| Abstract: | In this paper a method is presented for measurement of the apparentfree space (AFS) directly involved in the xylem vessel translocation. The AFS of xylem vessel bundles was measured by perfusing tomatointernodes with solutions of a non-absorbed (inulin-carboxylicacid), a weakly absorbed ( -aminoisobutyric acid), and a stronglyabsorbed compound (L--alanine). The xylem cross-section actually involved in the longitudinalflow of these substances plus the area which is in rapid (10to 15 min), diffusional, reversible equilibrium with the flowarea was found to amount to 2?5 times the cross-section of thexylem vessels. Since all apoplasmic compartments of the xylemconstitute only 70% of the AFS of the xylem vessels, 30% ofthe AFS seems to be situated outside the xylem. |
