Leaves of young barley (Hordeum vulgare cv Arivat) seedlings were examined anatomically, physiologically and cytologically to infer the pathway of transpirational water movement and to understand the basis for the selective responsiveness of the growing region to osmotic stress. Vessels with open lumens were found to extend from the intercalary meristem to the expanded blade, and all vessels are present in 5 functional vascular bundles (FVB) which are separated by 20 to 30 closely packed mesophyll cells and 2 to 3 immature vascular bundles (IVB). Heat pulse transport data confirmed the anatomical suggestion that water will move throughout the leaf in open vessels and they showed also that osmotic stress will reduce water transport within 1 min, which is before transpiration is lowered. Water representing about 2 per cent of the total tissue water was obtained by centrifuging cut sections of the growing region at 5 X g against an adsorptive surface. This water is probably xylem plus cell wall water because it is easily removed, its volume is 2X that calculated to be in the vessels, and it exchanges more readily with the water in the nutrient solution than the bulk tissue water. This lack of free exchange indicates apoplastic water is somewhat separated from mesophyll cells, and it is hypothesized that osmotic stress causes sudden growth cessation and initation of metabolic changes because (a) reduced water availability together with ongoing transpiration will cause a sudden reduction in the xylem's water potential, (b) there is a lateral transmission of this reduced water potential through walls of all cells in the growing region, and (c) cells can respond in some way to changes in water potential around them. Most cells in the expanded blade are considered unresponsive to osmotic stress because transpirational water will move predominantly from the 5 FVB through the closest stomata, so only cells closest to those bundles will be altered rapidly by stress.