Abstract

<jats:p> A zone of minimum vessel number and vessel cross-sectional area (the constricted zone) occurs in the nodal region of each of the three vascular traces serving a leaf of Populus deltoides Bartr. When the area data are transformed to Σr<jats:sup>4</jats:sup>, they can be used in the Poiseuille equation to estimate hydraulic conductivity. The decrease in xylem conductivity frequently observed at vascular junctions, such as the nodal constricted zone, may be accounted for by a restriction in hydraulic conductivity. </jats:p>