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

Although an initial stage of vascular differentiation precedingprocambium has been demonstrated in ferns, its presence in seedplants has not been accepted generally. In the shoot apex ofcarrot, a short cylinder of provascular tissue is recognizedas the initial stage of vascular differentiation. This firstbecomes apparent through the enlargement and vacuolation ofpith and cortical tissue rather than as a result of specificchanges in the provascular tissue itself. Procambium in discretestrands differentiates acropetally in the provascular tissuein relation to developing leaf primordia. Provascular tissueis not recognized above the axil of the youngest leaf primordiumbut it is distinct at or above the level at which the traceof the youngest primordium diverges. Support for the recognitionof provascular tissue is provided by a positive reaction tohistochemical tests for carboxylesterases in this tissue aswell as in procambium and later stages of vascular differentiation.Copyright1999 Annals of Botany Company. Provascular tissue, carboxylesterase, shoot apex, vascular differentiation, carrot (Daucus carotaL.).