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In the previous papery, it was reported that the initiation rate of the foliage and floral leaves at the Pharbitis shoot apex was accelerated by a flower-inductive dark period. Wetmore et al.2>, Jacobs and Raghavan3>, and Sunderland4> reported that the first sign of flower initiation is an increased rate of cell division at the shoot apex. Recently, Bernier5> suggested that the floral induction is linked with a recovery of a highly meristematic state and with a pronounced stimulation of DNA synthesis and cell division at the shoot apex. In the present experiments with the seedlings of Pharbitis nil, strain VIOLET, it was investigated whether or not the acceleration of the initiation rate of leaf primordia caused by a flower-inductive dard period is preceded by the stimulation of cell division at the shoot apex. The seedlings were grown under artificial light from daylight fluorescent lamps in air-conditioned rooms kept at 24-25, and exposed to a single dark period of 24 or 16 hours 5 days after the H2SO4-treatment1 of the seeds. Control plants were illuminated continuously without giving any dark period. Before and after the inductive dark period, 10 to 20 shoot apices of terminal buds were daily excised under a dissecting microscope, and stained with aceto-orcein by a routine squash method. Squashed preparations gave sharply defined mitotic figures, which appear a brilliant red, and they were observed with a 100>< oil-immersion lens at a total magnification of 1000x. Mitotic figures, pro-, meta-, ana-, and telo-phase, were counted, respectively. The developmental stages of the excised shoot apices were also observed using a dissecting microscope, and "the shoot apex ages" defined in the previous papery were recorded. The results are shown in Fig. 1. The number of mitotic figures at the flowerinduced shoot apices began to increase 24 hours after the end of a 24-hour dark period. The difference in numbers of mitotic figures between flower-induced and noninduced shoot apices rapidly became larger in process of time. On the other hand, the difference in the leaf initiation rates did so 48 hours after the dark period. At non-induced apices, both the number of mitotic figures and the leaf initiation rate remained almost constant throughout the experimental period. Similar results were obtained with a 16-hour dark period, and also with the second axillary bud. It seems clear that the stimulation of cell division precedes the acceleration of leaf initiation rate at the flower-induced shoot apex. The average number of metaphase and anaphase was about 14 per shoot apical meristein in the preparations sectioned with a routine paraffin method, while 28 per shoot apex in the present method. So,

shokubutsugaku zasshi/botanical magazine tokyo

Studies on the Flower Initiation in Pharbitis Seedlinds, with Special Reference to the Early Stimulation of Cell Division at the Flower-induced Shoot Apex