| Summary: | Early vascular differentiation has been experimentally investigated in the shoot apex of carrot (<i>Daucus carota </i>L. var. <i>sativa</i> DC.). As in ferns, there is initial vascular tissue--provascular tissue--in the shoot apex. A distinct provascular ring was observed in the shoot apex of carrot and extended above the attachment of the youngest leaf trace when this could be identified in the late plastochron. Histochemical tests indicate that carboxylesterase, which is mainly characteristic of vascular tissue, is present in this tissue. Surgical experiments further reveal that, as in ferns, the formation of provascular tissue is independent of the leaf primordia, but that unlike ferns, further maturation of the provascular tissue depends upon an influence from the leaf primordia. Finally, auxin replacement experiments reveal that IAA produced by the developing leaf primordia is one of the influences that affect the maturation of provascular tissue. Exogenous IAA applied in lanolin or more effectively in resin beads, enhanced provascular tissue development and promoted the final maturation of vascular tissue. These results suggest that there is insufficient IAA for vascular maturation in the shoot apex when leaf primordia are suppressed and support the hypothesis that auxin production has shifted from axial to lateral centres in the evolution of seed plants. Furthermore, the surgical experiments have been extended to two other dicotyledons, potato (<i>Solanum tuberosum</i> L.) and lupin (<i>Lupinus albus</i> L.). Although there are variations in the normal shoot apices in these two species, the result of suppression of leaf primordia is similar to that in carrot. In conclusion, the difference of the present results from comparable observations in ferns, in which the final maturation of vascular tissue from provascular tissue does not require the influence of leaf primordia, is interpreted as a reflection of the long separation of fern and seed plant evolutionary lines.
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