The flow and dispersion of water in the vascular network of dicotyledonous leaves.

The relationships between the geometric characteristics of, the local flow rates of xylem sap in, and relative pressures in the reticulate anastomosing vascular network of dicotyledonous leaves of Populus Balsamifera L. are reported. The conducting channels of cellulosic microcapillaries are covered by sheaths of chloroplast free cells through the walls of which water withdrawn from vascular bundles percolates to reach evaporation sites. Along the mid-rib and branch generations, the population and cross-section areas of the microcapillaries decrease with distance but not in a monotonic manner. Lateral withdrawal rates from the veins were highest at the base of the leaf lamina. More than 50% of the inlet stream had dispersed out of the conduits within the first 25% of the leaf lamina area from the petiole junction. Absolute values of pressure gradients generally decreased in the apical direction along the vein.