"Water is often the most limiting factor to plant growth. Therefore, plants have developed an effective system to absorb, translocate, store and utilize water. To understand water transport in plants, one first needs to understand the plants' plumbing. Plants contain a vast network of conduits, which consists of xylem and phloem tissues. This pathway of water and nutrient transport can be compared with the vascular system that transports blood throughout the human body. Like the vascular system in people, the xylem and phloem tissues extend throughout the plant. These conducting tissues start in the roots and transect up through the trunks of trees, branching off into the branches and then branching even further into every leaf.

"The phloem tissue is made of living elongated cells that are connected to one another. Phloem tissue is responsible for translocating nutrients and sugars (carbohydrates), which are produced by the leaves, to areas of the plant that are metabolically active (requiring sugars for energy and growth). The xylem is also composed of elongated cells. Once the cells are formed, they die. But the cell walls still remain intact, and serve as an excellent pipeline to transport water from the roots to the leaves. A single tree will have many xylem tissues, or elements, extending up through the tree. Each typical xylem vessel may only be several microns in diameter.

"The physiology of water uptake and transport is not so complex either. The main driving force of water uptake and transport into a plant is transpiration of water from leaves. Transpiration is the process of water evaporation through specialized openings in the leaves, called stomates. The evaporation creates a negative water vapor pressure develops in the surrounding cells of the leaf. Once this happens, water is pulled into the leaf from the vascular tissue, the xylem, to replace the water that has transpired from the leaf. This pulling of water, or tension, that occurs in the xylem of the leaf, will extend all the way down through the rest of the xylem column of the tree and into the xylem of the roots due to the cohesive forces holding together the water molecules along the sides of the xylem tubing. (Remember, the xylem is a continuous water column that extends from the leaf to the roots.) Finally, the negative water pressure that occurs in the roots will result in an increase of water uptake from the soil.

"Now if transpiration from the leaf decreases, as usually occurs at night or during cloudy weather, the drop in water pressure in the leaf will not be as great, and so there will be a lower demand for water (less tension) placed on the xylem. The loss of water from a leaf (negative water pressure, or a vacuum) is comparable to placing suction to the end of a straw. If the vacuum or suction thus created is great enough, water will rise up through the straw. If you had a very large diameter straw, you would need more suction to lift the water. Likewise, if you had a very narrow straw, less suction would be required. This correlation occurs as a result of the cohesive nature of water along the sides of the straw (the sides of the xylem). Because of the narrow diameter of the xylem tubing, the degree of water tension, (vacuum) required to drive water up through the xylem can be easily attained through normal transpiration rates that often occur in leaves."

Alan Dickman is curriculum director in the biology department at the University of Oregon in Eugene. He offers the following answer to this oft-asked question:

"Once inside the cells of the root, water enters into a system of interconnected cells that make up the wood of the tree and extend from the roots through the stem and branches and into the leaves. The scientific name for wood tissue is xylem; it consists of a few different kinds of cells. The cells that conduct water (along with dissolved mineral nutrients) are long and narrow and are no longer alive when they function in water transport. Some of them have open holes at their tops and bottoms and are stacked more or less like concrete sewer pipes. Other cells taper at their ends and have no complete holes. All have pits in their cell walls, however, through which water can pass. Water moves from one cell to the next when there is a pressure difference between the two.

"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."

Updated on February 8, 1999

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 REDWOOD TREES. Old growth redwoods, such as these giants from Rockefeller Forest in California's Humboldt Redwoods State Park, reach heights of 100 meters or more.

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.

Capillary action and root pressure can support a column of water some two to three meters high, but taller trees--all trees, in fact, at maturity--obviously require more force. In some older specimens--including some species such as Sequoia, Pseudotsuga menziesii and many species in tropical rain forests--the canopy is 100 meters or more above the ground! In this case, the additional force that pulls the water column up the vessels or tracheids is evapotranspiration, the loss of water from the leaves through openings called stomata and subsequent evaporation of that water. As water is lost out of the leaf cells through transpiration, a gradient is established whereby the movement of water out of the cell raises its osmotic concentration and, therefore, its suction pressure. This pressure allows these cells to suck water from adjoining cells which, in turn, take water from their adjoining cells, and so on--from leaves to twigs to branches to stems and down to the roots--maintaining a continuous pull.

Image: GARY ANDERSON, University of Southern Mississippi XYLEM TYPES. Some vessel elements have complete perforations (1) and others have no end walls (2). Tracheids (3) have overlapping walls and pits.

To maintain a continuous column, the water molecules must also have a strong affinity for one other. This idea is called the cohesion theory. Water does, in fact, exhibit tremendous cohesive strength. Theoretically, this cohesion is estimated to be as much as 15,000 atmospheres (atm). Experimentally, though, it appears to be much less at only 25 to 30 atm. Assuming atmospheric pressure at ground level, nine atm is more than enough to "hang" a water column in a narrow tube (tracheids or vessels) from the top of a 100 meter tree. But a greater force is needed to overcome the resistance to flow and the resistance to uptake by the roots. Even so, many researchers have demonstrated that the cohesive force of water is more than sufficient to do so, especially when it is aided by the capillary action within tracheids and vessels.

In conclusion, trees have placed themselves in the cycle that circulates water from the soil to clouds and back. They are able to maintain water in the liquid phase up to their total height by maintaining a column of water in small hollow tubes using root pressure, capillary action and the cohesive force of water.

Mark Vitosh, a Program Assistant in Extension Forestry at Iowa State University, adds the following information:

Image: PACIFIC LUTHERAN UNIVERSITY XYLEM. Water travels from a tree's roots to its canopy by way of this conductive tissue.

There are many different processes occuring within trees that allow them to grow. One is the movement of water and nutrients from the roots to the leaves in the canopy, or upper branches. Water is the building block of living cells; it is a nourishing and cleansing agent, and a transport medium that allows for the distribution of nutrients and carbon compounds (food) throughout the tree. The coastal redwood, or Sequoia sempervirens, can reach heights over 300 feet (or approximately 91 meters), which is a great distance for water, nutrients and carbon compounds to move. To understand how water moves through a tree, we must first describe the path it takes.

Water and mineral nutrients--the so-called sap flow--travel from the roots to the top of the tree within a layer of wood found under the bark. This sapwood consists of conductive tissue called xylem (made up of small pipe-like cells). There are major differences between hardwoods (oak, ash, maple) and conifers (redwood, pine, spruce, fir) in the structure of xylem. In hardwoods, water moves throughout the tree in xylem cells called vessels, which are lined up end-to-end and have large openings in their ends. In contrast, the xylem of conifers consists of enclosed cells called tracheids. These cells are also lined up end-to-end, but part of their adjacent walls have holes that act as a sieve. For this reason, water moves faster through the larger vessels of hardwoods than through the smaller tracheids of conifers.

Both vessel and tracheid cells allow water and nutrients to move up the tree, whereas specialized ray cells pass water and food horizontally across the xylem. All xylem cells that carry water are dead, so they act as a pipe. Xylem tissue is found in all growth rings (wood) of the tree. Not all tree species have the same number of annual growth rings that are active in the movement of water and mineral nutrients. For example, conifer trees and some hardwood species may have several growth rings that are active conductors, whereas in other species, such as the oaks, only the current years' growth ring is functional.

This unique situation comes about because the xylem tissue in oaks has very large vessels; they can carry a lot of water quickly, but can also be easily disrupted by freezing and air pockets. It's amazing that a 200 year-old living oak tree can survive and grow using only the support of a very thin layer of tissue beneath the bark. The rest of the 199 growth rings are mostly inactive. In a coastal redwood, though, the xylem is mostly made up of tracheids that move water slowly to the top of the tree.

Image: PURDUE UNIVERSITY STOMATA. These pores in leaves allow water to escape and evaporate--a process that helps to pull more water up through the tree from its roots.

Now that we have described the pathway that water follows through the xylem, we can talk about the mechanism involved. Water has two characteristics that make it a unique liquid. First, water adheres to many surfaces with which it comes into contact. Second, water molecules can also cohere, or hold on to each other. These two features allow water to be pulled like a rubber band up small capillary tubes like xylem cells.

Water has energy to do work: it carries chemicals in solution, adheres to surfaces and makes living cells turgid by filling them. This energy is called potential energy. At rest, pure water has 100 percent of its potential energy, which is by convention set at zero. As water begins to move, its potential energy for additional work is reduced and becomes negative. Water moves from areas with the least negative potential energy to areas where the potential energy is more negative. For example, the most negative water potential in a tree is usually found at the leaf-atmosphere interface; the least negative water potential is found in the soil, where water moves into the roots of the tree. As you move up the tree the water potential becomes more negative, and these differences create a pull or tension that brings the water up the tree.

A key factor that helps create the pull of water up the tree is the loss of water out of the leaves through a process called transpiration. During transpiration, water vapor is released from the leaves through small pores or openings called stomates. Stomates are present in the leaf so that carbon dioxide--which the leaves use to make food by way of photosynthesis--can enter. The loss of water during transpiration creates more negative water potential in the leaf, which in turn pulls more water up the tree. So in general, the water loss from the leaf is the engine that pulls water and nutrients up the tree.

How can water withstand the tensions needed to be pulled up a tree? The trick is, as we mentioned earlier, the ability of water molecules to stick to each other and to other surfaces so strongly. Given that strength, the loss of water at the top of tree through transpiration provides the driving force to pull water and mineral nutrients up the trunks of trees as mighty as the redwoods.

Original answer posted on February 1, 1999