Spirogyra reproduces by vegetative, asexual and sexual methods.
The most common mode of reproduction in Spirogyra is by vegetative method (fragmentation). However, the asexual reproduction is very rare in Spirogyra and found only in few species.
It is the method of reproduction through the existing vegetative structures of Spirogyra without any involvement of sex organs.
Asexual reproduction in Spirogyra takes place by the formation of akinites, alphanospores and azygospores.
Akinete is a thick walled, resting spore which contains abundant food material. During unfavorable condition, the entire protoplasm of a vegetative cell in some species like Spirogyra farlowii loses water and contracts. It rounds off and become thickened by the addition of thick wall layers around it to form akinete.
Aplanospores are the thin walled non motile spores which develop inside the vegetative cell due to the loss of water from the protoplasm. They contract and round off and secretes a thin wall around it. On returning to the favourable conditions the wall dissolves and produce new individual.
Under failure of normal gametic union, Parenthospores are formed by the secretion of thick walls around and behave as zygospores. These zygospores on germination, forms new filament.
Note that the difference between vegetative and asexual reproduction is that vegetative reproduction takes place through already existing vegetative structures but asexual reproduction involves formation of particular structure especially for reproductive purpose.