Fusion of plant protoplasts (and animal cells) can be induced by chemicals (PEG) and electric field. In cell-cell fusion three stages are recognized; pointadherence stage, face-adherence (or membrane-fused) stage and fused-sphere stage. The temperature effect on fusion has been demonstrated. Study on electro-fusion has suggested that transient of membrane from a "resting" state to a transient "fusion-susceptible" state is an important event in the induction. A threshold membrane potential is postulated for the transition.