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The rate of flow of a dilute KCl solution through sections of stem, branches, and twigs was measured and expressed in microlitres per hour, under conditions of gravity flow, per gram fresh weight of leaves supplied by that section of xylem. This is called leaf-specific conductivity (LSC). It is not uniform throughout the tree, LSC of the stem being higher than that of branches. Furthermore, vascular junctions, such as the path from stem to branch, represent hydraulic constrictions. Distribution of LSC in the tree is primarily based on varying vessel diameters. Vessel diameters increase from top to bottom in the tree stem. They are smaller in branches than in the main stem, and there is a distinct constriction of diameters at the base of each branch. Functionally this means that when transpiration begins the pressure has to drop more rapidly in the xylem of lower lateral leaves than in those at the top of the tree. It also means that under conditions of water stress peripheral parts of the tree are more vulnerable than the trunk.

MATERIALS AND METHODSXylem vessels in grapevines Vitis labrusca L. and Vitis riparia Michx. growing in New England contained air over winter and yet filled with xylem sap and recovered their maximum hydraulic conductance during the month before leaf expansion in late May. During this period root pressures between 10 and 100 kilopascals were measured. Afthough some air in vessels apparently dissolved in ascending xylem sap, results indicated that some is pushed out of vessels and then out of the vine. Air in the vessel network distal to advancing xylem sap was compressed at about 3 kilopascals; independent measurements indicated this was suffiLcient to push air across vessel ends, and from vessels to the exterior through dead vine tips, inflorescence scars, and points on the bark. Once wetted, vessel ends previously air-permeable at 3 kilopascals remained sealed against air at pressures up to 2 and 3 megapascals. Permeability at 3 kilopascals was restored by dehydrating vines below -2A megapascals. We suggest that the decrease in permeability with hydration is due to formation of water films across pores in intervascular pit membranes; this water seal can maintain a pressure difference of roughly 2 megapascals, and prevents cavitation by aspirated air at xylem pressures less negative than -2A megapascals.Xylem vessels in the wild grapevines Vitis labrusca L. and Vitis riparia Michx. are gas-filled during winter. Prior to leaf expansion in spring, the vessels become filled with water by root pressures that can reach 500 kPa at the base of vines (7). Once leaves are expanded and transpiring the vessels must be able to withstand negative xylem pressure potentials; any remaining trace of gas would nucleate cavitation, thus disrupting water transport. Water conduction in these refilled vessels is critical since new vessels are not differentiated until well after leaf expansion (JS Sperry, personal observation). In fact, vessels remain functional for up to 7 years (9) and thus go through several cycles ofemptying and filling. Vessels in grape are among the largest known, with maximum diameters of 0.5 mm, and lengths of over 8 m (14). How is all the gas so successfully removed from such large vessels?We have documented the spring filling of grapevine vessels and have investigated three possible mechanisms for it: (a) condensation of water vapor-if the vessels contain water vapor, filling would occur readily when root pressures approached vapor pressure (about 0.23 kPa at 20C); (b) dissolving of gas-sufficiently prolonged and elevated root pressure may be sufficient to dissolve the gas; and (c) expulsion of gas-root pressure may push gas out of the vessels, and eventually out of the vine. Initial studies were made on Vitis labrusca L. growing at the Harvard Forest in Petersham, Massachusetts. Subsequent work was done on Vitis riparia Michx. in the vicinity of Burlington, Vermont. Repeated observations and measurements ofroot pressure were made at a single site on the banks of the Winooski river. Root pressure wa...

1. Vessel-length distribution in stems of some American woody plants. Can. J. Bot. 59: 1882-1892. Vessel-length distributions in some trees, shrubs, and a vine have been calculated from measurements of particle penetration and of air-volume flow through the xylem. In shrubs and diffuse-porous species, longest vessels were about 1 m long, but most of them were much shorter, the largest percentage in the 0-10 cm length class. In the two ring-porous species investigated (Quercus rubra and Fraxinus americana), the longest vessels often were as long as the tree's stem, but most of them were much shorter. In the grapevine (Vitis labrusca) which has large-diameter vessels (ca. 30Q km) a small percentage of the vessels was 8 m, but most of them were less than 5 m long. In a given species, lengths of the longest vessel were quite variable, but the distribution of the short lengths was more constant. In general, vessel lengths are correlated with vessel diameters: wide vessels are longer. Even in diffuse-porous species, the slightly narrower latewood vessels are somewhat shorter than the wider earlywood vessels. The method is a simplified version of that described by Skene and Balodis, but using a programmable desk calculator. It works best with diffuse-porous species in which vessels are randomly distributed in the stem, and less well in species with wide vessels, because as vessels reach the length of the stem itself, they cannot be randomly distributed.