Plants need much more water to survive than animals do, a sunflower must take in 17 times more water than a human of comparable weight. At least 500 pounds of water must be lifted from the roots to the leaves to make a single pound of organic tissue. This tremendous amount of water is required to undergo photosynthesis each and every day of a plant's life. But how does a tree lift hundreds of gallons of water hundreds of feet in the air against the force of gravity? Scientists have developed two theories to explain this "impossible" phenomenon ....