The Rise of Sap in a Tropical Liana.

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Abstract
Pressure-flow relations have been studied in the stem of a tropical liana (Tetracera). It was shown by potometer measurements that whenever an air break is introduced into the vessels of a transpiring vine, e.g., by a double cut, or by cutting it off completely, the pressure above the cut always drops drastically. When a cut liana takes up water from a burette and then is given air the air is drawn in at the same rate as the water but soon stops abruptly, i.e., the receding water columns hang up. From this point on we observe the above-mentioned pressure drop. The air which enters the severed vessels is evidently stopped by the pit membranes in the vessel walls which do not allow an air-water inter-face to pass. As the vessels are of limited length, the air does not spread. The vessels represent the paths of least resistance to the transpiration stream, and whenever vessels are severed a sub-atmospheric hydrostatic pressure will empty them, thus rendering them inoperative for water conduction. This means a locally increased flow resistance which results in the recorded pressure drop. There are good indications that the drop may reach cohesive values. The results demonstrate that air breaks in the vessels and tracheids produced by various cuts stay confined in the severed units and that the transpiration stream simply moves around these obstacles in the intact units, as postulated by Dixon. Hence the idea of cohesive lift is not incompatible with air breaks in the transpiration stream.

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