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It is a dark blue green filamentous alga. It floats in ditches and shallow pools of water and on wet rocks and walls. Filaments of oscillatoria are entangled in masses which float on water. Each filament is slender, un-branched and cylindrical, consisting of a row of short cells. The individual cells are the oscialltoria plants and the filament is regarded as a colony. All the cells of the filament are alike except end cell which is usually convex and there is no differentiation into the base and the apex. Some empty cells occur in some of the filaments.

The protoplast of each cell is differentiated into two regions: a coloured peripheral zone the chromoplasm and an inner colourless zone the central body. Colour is due to the presence of chlorophyll and phycocyanin (a blue pigment), which diffuse through the chronmoplasm. There is no plastid. True nucleus is also absent. The central body however is regarded as an incipient nucleus with only some chromatin but without nuclear membrane and nucleolus. Cell division takes place in one direction only. Each filament remains enveloped in a thin mucilaginous sheath. Under the microscope a slow swaying or oscillating movement of the filaments with ends tossing from side to side may be distinctly seen. This is characteristic feature of oscillatoria. The filaments may sometimes exhibit a twisting or rotating motion.

Reproduction: In blue green algae reproduction takes place by vegetative method only. They do not bear any kind of ciliated body. Gametes and zoospores are altogether absent. In oscillatoria the filament breaks up into a number of fragments called hormogonia. Each hormogonium consists of one or more cells and grows into a filament by cell division in one direction. The hormogonium has a capacity for locomotion.

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