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Figure 7.4 Most root apices contain a quiescent centre of very slowly dividing cells. (a) Diagram of longitudinal section through a maize (Zea mays) root tip. The quiescent centre is shaded dark green. (b) Autoradiograph of transverse section through root apex of Vicia faba (broad bean), fed for 24 h with radioactive [3H]thymidine which specifically labels DNA in nuclei of dividing cells. The quiescent centre has significantly fewer labelled cells (dark silver grains). (Based on Clowes 1959; Waisel et al. 1996)