacuole which is filled up with cell sap. The pigments present
in Vaucheria are – Chlorophyll-e, Chlorophyll-a,
β-carotene, siphonein, siphonoxanthin, etc. The cytoplasm also contains reserve
food in the form of colourless oil droplets.
C. REPRODUCTION: Vaucheria reproduces both by vegetative, asexual
and sexual methods of reproduction.
1. Vegetative Reproduction:- Vegetative reproduction takes place
by fragmentation. In this, the thallus accidentally breaks
into short segments, each of which becomes thick walled and finally develops
into new plant.
2. Asexual Reproduction:- It takes place by the production of
different types of spores –
(a) Zoospores – It is the most common method of
reproduction of aquatic species . The zoospores ofVaucheria are
large, multinucleate and multi-flagellate motile spores formed singly within
the elongated club shaped zoosporangia developed at the
tip of the side branch.
The mature zoospore escapes through a narrow
aperture which is formed by the gelatinization of the wall at the distal end of
the zoosporangium. The zoospore takes a short period of 5-15 minutes of rest
after liberation and starts germinating.
(b) Aplanospores – These are non-motile asexual spores
produced normally by the terrestrial species. These spores are developed when
the terrestrial species are exposed to draught or when the species are
transferred from light to darkness or from running to still water. The
aplanospores are more or less rounded or elongated structures developed at the
ends of short lateral or terminal branches known asaplanosporangia.
Very rarely the aplanospores grows into a new plant.
(c) Akinetes – In some aquatic and terrestrial
species when exposed to greater dessication or low temperature, the branched
filaments divides into rows of short segments by thick, gelatinous cross walls.
These are resting spores and are known as cysts or hypnospores or akinetes.
For a time the cysts may remain connected by a parent membrane of the filament,
giving it the appearance of another alga Gongrosira. This stage
of Vaucheria is thus known as Gongrosira stage.With
the return of conditions favourable for growth, each cysts may germinate into
new plant.
3. Sexual Reproduction:- The sexual reproduction of Vaucheria is
of oogamous type. Most of the species of fresh water and
terrestrial habitats are homothallic, while a few marine species
are heterothallic.
In homothallic species, both antheridia and oogonia are
produced adjacent to one another, either on a common lateral branch or on
adjoining branches.
(a)
Antheridia:- The
antheridium is slender, curved, hooked-like tubular structure opening by a
terminal pore. It is formed at the end of short lateral branch. The tip of the
branch producing an antheridium becomes densely filled with cytoplasm containing
many nuclei and few chloroplasts. It is then cut
off from the other parts of the vegetative body by a transverse wall. The
protoplast of an antheridium becomes divided into number of uninucleate
fragments, each of which is metamorphosed into a biflagellate sperm or antherozoid.
Each
antherozoid is a colourless, pear shaped structure provided with two laterally
flagella of equal length. When the antheridium is fully developed, the antherozoids
are liberated into the surrounding water through a terminal pore by the
gelatinization of the antheridial wall at the apex.
(b) Oogonia:- The oogonium is a tumid, round or oval,
sessile or short stalked body and has a wall with rounded beak, which opens at
maturity to receive the sperm. It is formed adjacent to the antheridium
and begins to develop simultaneously with the accumulation of the colourless,
multinucleate mass of cytoplasm, called waterplasm.
The protoplasm i.e., the contents of the
oogonium round off to form a single uninucleate egg or ovum.
(c)
Fertilization:- During fertilization both antheridia and oogonia open,
usually simultaneously or with the difference of time ranging from a few
minutes to one or two hours. By the opening and rupturing of oogonial tip a
small drop of cytoplasm oozes out in the form of a twisted mass. Many sperms
escape from the terminal opening of the antheridium. A few of them enter into
the oogonium through the apical pore, but only one of them fuses with the egg
resulting in the formation of diploid zygote (2n).
After
fertilization the zygote develops a thick wall (usually 3-7 layers) and
undergoes a resting period of a few months. When resting period is over, it
undergoes meiotic division. As a result haploid nuclei are formed and from each
haploid nucleus a haploid coenocytic filament is produced.
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