VAUCHERIA - STRUCTURE OF VEGETATIVE BODY AND REPRODUCTION

A. CLASSIFICATION:

Division – Xanthophyta

        Class – Xanthophyceae

                  Order – Heterosiphonales

                            Family – Vaucheriaceae

                                   Genus – Vaucheria

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 of Vaucheria 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 as aplanosporangia. 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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