Chlorophycae - Volvocales - Chlamydomonas

Phylum Thallophyta - The Algae - Volvocales - Chlamydomonas 
This organism is very common in fresh waters, sometimes forming a green scum over the surface, and sometimes co louring whole }lands a turbid green. The vegetative cell is usually oval and is enclosed in a cellulose wall with the protoplast closely applied to it. The cellulose wall distinguishes Chlamydomonas from Euglena. The chloroplast is basin-shaped, with its base at the hinder end of the cell, the nucleus being situated in the centre. There is a single pyrenoid in the lower part of the chloroplast, which acts as a centre around which starch is formed. The pyrenoid itself stains brown with iodine and is usually more conspicuous than the nucleus.
At the anterior end of the cell two long flagella pass out through the cell wall, each arising from a basal granule, and between them the cell wall is often slightly protruded in the form of a beak. Close to where the flagella arise two contractile vacuoles are formed.
When fod and oxygen are plentiful the organism swims actiely about by means of its flagella which, by their rapid circular motion, act as tractor propellers, drawing the organism along at a good speed, flagella foremost, with a spiral movement.
The water and inorganic salts required for life are absorbed over the whole surface of the cell. The 'water contains dissolved carbon dioxide which, in the presence of light and chlorophyll, is combined with water to 'orm sugar and starch. The starch is formed as little grains, clustered round the pyrenoid. The cell is attracted towards light of moderate intensity, the response being apparently due to the sensitiveness to light of a tiny spot of red carotinoid pigment at the forward edge of the chloroplast. This eye pot or stigma is covered by a minute lens in the thickness of the cell wall d is supposed to function as an extremely primitive eye.
Reproduction in Chamydomonas
Reproduction may be sexual or asexual. In sexual reproduction the actie cell comes to rest, the flagella are withdrawn and the contents of the cell divide up int four, eight or sixteen parts, which become zoospores. A zoospore is a motile cell, very similar to the parent cell but smaller. It may either become active and swim away or may re­main enclosed by the parent wall and grow and divide again, so that under some conditions a large number of zoospores, usually without flag­ella, may be found clustered together inside an envelope of mucilage. This aggregation of cells is spoken of as the palmella stage, from its re­semblance to another Alga of that name. The cells easily revert, however, to the motile condition.
In sexual reproduction the active cell comes to rest and the flagella are withdrawn. The contents then divide up into either thirty-two or sixty-four gametes. These also resemble the parent cells, except that they are much smaller and are devoid of a cell wall. Under suitable conditions they are liberated by the bursting of the parent cell. They then fuse in pairs, each pair forming a zygote. In fusion the two anterior ends come together and the contents of the one gamete pass into the other. The zygote then secretes a thick wall and thus becomes a cyst. There is usually no difference in the size or structure of the gametes, and hence they are said to be isogamous. In some species, however, the gametes are alike in structure but differ in size, and they are then termed heterogamous or anisogamous.
In Chlamydomonas braunii the gametes are dissimilar in Size. Macrogametes are produced in fours from the parent cell, which they resemble; while the microgametes, which are formed in eights, are only about half the size and more elongated. The macrogamete soon comes to rest, withdraws its flagella and is sought after by the active microgamete. Both possess a cell wall, and after union by their apical ends the membranes coalesce and the contents of the microgamete pass over into the macrogamete. During this process the flagella disappear and a close-fitting Zygote membrane is secreted by the zygote. In this species, there­fore, chlamydomonas approaches an oogamous condition.
The life-history of Chlamy­domonas may be represented in the form of a diagram showing the succession of stages which occur both in sexual and asexual reproduction.

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