Ring meristem

It is intriguing that early developmental stages in A. celastrifolia closely resemble the distantly related Nelumbo (Nelumbonaceae) (Hayes et al., 2000). In particular, the clear-cut border between androecial ring-wall and central floral meristem on which the carpels will be formed is striking. In contrast, in polyandrous and multicarpellate Ranunculaceae carpel initiation commences more or less gradually... 

So far the Sinapis story can be interpreted as follows The shoot apical meristem of 2-month-old plants is presumably cytokinin-limited. Exposure to LD causes the production, in the leaves, of a signal which is then transported to the root system. There it alters the course of cytokinin metabolism and/or release. The increase in cytokinin levels in the transpiration stream (xylem) causes an increase in leaf cytokinin levels by 16 h. Some of the leaf cytokinins are then re-exported in the phloem sap to the apical meristem, where they cause a mitotic activation at 26-30 h. Since the cytokinin level in root exudate is altered as early as h 9 (Fig. 2), the initial leaf-to-root signal is apparently produced and transported extremely rapidly, i.e. within the first hour of the photo-extension period of the LD. The nature of this signal is unknown, but bark-ringing experiments indicate that it moves in living tissues (Lejeune, unpublished). 

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