In this method of sexual reproduction, two filaments of Spirogyra lie parallel to each other and form tubular outgrowths from the walls of their corresponding cells facing each other. These outgrowths further grow and touch each other. The middle wall between them disintegrates and they form the conjugation tube. The entire protoplasmic content of these cells contracts to form single gametes. The gamete of one particular filament passes through the conjugation tube to the corresponding cell of other filament and fuses with the gamete present in it. The fusion of two gametes with each other results in the formation of zygospore.