Imagine being handed a very long straw, and told to lean out of a 35-storey building and drink from a glass on the sidewalk below. Sounds like a challenge? Well if you are a California redwood, it’s just part of everyday life. Reaching as much as 350 feet in the air, and so just as far down for water, these are the tallest trees on the planet. Even for relative dwarfs like western hemlock, or ponderosa pine, where the tallest are a mere 250 feet tall, the task is daunting.
The basic problem is easy to see. We could take a tall tube, with the lower part standing in water, and attached the top to a pump to remove the air. Even if we create a perfect vacuum at the top of the tube, the water will rise a mere 33.8 feet, before stopping. This is because, with a vacuum at the top, the water is lifted by the pressure of the atmosphere, which at sea level is normally 14.7 pounds per square inch. That is sufficient pressure to lift water 34 feet up, but no more. So how do these majestic trees manage to draw water up to such extraordinary heights, despite this apparent limitation?
Ever since botanists began to look at the workings of plants, this problem has been apparent. Following the discovery of human circulation, a similar concept was accepted for plants, although there was no physical equivalent of the pumping heart driving fluids around our bodies. As late as 1905, it was thought that plants had an active function – of an unknown nature – pumping water and sap around the tree. Professor E.J. Ewart, an Australian botanist, climbed 300-foot Australian mountain ash (Eucalyptus regnans) to measure the pressure inside branches. This tree rivals the Redwood for height, and Ewart concluded that the ability to lift water to those heights required living wood, and thus some activity by the tree that was only possible in life – the exercise of a ‘vital force’.
On the other side was an unconventional botanist, Henry Horatio Dixon, at Trinity College in Dublin, and his physicist friend, John Joly. At a time when botany was mostly a descriptive science, Dixon stood apart in taking a deep interest in the physiology of plants. The two friends published a paper in 1995 showing that evaporation from leaves could drive the uptake of water, and Dixon followed this up with a book in 1914, Transpiration and the Ascent of Sap in Plants. Supporters of the ‘vital force’ greeted his work with skepticism, but his ideas prevailed and the realization that water rose through plants entirely by physics, with no ‘life-force’ needed, became the accepted explanation.
What exactly was it that Dixon and later scientists discover? The answer is a little complex, but let’s take it step by step.
Water into the roots
The first step is to move water from the soil into the roots. This is made possible by differences in the concentration of minerals in soil water – which is very dilute – and cell sap – which is much more concentrated. Separating them is the cell membrane, which has molecular holes in it large enough to allow water to enter, but small enough to keep the dissolved minerals inside the cell. Water flows from a place of low mineral concentration to a place of higher mineral concentration, in a process called osmosis, generating pressure inside the root cells. That pressure is strong enough to lift concrete sidewalk slabs, and crack roadways, or, in the strangler fig, to crush boulders, and it can be 5 times the pressure inside a car tire.
A significant part of that pressure is lost to friction in moving the water through the narrow pores of the cells, so in practice the effective root pressure is quite small. If you chop down a tree, its stem doesn’t start to spurt water. We do see significant root pressure in spring in many trees, such as sugar maple, where rising sap will flow from a wound. Even so, root pressure might be enough to supply a small plant, but it certainly can’t pump water to the top of a redwood tree.
At the other end from the roots are the leaves, and there we see the loss of water by evaporation, in a process called by botanists, transpiration. It was the discovery of this process that put Dixon on the trail of the correct explanation of water movement in plants. That classic corn plant in a field in the Midwest, beloved by Botany texts, moves around 50 gallons a day from the soil into the air, and large trees, especially in hot climates, can transpire 300 gallons a day. Almost all that water movement is needed just to bring up minerals, and only about one-twentieth of it is actually used by the plant in cell growth.
How much pull the evaporation of water from the leaves creates depends on the humidity of the surrounding air. In humid conditions it is slight, as the rate of evaporation into areas already almost saturated with water is low. In less humid weather, evaporation is more substantial, as the escaping water molecules pull more after themselves. Thermal energy from the sun warms the water molecules and gives them the energy to escape as vapor – the leaf takes no active part in that. Dixon’s work showed that the pull created by transpiration is in theory enough to lift water to the top of the tallest tree – but there is a problem, and it’s a big one – one that kept plant physiologists arguing for much of the last century.
Going back to our column of water supported by atmospheric pressure, we could use a pump and try to pull water above that 34 feet. However, if we do that, we immediately see that the water column collapses under its own weight, and cavitates.
Water rises through trees in the narrow cells of the young wood, called xylem. These hollow tubes are created by cells that then die, so they are empty spaces that fill with water from the pressure of the water in the roots. Transpiration in the leaves draws that water up, and as it does so, the columns of water are stretched. Despite that, water moves with some speed through the hollow cells. In the largest xylem cells found, speeds of 50 feet an hour are possible, and even in average-sized cells, water is moving 5 to 20 feet per hour – a pretty impressive rate through what looks like a solid material.
So why is it that a mechanical pump cannot raise water far before it cavitates, but this does not happen in xylem cells? As much as 300 atmospheres of negative pressure can be found in these water columns, but water still ascends the tree, and doesn’t collapse through cavitation. How is this possible?
The secret is in the small size of the vessels. In very small tubes, the cohesive forces of the water molecules, one for the other, combined with adhesion to the walls, and surface tension, are sufficient to keep the water columns intact, resisting gravity. This is true even though there is significant negative pressure in the vessels. This is the same force that causes water to rise up around the edge of a glass, forming what is called a meniscus.
At such negative pressure though, cavitation will sooner or later occur even in small tubes, and should spread until the whole system collapses. Yet it doesn’t do that, and even sawing through large parts of the trunk doesn’t prevent water continuing to rise. The explanation lies in the unique anatomy of the xylem. Rather than a simple set of long tubes, it is a complex network of smaller vessels, connected by tiny holes, called pits. These have a valve-like structure to them, so that when the negative pressure does cause some part of the water to cavitate, the pits close, keeping the cavitation inside a single cell, while the water continues to flow through the surrounding cells. It is even possible to attach a microphone to a tree and hear distinct ‘pings’, as cells cavitate and the pits snap shut. Over time, water gradually moves back into the cavitated cell, restoring its function. The system continuously repairs itself mechanically, and operates because of the built-in redundancy of many millions of cells.
During a growing season, the function of the xylem naturally deteriorates. This is why almost all trees replace their functioning cells every spring, leaving the older ones to form the structural woody core of the tree, which does not to conduct water. Only the outermost layers of cells just below the bark are moving all the water through even the largest tree.
This picture of water movement, called the Cohesion-Adhesion Theory, remained largely theoretical until quite recently. In 2008 Abraham Stroock and Tobais Wheeler, at Cornell University, created an artificial ‘tree’ in a plate-sized piece of gel similar to that used in contact lenses. By forming minute pores through the gel, Stroock, who is a professor of bio-molecular engineering, demonstrated that pressures like those inside trees could be produced with intact water columns, and that water could be moved through the gel by purely physical means, just as it is in trees. Their work finally proved what Dixon, a hundred years earlier, had predicted. There is no ‘vital force’ moving water up those redwoods and other tall trees, just a brilliant piece of bio-engineering facilitated by millions of years of evolutionary trial and error.