It is a green free floating filamentous alga present in ponds, ditches and springs. Each spirogyra filament is un-branched and consists of single row of cylindrical cells. The walls are made up of cellulose and pectin. Pectin swells in water into a gelatinous sheath. Filament shows no difference in base and apex. Each cell has a lining layer of protoplasm, one or more spiral bands of chloroplasts with smooth, wavy or serrated margins and a distinct nucleus situated in the middle. The spiral chloroplasts are the characteristic feature of spirogyra. Each chloroplast has a number of nodular proplasmic bodies called pyrenoids around which minute starch grains are deposited. If the filament happens to break up into pieces, they grow up into new filaments by cell division.
Reproduction: It takes place by sexual method. It consists of fusion of two similar gametes i.e. isogametes and the process is called conjugation which takes place between the cells of two filaments. Sometimes conjugation takes place between two adjoining cells of the same filament.
(1) Scalari form Conjugation: When two filaments come to lie in contact in the parallel direction they form tubular outgrowths from their opposite or corresponding cells. These tubular outgrowths called conjugation tubules give the whole structure the approach of a ladder.
Their end or partition walls dissolve and an open conjugation tube is formed. The protoplasmic contents of each cell lose water, contract and become rounded off in the centre. Each contracted mass of protoplasm forms a gamete. All gametes are alike in appearance but gametes of filament (male) creep through the conjugation tubes into the corresponding cells of adjoining filament (female) and fuse with the gametes of that filament. Fusion of two gametes results in the formation of a thick walled zygospore which soon turns black zygospores are formed in a series in one filament (female) while the other filament (male) becomes empty except for a few vegetative cells here and there.
(2) Lateral conjugation: It takes place between the cells of same filament
(i) Chain type: Commonly an outgrowth or conjugation tube is formed on one side of partition wall and passage the gamete (male) of one cell passes into the gamete (female) of the neighbouring cell.
(ii) Direct Method: In some species male gamete pushes the partition wall and pierces it in the middle. By the opening thus formed the male gamete passes into neighbouring cell and fuses with the female gamete.
In lateral conjugation the gametes of alternate cells only move to the neighbouring cells and thus later on the zygote bearing cells are seen to alternate with the empty cells in the same filament. Sometimes conjugation does not take place, gametangium may become directly converted into a zygospore like body called a zygospore or partheuospore which germinates like the zygospore.