The Quiescent Centre and Rates of Mitosis in the Root Meristem of Allium sativum

The root apices of Allium sativum have been examined by continuous- and pulse-labelling with tritiated thymidine and by colchicine treatment to measure the time parameters of the mitotic cycle in various parts of the meristem. There is a quiescent centre of 30–50 cells whose average rate of mitosis is low because the G 1 period is extended to about 140 h compared with about 4 h in the othe regions of the meristem. The stele just above the quiescent centre and at 200 microns above it and the cap initials just below the quiescent centre are very similar in their mitotic cycles, the total lengths of which are about 30 h of which nearly half is taken up by DNA synthesis. Allium thus differs from Zea in having root cap initials whose mitotic cycle is not telescoped by the elimination of the G 1 phase. These facts are discussed in relation to the radiosensitivity of the meristem.