For all its mass, a tree is a remarkably delicate thing. All of its internal life exists within three paper-thin layers of tissue--the phloem, xylem, and cambium--just beneath the bark, which together form a moist sleeve around the dead heartwood. However tall it grows, a tree is just a few pounds of living cells thinly spread between roots and leaves. These three diligent layers of cells perform all the intricate science and engineering needed to keep a tree alive, and the efficiency with which they do it is one of the wonders of life. Without noise or fuss, every tree in a forest lifts massive volumes of water--several hundred gallons in the case of a large tree on a hot day--from its roots to its leaves, where it is returned to the atmosphere. Imagine the din of commotion, the clutter of machinery, that would be needed for a fire department to raise a similar volume of water.