"Because these cells are dead, they cannot be actively involved in pumping water. It might seem possible that living cells in the roots could generate high pressure in the root cells, and to a limited extent this process does occur. But common experience tells us that water within the wood is not under positive pressure--in fact, it is under negative pressure, or suction. To convince yourself of this, consider what happens when a tree is cut or when a hole is drilled into the stem. If there were positive pressure in the stem, you would expect a stream of water to come out, which rarely happens.
"In reality, the suction that exists within the water-conducting cells arises from the evaporation of water molecules from the leaves. Each water molecule has both positive and negative electrically charged parts. As a result, water molecules tend to stick to one another; that adhesion is why water forms rounded droplets on a smooth surface and does not spread out into a completely flat film. As one water molecule evaporates through a pore in a leaf, it exerts a small pull on adjacent water molecules, reducing the pressure in the water-conducting cells of the leaf and drawing water from adjacent cells. This chain of water molecules extends all the way from the leaves down to the roots and even extends out from the roots into the soil. So the simple answer to the question about what propels water from the roots to the leaves is that the sun's energy does it: heat from the sun causes the water to evaporate, setting the water chain in motion."
Ham Keillor-Faulkner is a professor of forestry at Sir Sandford Fleming College in Lindsay, Ontario. Here is his explanation:
Image: CHERYL MATTHEWS, Humboldt Redwoods Interpretive Association
To evolve into tall, self-supporting land plants, trees had to develop the ability to transport water from a supply in the soil to the crown--a vertical distance that is in some cases 100 meters or more (the height of a 30-story building). To understand this evolutionary achievement requires an awareness of wood structure, some of the biological processes occurring within trees and the physical properties of water.
Water and other materials necessary for biological activity in trees are transported throughout the stem and branches in thin, hollow tubes in the xylem, or wood tissue. These tubes are called vessel elements in hardwood or deciduous trees (those that lose their leaves in the fall), and tracheids in softwood or coniferous trees (those that retain the bulk of their most recently produced foliage over the winter). Vessel elements are joined end-to-end through perforation plates to form tubes (called vessels) that vary in size from a few centimeters to many meters in length depending on the species. Their diameters range from 20 to 800 microns. Along the walls of these vessels are very small openings called pits that allow for the movement of materials between adjoining vessels.
Tracheids in conifers are much smaller, seldomly exceeding five millimeters in length and 30 microns in diameter. They do not have perforated ends, and so are not joined end-to-end into other tracheids. As a result, the pits in conifers, also found along the lengths of the tracheids, assume a more important role. They are they only way that water can move from one tracheid to another as it moves up the tree.
To move water through these elements from the roots to the crown, a continuous column must form. It is believed that this column is initiated when the tree is a newly germinated seedling, and is maintained throughout the tree's life span by two forces--one pushing water up from the roots and the other pulling water up to the crown. The push is accomplished by two actions, namely capillary action (the tendency of water to rise in a thin tube because it usually flows along the walls of the tube) and root pressure. Capillary action is a minor component of the push. Root pressure supplies most of the force pushing water at least a small way up the tree. Root pressure is created by water moving from its reservoir in the soil into the root tissue by osmosis (diffusion along a concentration gradient). This action is sufficient to overcome the hydrostatic force of the water column--and the osmotic gradient in cases where soil water levels are low.