Surgical Experiments on the Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carota L.)

Surgical techniques were applied to the shoot apex of carrot ( Daucus carota L.) to test the interpretation that provascular tissue is the initial stage of vascular differentiation and to localize the sources of the influences that control its differentiation. If the apex is isolated laterally by vertical incisions leaving it at the summit of a plug of pith tissue, vascular differentiation proceeds normally and an independent vascular system is formed in the pith plug. If all leaf primordia are systematically suppressed, provascular tissue continues to differentiate as an acropetal extension of the pre-existing vascular system but no further differentiation occurs. When the apex is isolated laterally and all leaf primordia are suppressed, provascular tissue continues to be formed acropetally and is extended basipetally into the pith plug by redifferentiation of pith cells, but no further differentiation occurs. This tissue reacts positively to histochemical tests for esterase indicating its vascular nature. If only one leaf primordium is allowed to develop on an isolated shoot apex, its vascular system develops normally and extends basipetally into the pith plug, but there is no extension of provascular tissue into the pith plug. These results support the interpretation that the initial stage of vascular differentiation is controlled by the apical meristem but that further maturation of vascular tissue depends upon influences from developing leaf primordia.