  One approach to the understanding of fusion in cells and model membranes
involves stalk formation and expansion of the hemifusion diaphragm. We predict
theoretically the initiation of hemifusion by stalk expansion and the dynamics
of mesoscopic hemifusion diaphragm expansion in the light of recent experiments
and theory that suggested that hemifusion is driven by intra-membrane tension
far from the fusion zone. Our predictions include a square root scaling of the
hemifusion zone size on time as well as an estimate of the minimal tension for
initiation of hemifusion. While a minimal amount of pressure is evidently
needed for stalk formation, it is not necessarily required for stalk expansion.
The energy required for tension induced fusion is much smaller than that
required for pressure driven fusion.
