Abstract
Transition to flowering is described in gibberellic acid (GA) - treated and control plants of Helianthus annuus. Hormonal treatment accelerates reproductive development without reducing the number of leaves developed before flowering. Studies of [3H]thymidine incorporation in the apex show that a non-synthesizing summital group of cells, the central zone, is present in the vegetative as well as the transitional apex. During transition to the floral apex the size of the central zone is gradually diminished, as its peripheral and basal cells undergo synthetic activity and the apex develops a domed shape. In GA-treated shoots the order is changed so that development of a dome precedes activity in the central zone. Cells of the second tunica layer of the central zone are the last to incorporate thymidine. They are conspicuously enlarged and distinct before development of the inflorescence. It is suggested that this layer has a specialized role in flowering.
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