Abstract
In two rosette plants, Hyoscyamus niger and Samolus parviflorus, gibberellic acid causes a substantial increase in cell division in the tissues immediately underlying the apex. As the treatment continues an elongate stem is produced and, for at least 72 hours, the period considered in this study, the subapical zone of cell division expands commensurate with the growth in length. Since there was no cell elongation during the 72-hour interval, the entire increase in length was due to the production of new cells. Hence, it was possible to determine the number of cells produced in a certain time interval and compare this to the observed mitotic activity (number of mitotic figures counted). In the pith region of Samolus, peaks in mitotic activity at 24 and 48 hours were observed and ascribed to a partial synchronization of cell division initiated by the application of GA. The 24-hour interval between the two peaks probably reflects the average pith cell-generation time. Based on the above information additional calculations were made to determine the total number of cells dividing in the first 24 hours after application of GA and the karyokinesis time for pith cells.
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