In this article we will discuss about the vegetative body and reproduction of volvox.
Vegetative Body of Volvox:
The plant is a free-swimming coenobium. The coenobium is hollow, spherical or ovoid, appears in nature as large as a pinhead, and is composed of thousands of cells arranged in a layer joined to one another by a mucous sheath. Each cell has got a cellulose wall and a thick mucous sheath.
It is more or less oval in shape, and has– (1) A central nucleus, (2) One cup-shaped or stellate chloroplast with one or more pyrenoids, at the posterior end, (3) Two equal flagella at the anterior end, directed towards the surface of the coenobium, (4) 2-6, often scattered, contractile vacuoles, and (5) An eyespot, in the form of a reddish or brownish dot at the anterior end.
The eyespot is a photo-receptive organ, intimately concerned with directing the movement of the flagella. Contractile vacuoles alternately expand and contract, and probably serve as excretory organs. It is due to the movement of these flagella of all the cells that the colony moves in water.
Reproduction in Volvox:
Reproduction is asexual at the beginning of the growing season, but sexual at the end.
A colony develops usually from 4-10 asexual reproductive cells, called gonidia, which become many times larger than the ordinary cells, lose their flagella and lie within the globular mucilage sac that projects towards the interior of the colony. Each of them, by repeated divisions, gives rise to a spherical group of cells and forms a daughter colony, which swims about in the cavity of the parent colony. The daughter colonies are ultimately liberated due to rupture or decay of the mother colony.
During the formation of daughter colonies, the plakea undergoes an inversion just as in Eudorina.
The sexual reproduction is oogamous due to the fusion of a motile antherozoid with a non-motile egg. Though the gametes may be formed in any cell of the colony, the gamete-formation is usually restricted to certain cells only. Oogonia and antheridia are highly enlarged cells within a mucous sheath. The Oogonium contain and a single spherical egg. The antheridium divides repeatedly and forms a bundle of 16, 32, 64, 128 or more, small, fusiform, yellowish, biflagellate bodies, the antherozoids.
The colony may be monoecious, or dioecious, and the same individual may or may not form gonidia.
The mature antherozoids escape from the colony, either as free-swimming individuals, or as a free-swimming colony that dissociates into individual male gametes later on. The antherozoids, attracted by an egg, make their way through the gelatinous envelope, which surrounds the egg, but only one of them finally unites with the egg and forms the zygote (oospore).
There are various views regarding the germination of zygote. According to the opinion of Pocock, the diploid nucleus of the zygote divides into a number of daughter nuclei, the first one being reductional and gives rise to biflagellate zoospores, of which only one survives. The protoplast of this swimming zoospore now divides repeatedly giving rise to a new daughter colony.
The zygote develops a thick wall and is retained within the colony until its decay. Then it falls to the bottom of the pool and undergoes a period of rest. Finally, the wall of the zygote breaks and the protoplast comes out. It divides and soon forms a free- swimming colony of the Volvox plant.
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