In Spirogyra, sexual reproduction is accomplished by conjugation. It is of two type
(i) Lateral conjugation: When conjugation takes place between two adjacent cells of same filament, ie, it involves only a single filament (homothallic). It is of two types Direct lateral conjugation and Indirect lateral conjugation.
(ii) Scalariform conjugation (Ledder type) involves two different types of filaments. It takes place at night between recently dividing cells and is completed in three phases - Maturation phase, gametic union phase and zygospore contraction phase.