The Growth Fraction of the Quiescent Centre

SUMMARY Evidence used to support a view that the quiescent centre of roots of Avena sativa, unlike those of other species, has a growth fraction of unity, i.e. all its cells are cycling, has been examined in the light of further investigations. Measurement of potential cell‐doubling times before and during a period of DNA labelling by tritiated thymidine in the quiescent centre and in the surrounding meristem show that there is no reason to reject the notion that many cells of the quiescent centre are out of cycle. Observations on planes of division in the proximal tier of the root cap, which were also used to support belief in a fundamental difference in cell kinetics between Avena and other species are also interpreted differently as a result of additional work.