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Labelling index (LI) was determined in the cap, epidermis, cortex and stele over the apical mm of newly emerged, 0.2, 0.5 - 2 and 4 cm long secondary roots of Vicia faba. This parameter increased in each of the tissues investigated as the roots lengthened. These results have been discussed with respect to the onset of meristematic activity in more of less quiescent lateral root primordia about the time they elongate to form secondary roots. LI was also different in the epidermis, corted and stele in roots of any one length. These differences were more evident in the more basal parts of the laterals examined and they increased with root elongation. On the basis of these results and others in the literature, it is suggested that changes in LI reflect one of the initial events taking place at the onset of cell differentiation and that it may be possible to study differentiation by examining cells from comparable parts of lateral roots which are at different stages of development. [ABSTRACT FROM AUTHOR]