Illustrated account of life history of Volvox

It is fresh water colony forming free-swimming green alga found ponds and other pools of water during and after rains. Volovox has reached highest degree of colony formation. Each colony or coenobium consists of a few hundreds to several thousands cells arranged in peripheral layer to form a hollow sphere containing water or a dilute solution of gelatinous material. Each cell of colony has gelatinous sheath and cells are held together in a colony by sheaths secreted by individual cells. Cells are connected by delicate strands of cytoplasm. Individual colony freely swims about in water. Each volvox cell is like chlamydomonas. Each mature colony has two kinds of cells: numerous small vegetative cells and few large cells among them. Vegetative cell has two cilia protruding outwards and vibrating 2 to 5 contractile vacuoles, a central nucleus a cup shaped or plate like chloroplast with one pyrenoid and one eye spot. Vegetative cells do not divide. Large cells of colony are

Reproductive cells. These cells may behave as asexual in beginning of season and as sexual cells at close of the season.

ASEXUAL REPRODUCTION IN VOLVOX:

The enlarged cells called gonidia of mother colony after retracting their cilia and pushing back to the posterior side divide and re-divide in the longitudinal place and give rise to large number of cells in one plant, thus forming new young daughter colonies with mother colony. When cell division cease the cells turn round, develop cilia and form again hollow spheres. These are seen to float and slowly revolve within the much enlarged hollow portion of the mother colony. Soon by ruphire they come out of the membrane of mother colony or trough a pore in it and swim away as independent colonies.

Sexual Reproduction: In volvox sexual reproduction is oogamous. In monoecious species both types of gametes (male and female) are borne by same colony while in dioecious species these are borne by separate colonies.
The said gametes are borne by enlarged cells called gametangia (gamete bearing cells) which lie in posterior side of the colony. Some of these cells are antheridia or male reproductive organs, the protoplasm of which divides many times and produces a cluster of minute biciliate male gametes called antherizooids or sperms; while other cells are oogonia or female reproductive organs, the proplast of which forms a single large female gamete called egg or ovum. Egg is large passive and non motile while sperms are very minute, active and motile sperms may be in a plate like colony escaping from mother colony as a unit or these may be arranged to form a hollow spore. In former case the unit as it approaches an egg breaks up into individual sperms and in the latter case sperms are liberated singly. Mode of fertilization is oogamous. Sperms swim and enter by gelatinous sheath into organism lying in mother colony and one of the fuses with egg. Thus the fertilization is affected.

ZYGOTE:- After fertilization the zygote clothes itself with a thick spiny wall. It is set force from the mother colony only after the decay or disintegration of latter zygote sinks to bottom of pool of water and then after a period of rest is germinates with the approach of favourable reason. The protoplast of the zygote undergoes reduction division prior to germination. In some species the protoplast of the zygote divides and forms a new colony directly, whiten in others it forms a single bicliate zoospore which escapes by the rupture of the zygote wall and swims away. The free swimming zoospore then divides and forms new colony.