The effect of photoperiodic treatments on mitotic activity in Pharbitic nil Chois

The short-day plant, Pharbitis nil, requires only a single inductive cycle with a 16-hour dark period for flowering. The mitotic activity in the shoot apices was studied directly after the termination of the inductive photoperiod. A pronounced rise in the mitotic index was found in the 2nd and the 8th-14th hours. Control plants grown under noninductive conditions (continuous light, a light interruption in the middle of the dark period) did not flower and did not show an increased mitotic index. The increased mitotic activity in the shoot apices of Pharbitis seems to be causally connected with the phytochrome-controlled entry of the plants into the state of generative induction.