In a nearby park, just down the block, or maybe in your own backyard, you'll find an engineering marvel that has stumped scientists for centuries: a tree. It may look ordinary, but its plumbing is extraordinary. Beginning at the roots, traveling the length of the trunk and branches, and tapering to microscopic channels in the leaves, a series of inert pipes called the xylem carries water from deep underground to the tops of trees at rates as fast as 150 feet per hour. The dead cells of the xylem can lift a hundred gallons a day, against the force of gravity, to dizzying heights. And biologists still aren't sure exactly how it's done. "Water transport in trees may seem impossible," says Barbara Bond, a forest physiologist at Oregon State University in Corvallis. "Somehow, these organisms find a way to do it anyway."
Pound for pound, trees and other plants need much more water to survive than animals do. Their thirst is driven by photosynthesis: the process by which the green parts of a plant use the energy of sunlight to transform water and carbon dioxide into oxygen and carbohydrates. Yet photosynthesis itself consumes only a small fraction of the water taken up by plants. More than 90 percent "leaks" out into the air through pores in leaves that trap carbon dioxide. The volumes involved are formidable. A mere sunflower must imbibe 17 times more water each day than a human being of comparable weight.
There are two ways to propel that much water: either pull it from above or push it from below. The pull mechanism has long been the favorite of physicists and biologists alike. First proposed in the late 1800s, the theory relies on a property of water that's not usually associated with fluids: its tensile strength. Water molecules tend to stick together because each molecule has positively and negatively charged poles that form weak hydrogen bonds with poles of the opposite charge on other water molecules. Hydrogen bonds help water stay in the liquid phase when low pressures or high temperatures would drive less polar fluids into gas. The bonds can actually stretch before liquid water will vaporize or boil to steam.
According to the pull theory, that's what happens to the water in trees. Instead of making a clean break, water evaporating from treetops tugs on the water molecules that are still in the leaves. Because all the water molecules in the xylem are connected by hydrogen bonds, that tug extends all the way down the trunk to the roots. The tree itself doesn't have to push or pull; all the energy for lifting water comes from the evaporative power of the sun. "It all has to do with the way water molecules interact," says plant biologist Michele Holbrook of Harvard University.
Biomechanical models suggest that the lift requires a sucking force much stronger than a vacuum—a phenomenon known as negative pressure. According to the pull theory, the pressure inside the xylem tubes of the tallest trees could be as low as -20 atmospheres. That's about the same amount of pressure held in an automobile tire—except that negative pressure would deflate a tire, rather than inflate it. But Holbrook and other experts are still struggling to find a reliable method of measuring xylem pressure. Despite the ingenious application of medical imaging, centrifuges, pressure chambers, and specialized syringes that are like ivy IVs, conclusive figures have remained elusive. You can't just wrap a blood-pressure cuff around a poplar and wait for the first pulse.
Without proof that sizable negative pressures exist in the xylem, the pull theory remains speculative. And it has been challenged, most persistently by botanist Martin Canny of the Australian National University in Canberra. Canny is one of the few members of the push camp. He maintains that positive pressure in the roots of trees can force water up the xylem from below. Roots build up pressure when the subterranean xylem draws water from the surrounding soil. Canny thinks that pressure can exert enough upward force to vanquish the downward thrust of gravity. "[T]hat is the pressure step that the root pump must overcome to put water in the bottom of the pipes," Canny writes. "Once it is there, no further lifting is needed."
Of course, trees could be moving water by both pulling and pushing. But pull proponents note that no one has found evidence of root pressure in excess of five atmospheres and that some trees, such as pines and other conifers, don't exhibit root pressure at all. So water transport is probably more pull than push. But it would be nice to have proof—especially since a crop of new studies shows that the transport mechanism could have profound effects on the sizes and shapes of trees.
Barbara Bond, for example, invokes the pull theory to explain why even the tallest trees don't grow any higher than about 350 feet. Both the force of gravity and the resistance caused by friction increase as the length of a xylem tube increases, she reasons, so xylem pressure in trees of the same species should be lower in the larger representatives. But the lower the pressure, the higher the chance of cavitation—the formation of air bubbles in the water column. Cavitation is disastrous for water transport because it breaks hydrogen bonds. In order to limit bubble formation, Bond proposes, big trees close the pores essential to photosynthesis in mid-afternoon, when evaporation rates are highest. Without those extra few hours of photosynthesis, a tree can't continue to grow skyward. In studies of forest canopies, Bond has found evidence that rates of photosynthesis in Douglas fir and ponderosa pine do decline as they age. "Some people have taken exception to the idea that photosynthesis slows down as trees get older and larger," she says. "But there are absolute physical consequences of distance from the ground that a plant can't avoid."
Ecologist Brian Enquist of the University of Arizona in Tucson has also used the pull theory to argue that hydraulic constraints predict both upper and lower size limits on plants. The xylem vessels at the base of a tree can't grow beyond a certain diameter, he says, because bubbles are more likely to form in big tubes. And to minimize friction, xylem tubes need to taper as they get farther from the ground—up to a point. There seems to be a minimum capillary size beyond which tapering actually interferes with fluid flow.
"The tubes leading right up to the leaf can only be so small," says Scot Zens, a forest ecologist at Dartmouth College in New Hampshire. "That's true in the capillaries of human beings and fish as well. Blood or water, there's the same size limit at the end of the vessel."
Although the central conduits and branching veins of plants do in fact resemble the human circulatory system, trees don't have a tireless, pumping heart to push their vital fluids along. But Holbrook's latest work implies that plants are capable of fine-tuning water transport in ways that current theories don't anticipate. She has shown that membranes linking a tree's xylem vessels may swell or shrink in response to the concentration of charged molecules, called ions, dissolved in the water of the xylem. In one set of experiments, flow rates more than doubled because of changes in ion concentrations. Holbrook thinks living cells near the xylem might be manipulating ion levels to prevent or repair cavitation and send water to a plant's thirstiest parts.
"Now there's the possibility of active control," she says. "It's not just a bunch of dead tubes anymore."
For a thorough overview of the vascular tissues of plants, visit www.biologie.uni-hamburg.de/b-online/e06/06b.htm.
