The Rise of Sap in Tall Grapevines.

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Abstract
The hydraulic relations in tall vines of Vitis labrusca have been investigated. The pressures in the xylem vessels were measured directly, using a manometer punch. When filled with water the vine stem acts like a simple water hose, exhibiting a normal hydrostatic gradient. Most vessels in the stem have functional valves spaced 60 cm apart which will pass water freely, but not an air-water interface. In the spring before the leaves are out the sap pressure is often 3 to 4 atmospheres and a normal hydrostatic gradient of 0.1 atmosphere per meter elevation obtains in the stem. During transpiration the pressure at ground level is often only a fraction of an atmosphere and a hydrostatic gradient is not found. Yet pressures were never as low as water vapor tension, which would have been indicative of cohesive columns. The sap at all levels is saturated with atmospheric N and hence travels up the vessels without diffusion contact with a low pressure gas phase. Sap could not be withdrawn from the stem at low positive pressures, but came out easily when the pressure approached atmospheric. Leafy shoots consumed water at an undiminished rate when vacuum was applied to the water, but at this pressure trains of cavitation bubbles emerged from many vessels. Vessels in the stem did not support experimentally produced tensions. A considerable volume of air or helium was aspirated by the cut stems of vines, severing every vascular water column carrying the transpiration stream. Yet this did not even temporarily slow down the rate of water consumption in a vine taller than ten meters. Most of the air travels up the vine, but how far is not known. A tall vine wilted by a large amount of aspirated air will resume water consumption at a much reduced rate and become turgescent again. Such a wilted vine will also draw copper sulfate up into the leaves. Potometer experiments showed that high tensions across a resistance are readily obtained also where the vessels are filled with air, and such experiments consequently do not prove cohesion in the xylem vessels. Several of the above and earlier findings are, at least for the present, not explainable in terms of the cohesion theory.

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