An analysis of resistance to water flow through wheat and tall fescue leaves during pressure chamber efflux experiments
Abstract. This is a physical analysis of water movement in wheat (Triticum) and tall fescue (Festuca arundinacea) leaves placed in the Scholander pressure chamber. It takes into account the efflux resistances of water movement through the xylem and water flow across the cell membranes. Xylem resistance was estimated using Poiseuille's law. 
 
Leaves which had been pressurized in the chamber were embedded, sectioned, examined under a light microscope and photographed. Cells were intact but distorted and xylem vessels were intact. Cells in portions of the blade squeezed by the chamber sealing grommet were crushed, but xylem vessels remained intact. 
 
By applying pressure several tenths of a megapascal in excess of the balance pressure, water was forced from each leaf through the severed end which protruded from the chamber. Efflux curves were drawn by plotting the total water expressed as a function of time after the pressure increase. Water efflux from the shortest wheat leaf lasted only 10 min while efflux from the longest continued for up to 40 min. The efflux from a tall fescue leaf which was rehydrated and cut to a shorter length was much more rapid than efflux from the original leaf. 
 
Experiments combined with mathematical analysis suggested that the effect of leaf length on efflux is related to a high resistance to water flow through vascular bundles. Xylem resistance would be sufficient to produce this effect if it were 10 times greater than that predicted by Poiseuille's law. Both the observations of water flow from the cut end of the leaf and the mathematical model suggested very little water flows from bundles with vessels of diameter less than 12 μm. The apparent explanation is high resistance to water flow through these small diameter vessels.