Algae or spirogyra can reproduce through 2 different ways.
(A) asexual mode : By fragmentation it can produce asexually.It breaks into smaller fragments and each fragment by mitosis grows into individual plants.
(B) sexual mode : By forming conjugation tubes.Male and female gamete fuses together in the conjugation tube and form zygote.This zygote gets enclosed within thick walls and becomes dormant and now known as zygospore.On getting favourable conditions , this germinates and grows into the new spirogyra plant.