THE flower buds of certain varieties of Phaseolus vulgaris are inhibited in their development and eventually abscise during the Cambridge summers. The results of preliminary experiments suggested that long photoperiods are largely responsible for this behaviour1 and this has since been confirmed. The effect has been shown to be truly photoperiodic in experiments performed in controlled environment cabinets where (1) the light period was extended beyond 11 h using low intensity incandescent light, and (2) long dark periods were interrupted by short periods of light2.