  We describe the growth of vesicles, due to the accretion of lipid molecules
to their surface, in terms of linear irreversible thermodynamics. Our treatment
differs from those previously put forward by consistently including the energy
of the membrane in the thermodynamic description. We calculate the critical
radius at which the spherical vesicle becomes unstable to a change of shape in
terms of the parameters of the model. The analysis is carried out both for the
case when the increase in volume is due to the absorption of water and when a
solute is also absorbed through the walls of the vesicle.
