The transport system that drives sap ascent from soil to leaves is extraordinary and controversial. Like their animal counterparts, large multicellular plants need to supply all their cells with fuel and water. For animals, the solution was the evolution of a vascular system, with a pump to circulate an isotonic blood plasma that prevented cell rupture through the osmotic inflow of water. Plants took a different route to solve the problem of osmoregulation, encasing each cell in a rigid exoskeleton, the cell wall. But this rigidity brought with it a lack of mobility — for whole organisms and also for tissues and cells. Plant tissues were too rigid to evolve a pump mechanism for long-distance transport. So what force is responsible for the ascent of water in plants?

More than a century ago, H. H. Dixon (1896) proposed that a pulling force was generated at the evaporative surface of leaves and that this force was transmitted downward through water columns under tension to lift water much like a rope under tension can lift a weight. The cohesion–tension theory (C–T theory), as it is known, supposes both adhesion of water to conduit walls and cohesion of water molecules to each other. Francis Darwin, when commenting on Dixon's proposed theory, said: “To believe that columns of water should hang in the tracheals like solid bodies, and should, like them, transmit downwards the pull exerted on them at their upper ends by the transpiring leaves, is to some of us equivalent to believing in ropes of sand.”

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