The phloem exudate prepared from the cotyledonsof Pharbitisseedlings that had been exposed to a single dark period (of longer than 10 h) induced flowering in cultured apices excised from non-induced seedlings. The flower-inducing activity of the exudate increased as the seedlings were exposed to longer periods of darkness. The highest activity was associated with the exudate taken from cotyledons exposed to a single 16-h dark period. The activity of the exudate taken from cotyledons exposed to an inductive dark period was clearly reduced by interruption of the dark period. The addition of exudate taken from three cotyledons to 10 ml of medium resulted in the highest flower-inducing activity. About 50of cultured apex explants formed floral buds, even when the concentration of the exudate was reduced to 0.1 cotyledon equivalents per 10 ml of medium. The flower-inducing activity of the exudate appeared to be heat-stable.
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