Initial Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carotaL.)

Abstract

Although an initial stage of vascular differentiation preceding procambium has been demonstrated in ferns, its presence in seed plants has not been accepted generally. In the shoot apex of carrot, a short cylinder of provascular tissue is recognized as the initial stage of vascular differentiation. This first becomes apparent through the enlargement and vacuolation of pith and cortical tissue rather than as a result of specific changes in the provascular tissue itself. Procambium in discrete strands differentiates acropetally in the provascular tissue in relation to developing leaf primordia. Provascular tissue is not recognized above the axil of the youngest leaf primordium but it is distinct at or above the level at which the trace of the youngest primordium diverges. Support for the recognition of provascular tissue is provided by a positive reaction to histochemical tests for carboxylesterases in this tissue as well as in procambium and later stages of vascular differentiation.