A hypothetical plant growth substance that is said to cause flower initiation in many species. Florigen has yet to be isolated from plants although there is much circumstantial evidence for its existence. Thus although flowering often depends on the photoperiod experienced by the plant it is the leaves and not the growing apex that perceive photoperiod. Thus the stimulus must be transported from the leaves to the apex. The flowering stimulus can be transferred, by grafting, from one plant to another. If leaves from short-day plants grown under short days are grafted onto long-day plants grown under short days then the stimulus from the grafted leaves will promote flowering in the long-day plant. Such work shows that florigen is similar in long- and short-day plants. The rate of florigen movement has been measured by removing the leaves from a plant at varying intervals after exposure to an appropriate photoperiod and noting whether flowering still occurs. In some plants the rate appears to be slower than that expected if transport were via the phloem. See also vernalin. |
