Transpiration pull theory is the proposed mechanism by which trees draw water through their roots. Transpiration occurs when the leaves of a tree allow water to exit into the air by means of tiny holes called stomata. When the water exits the leaves, the combination of capillary action, cohesion and adhesion draws more water up through the plant's roots to replace the released water.
Capillary action is the tendency for liquids, such as water, to climb the sides of narrow tubes. Tree wood, which is called xylem, contains numerous capillary-like structures for drawing up the water. Cohesion and adhesion refer to the tendency for water to bond with itself and other substances, respectively.
Small structures called guard cells surround each individual stoma; these cells open and close the holes as necessary to regulate the amount of water exiting the leaves. The rate of transpiration varies with the seasons and time of day. During the winter, deciduous trees shed their leaves to avoid desiccating.
This mechanism of drawing water from the ground is entirely passive. Trees expend no energy hydrating themselves. However, this method places an upper limit on the height of trees, as the tension on the water column can break, resulting in air bubbles. These bubbles compromise the capillaries and damage the tree.