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.
