HEAVY LIFTING: Among the theories put forth by 19th-century scientists to explain the movement of water up a tree was the idea that pressure gradients inside leaves sucked water upward. Others insisted cellular conduits must function like pumps. Dixon and Joly, in 1895, discredited both of those proposals. They watched a branch move water against triple the normal atmospheric pressure and posited that tensile properties of water enable trees to lift the fluid without exerting any effort whatsoever. PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY BThe uppermost branches of a tree might sway several hundred feet in the air, yet they will receive a constant supply of water sucked out of the soil below. In the late 19th century, the world’s botanists were mired in fierce debate over this astonishing hydraulic feat, divided over whether and how trees expended energy to lift water against the force of gravity.
While theories and counter-theories flew, two Irish scientists, one of them a renowned physicist named John Joly and the other a young botanist named Henry Dixon, decided to test the strength of a tree’s water-lifting capabilities.
Inside of a tree, water courses from root to leaf through tiny conduits collectively called xylem, and almost all water not consumed by cellular processes evaporates, or transpires, through pores on the undersides of leaves. In a paper published in 1895 (Philos Trans R Soc B, 186:563-76), Dixon and Joly reported the results of an experiment to push back against that upward flow.
The pair encased a maple branch in a thick glass tube, sealed except for the end of the branch protruding down into a vial of water. ...
