Spirogyra is a free floating alga found in fresh water ponds. The body has a row of cylindrical cells joined end to end (filamentous alga). Each cell depending upon the species, may have 1 to 14, spiral ribbon shaped chloroplasts with many uni-seriately arranged pyrenoids. Central region of the cells has a large vacuole. The single nucleus is present in the centre of the cells supported by cytoplasmic strands.
The filament breaks into small fragments, at the point of transverse septum following a physico-chemical change. Each fragment having at least one complete cell grows into a new filament by repeated mitotic cell division.
It takes place by scalariform and lateral conjugation.
Scalariform Conjugation (conjugating filaments give a ladder-like appearance)
Two filaments come to lie very close to each other so that the cells of the two filaments pair septum to septum and face to face. The pairing cells of the two filaments form a contact with the help of a tube called the conjugation tube. Cytoplasmic contents of each cell round off to act as a gamete.
Gamete from one cell (male) passes to the other cell (female) through the conjugation tube, by amoeboid movement. The cells of each filament acts either as male or female. The contents of two gametes fuse in the female cell and form a diploid zygote.
Consequently, after the sexual fusion of gametes, all the cells of male filament are empty whereas each cell of the female filament has one thick-walled diploid zygospore. The zygospore develops a thick wall around itself and develops dark brown to black pigment to tide over the unfavourable period.
On the return of favourable conditions the diploid nucleus divides by meiosis into four haploid nuclei. Three of these nuclei degenerate. On germination, wall of the zygospore ruptures and a small tube like structure, containing one haploid nucleus comes out. The small tube develops into a long filament by repeated mitotic cell divisions.
Here, cells of only one filament are involved in conjugation wherein, male and female cells are arranged in alternate pairs i.e., two male cells alternate with two female cells all along the length of a filament. Conjugation tube is formed lateral to the septum separating a male and a female cell. Protoplasm of male cells migrate into female cells.
After fertilization, a filament would show two empty cells alternating with two cells each having thick-walled diploid zygospore. The zygospore under favourabe conditions, germinates as in scalariform conjugation to produce only one independent plant, because 3 haploid nuclei after meiosis, degenerate.