Life cycle of Vaucheria

What is Vaucheria?

Vaucheria is green freshwater alga found in ponds, ditches and wet soil. It is not free floating like spirogyra but is mostly attached to substratum by colourless rhizoids or holdfasts. It is thallus is single branched tubular filament. It contain many minute nuclei present in living layer of cytoplasm surrounding a large central vacuole. Such structure is called coenocyte. Septa appear in connection with reproductive organs.

Asexual reproduction in Vaucheria

It takes place by large solitary zoospore. During its development the apex of filament swells up, becomes club shaped and is separated from rest of filament by a septum. This club shaped body is called zoosporangium. Its protoplasmic contents become rounded off forming a single zoospore wall of zoosporangium, ruptures at the apex, and the zoospore escapes by terminal pore and begins to rotate. Zoospore is an oval body of large size. Central part of it is occupied by large vacuole and in surroundings zone of protoplasm.

There lie many small chloroplasts, giving the zoospore deep green colour. Whole surface of zoospore is covered with many short cilia arranged in pairs and under each pair lies a nucleus. Therefore zoospore is regarded as compound one. Zoospores escape and swim about freely in water by cilia and soon come to rest. Cilia area with drawn and cell wall is developed round them. After coming to rest zoospores germinate by produces colourless branches rhizoid and attaches the plant to the substratum.

Sexual Reproduction in Vaucheria

It takes place by the method of fertilization i.e. by sharply differentiated male and female organs. Male organs are antheridia and female organs oogamia and these are developed at scattered intervals as lateral outgrowths. In monoecious species of vaucheria antheridia and oogamia usually arise side by side on same filament, or on short lateral branches of it.

The outgrowth that forms oganium swells out, assumes a more or less rounded form and is cut off by basal septum. The apex of oogonium develops a beak either towards antheridium or away from it. The protoplasm of oogonium contains one nucleus and forms a single large female genete. i.e. the egg (ovum or oospore) which fills ooganium. Each antheridium arises as a short tubular branch by side of ooganium. The terminal portion of it is cut off by a septum then it becomes actual antheridium. As it matures it becomes much curved towards the oogonium. The protoplasm contains many chloroplasts and nuclei. Many male gametes or antherozoids are produced inside each antheridium. They are minute in size and are biciliated. Cilia point in opposite direction.

Fertilization:

Self fertilization is common but in diochious species cross fertilization is present. Antheridium bursts at the apex and many antherozooids called around the beak which opens at about the same time.

Several antherozooids may enter the oogonium through the beak but only one of them fuses with the ovum, while the rest perish. After fertilization ovum becomes invested with a thick cell wall and is known as oospore. Oospore undergoes period of rest and germinates into a new vaucheria filament.

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