Let's not forget the critical role of surface tension in xylem water relations

The widely supported cohesion–tension theory of water trans -port explains the importance of a continuous water column and the mechanism of long-distance ascent of sap in plants (Dixon 1914, Tyree 2003, Angeles et al. 2004). The evapora-tion of water from the surfaces of mesophyll cells causes the air–water interface to retreat into the cellulose matrix of the plant cell wall because the cohesion forces between water mol -ecules are stronger than their attraction to air. As a result, the interface between the gas and liquid phases places the mass of water under negative pressure (tension). This pulling force is then transmitted to soil water via a continuous water column since the strong hydrogen bonding of the water molecules also allows water to stay liquid under tension (Oertli 197 1). Related to these cohesive forces is surface tension, which characterizes how difficult it is to stretch the surface of a liquid.Most laboratory and field studies dealing with xylem cavita-tion and embolism repair assume that surface tension is equal to that of pure water and constant within and between species. Although surface tension is a crucial parameter in xylem water movement, few studies have tested whether this parameter dif -fers from that of pure water (Bolton and Koutsianitis 1980). In this issue, the study by Christensen-Dalsgaard et al. (2011) looked at the instantaneous surface tension of xylem sap extracted from branches of three tree species and its change over time. Using the pendant-drop method, they showed that in all three species studied, the instantaneous sap surface ten-sion was indeed equal to that of pure water. However, in one species,