The promeristem of Melilotus alba Desr. consists of two more or less distinct layers of initials. The procambium and cortex are derived from one of these layers, while the root cap and epidermis have a common origin from the second. A diurnal periodicity of divisions exists in root tips of Melilotus, with maxima at 12.00 hours (noon) and 24.00 hours (midnight). Meri-stematic activity extends from the initial region, or promeristem, through the level of maturation of the protophloem. Histochemical and autoradiographic studies of the root apices of M. alba, Medicago sativa L. and Trifolium pratense L. revealed that the roots of Melilotus lack well-defined quiescent centres, that those of Medicago have large, clearly defined quiescent centres and that, although young Trifolium roots initially lack quiescent centres, such centres eventually develop to include all of the so-called initial layers.
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