This paper is a review dealing with the various structural and metabolic changes that have been described in the shoot apex in transition to flowering. Evaluation of the significance of these changes can tentatively be made only in cases where a, the timing of physiological events in the plant, particularly the time of movement of the floral stimulus, is reasonably well known; b, the timing and localization of events in the shoot apex are known.For each of the best known cases (Xanthium, Pharbitis, Sinapis, Lolium, Chenopodium), the temporal sequence of events in the target cells of the apical meristem is described and the sequences are compared. Three successive phases of general occurrence are distinguished.1. The evocation phase, including the events occurring at the presumed time of arrival of the floral stimulus at the meristem. During this phase, RNA and protein molecules essential to the flowering process are synthesized.2. The mitotic phase, characterized by the release in mitosis of cells that are in the postsynthetic G2 phase of the mitotic cycle.3. The morphogenesis phase, including the events leading to the production of flower buds.The essentiality and role of the phases are discussed in relation with current views on cell differentiation.
Photoperiodic flowering of Arabidopsis: integrating genetic and physiological approaches to characterization of the floral stimulus