1) Far-red light or low-intensity light illumination following an inductive dark period of 16 hours has no significant influence upon flower initiation of Pharbitis seedlings.
2) Illumination with low-intensity light containing little far-red radiation promotes flowering response to some extent if it follows a dark period of 8-12 hours, but far-red light illumination does not.
3) The flower-promoting effect of low-intensity light illumination following a dark period of 12 hours or less decreases considerably if the far-red light of low intensity is mixed with it.
4) A second dark period separated from an inductive dark period by a brief exposure to red light does not inhibit flowering response when the first dark period is 16 hours, and promotes flowering strikingly when the first period is 12 hours. 5) The last phase of the inductive dark period appears to be relatively light stable, but less stable than the first phase.