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FEATURE ARTICLE

Transporting Water in Plants

Evaporation from the leaves pulls water to the top of a tree, but living cells make that possible by protecting the stretched water and repairing it when it breaks

Martin Canny

Figure 2. Stephen Hales's drawingClick to Enlarge Image

The tallest trees must lift water from the ground to leaves as high as 300 feet. Since the 1700s, many scientists have assumed that trees perform that feat through a wick-like mechanism in which continuous columns of water are "pulled" up a tree by way of negative pressures, or suction. Despite the broad acceptance of such a theory, it requires much higher negatives pressure than have ever been measured in plants and, in some cases, an absurdly high cohesive strength for water to keep it intact under such "pulling," or tensile stress. Canny offers a new theory that relies on a combination of factors: pumping water into the roots, water evaporating from leaves and living cells applying pressure to the tubes in which the water rises. Under Canny's theory, a plant can lift water 300 feet and more with very little suction.


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