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The organization of the meristem and rates of cell proliferation in its regions have been investigated by continuous labelling of DNA and stathmokinetic experiments. Mycorrhizas bearing intact fungal sheaths and those in which the apices had grown through the sheaths were compared with uninfected roots. Despite low overall rates of mitosis the restricted meristems of mycorrhizal roots have quiescent centres of non-cycling cells, the numbers of which fluctuate periodically. At one time the quiescent centre may comprise only the cells at the pole of the stele, at others these plus from one to seven tiers of axial cells belonging to the cortical complex distal to the stelar pole. This flux in cell cycling forces an exchange in roles between cortical cells and cap initials which produces further changes in the organization of planes and rates of cell division. Roots which have escaped from the fungal sheath have higher rates of mitosis and larger meristems than those with intact sheaths, but the cap meristems of the latter contribute over a quarter of the total production of cells by the whole apex compared with a much smaller fraction in escaped roots. The fungal sheath must therefore contend with the production of as many as 50 cap cells per day and this problem is discussed.
The New Phytologist © 1981 New Phytologist Trust