Day lengths of 2-4 h were suboptimal for flowering induction, those of 4-12 h optimal, those of 12.5-3 h supra-optimal, and those of 13.5 critical. The first sign of transition from a vegetative to a generative state was an abrupt increase in apical cell division. Preceding long days (LD), age of the plant and defoliation did not influence sensitivity to short days (SD).The degree of inhibition of SD-induction by interruption or by subsequent uninductive light periods depended on their length and number. Interruption of darkness for I pair of leaves or the apex during SD-induction indicated that the main receptor was the leaf and not the apex. If 2 leaf pairs and the apex were in SD- treatment and continuous light interrupted it for the lowest pair, flowering was remarkably inhibited, even when the lowest pair was removed immediately after treatment. So the inhibition must be rapidly transported to the apex. This factor may also be present in the apex before SD-treatment. The morphological changes in the apex must be regulated by a competition between substances inhibiting and promoting flowering, both arising in the leaves and independently transported to the apex.