Characters and origin of vessels with heterogenous structure in leaf and flower abscission zones

The lifetime of many plant organs does not exceed a few weeks or a few mont hs. These organs separate from the plant at the level of specialized abscis sion zones. The observation of xylem vasculature in abscission zones, a lar gely neglected subject, revealed original features when a vessel casting me thod was used. In all species of dicotyledons examined so far, flower and l eaf abscission zones possessed heterogenous metaxylem vessels adjoining pro toxylem and secondary xylem vessels with homogenous patterns of lignified t hickenings. Heterogenous metaxylem vessel thickenings were helical, reticul ate, or scalariform elements when in the abscission zone and pitted element s on the proximal and the distal sides. The origin and possible role of the se vessels are considered. Data obtained on the flower abscission zone of t omato (Lycopersicon esculentum Mill.) suggest that formation of heterogenou s vessels results from localized changes in the rhythm of cell differentiat ion and cell maturation inside the procambium-cambium continuum.