Heterocysts are slightly larger than the vegetative cell. It is mainly concerned with the nitrogen fixation and sometimes reproduction too. The heterocysts have thick two layered envelope with inner walls forms knob like projections into the cell cavity. These knobs like projections are known as the polar nodules which are rich in enzymes nitrogenase and hydrogenase. Reproduction by heterocyst is a rare but they germinate in certain cases. The heterocysts are liberated by breaking their walls.