Relatively brief treatment of imbibed seeds of positively photoblastic lettuce (Lactuca sativa L. cv. Cannington Forcing) with low concentrations of ethanol promotes germination. It is suggested that this effect is upon a cellular membrane, a view which is supported by the fact that a subsequent treatment with Ca2+ markedly reduces the ethanol promotion. Data from experiments involving movement and transfer of seeds provide no support for the involvement of a stable, leachable, endogenous inhibitor in the control of germination. It is concluded that whether or not the seeds break dormancy is determined by the interplay between forces generated in the developing embryo and the mechanical restraint provided by the tissues which envelop it.