Melvin T. TyreeThe transport system that drives sapascent from soil to leaves is extraordi-nary and controversial. Like their ani-mal counterparts, large multicellular plantsneed to supply all their cells with fuel andwater. For animals, the solution was the evolution of a vascular system, with a pumpto circulate an isotonic blood plasma thatprevented cell rupture through the osmoticinflow of water. Plants took a different routeto solve the problem of osmoregulation,encasing each cell in a rigid exoskeleton, thecell wall. But this rigidity brought with it alack of mobility — for whole organisms andalso for tissues and cells. Plant tissues weretoo rigid to evolve a pump mechanism forlong-distance transport. So what force isresponsible for the ascent of water in plants?More than a century ago, H. H. Dixon(1896) proposed that a pulling force wasgenerated at the evaporative surface of leavesand that this force was transmitted down-ward through water columns under tensionto lift water much like a rope under tensioncan lift a weight. The cohesion–tension theory(C–T theory), as it is known, supposes bothadhesion of water to conduit walls andcohesion of water molecules to each other.Francis Darwin, when commenting onDixon’s proposed theory, said: “To believethat columns of water should hang in thetracheals like solid bodies, and should, likethem, transmit downwards the pull exertedon them at their upper ends by the transpir-ing leaves, is to some of us equivalent tobelieving in ropes of sand.”Dixon proposed that plants transport
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