it is oval or sperical in shape and bounded by a cell wall containing hemi-cellulose. At the anterior end, the wall projects and thickens to form a colourless papilla. It has two flagella which are used for locomotion. There is the contractile vacuole for osmoregulation. It posses a nucleus and a cup shaped chloroplast which contains chlorophyll. It has a pyrenoid which is used for production of starch. A red eye spot is embedded on the anterior border of the chloroplast which responds to light stimulus
Buoyancy The density of fresh water is considerably less than that of the sea and such floatation is a greater problem for fresh water plankton. There is occasional flooding which submerges the plants. The water lilies, water lettuce, water hyacinth are able to stay afloat because of the air spaces in the lower part of the leaves as well as in the skins and roots.
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