The blockage in the G 1 phase of the cell cycle in the dormant shoot apex of ash

The shoot apex of the terminal bud was studied in four successive physiological states: during dormancy, when dormancy breaks, during the third week after the break of dormancy, and during a later typical period of active growth. DNA content was measured in Feulgen-stained nuclei of the axial zone, of the lateral zone, and of the rib meristem. The mitotic index was established for each zone of the meristem. During the period of dormancy, all the nuclei of the meristem are in phase G₁ of the cell cycle and are blocked at the same point common to all nuclei. When dormancy breaks, this blockage is removed simultaneously and all nuclei in the shoot apex resume their cell cycles starting at the same point. The cycles remain synchronized for awhile. In the axial zone they remain synchronized until the third week after resumption of active growth.