17.13: Water and minerals are distributed through the xylem.

Evaporation is relentless. Leave a glass of water on your desk for a day or two, and eventually all the water will evaporate away. Because there is less water in the dry air than in the glass, the liquid water continuously turns to vapor. If you pour the water onto a plate so that it spreads out, it will evaporate even faster, because more of the water is in direct contact with the air.

Evaporation steals the moisture from leaves, too. And although evaporation can cool leaves and help move water and ions up a plant, perpetual water loss to the atmosphere is a real problem for plants, especially trees. In one day, a big tree in a North American forest can lose 100 gallons of water or more! That is 100 gallons of water that the tree’s roots absorbed but the plant does not get to use for its physiological needs. Water loss through evaporation also presents a physical challenge for the plant. Most water is lost from the leaves, high in the tree, but all of the water comes from the ground. Somehow, water must be constantly delivered to the leaves—dozens of gallons or more every hour. Not only is this a large amount of water for the roots to absorb, but it is also a very heavy load to lift hundreds of feet into the air (FIGURE 17-32). How do trees manage to get the water up so high?

Water transport is the job of the xylem. As we saw earlier, the xylem is one of the two transport systems in plants. It is responsible for directing the flow of water and dissolved minerals from the roots to all of the plant’s cells.

In the absence of a pump like the one in animals’ circulatory systems, there are two possible explanations for how water can be transported from the roots to the leaves—which might be 200 or 300 feet off the ground. The first possibility is that the column of water is pushed up by pressure from below. This would require the water in the soil and roots to be under very high pressure, which it isn’t. The alternative explanation is that water is pulled up to the highest leaves. Although this possibility seems improbable, it turns out to be true.

There are three important components to the process by which water is moved throughout a plant (FIGURE 17-33).

• 1. Evaporation. Because of low water concentration in the air relative to that in the leaf, molecules of water, one by one, are vaporized and lost from the leaf.
• 2. Tension. Recall from Chapter 2 that hydrogen bonds cause water molecules to stick together. As one water molecule evaporates, it creates a tension, pulling the chain of water molecules stuck to it.
• 3. Cohesion. Through this cohesion, or stickiness, water molecules all the way down to a tree’s roots are pulled up as molecules evaporate, one by one, from the leaf.

The beauty of this system—called the cohesion-tension mechanism—is that the plant does not need to expend energy to pump water and minerals up from the roots to all the cells in every branch and leaf. Just as a mechanism evolved to take advantage of nitrogen-fixing bacteria to produce a usable source of nitrogen for plants, so, too, an efficient way evolved to avoid the energetic expense of moving heavy fluid around in the plant body. And so, even though losing water to evaporation is generally bad for plants, their use of the process to move water through the plant body turns it into a positive effect.

Although this system works well, it does have a disadvantage: it places a limit on how tall a tree can grow. Leaves at the top of very tall trees have trouble getting enough water. They must “pull” it up against the ever-increasing force of gravity pulling downward on the long column of water in the xylem. And although the collective strength of the hydrogen bonds causing all the water molecules to stick to each other is impressive, it is limited. As a consequence, the leaves at the top of the tallest trees resemble desert leaves: they are small, grow slowly, photosynthesize at a slow rate, and otherwise behave like a parched leaf. Researchers estimate that the physical constraint of lifting water against the force of gravity will restrict any tree to a maximum height of about 400 feet.

The sap in the xylem doesn’t always contain just water and dissolved minerals. Sometimes the fluid contains sugars, too. (Recall that phloem is generally the sugar-transporting pipeline.) Throughout the late summer and early fall, just before maple trees (Acer saccharum) lose their leaves, they begin storing the sugar from photosynthesis as starch in their roots, much like a bear storing body fat prior to hibernation. In late winter, the temperature rises and the starch in the roots begins to break down into sugar and is released into the xylem (FIGURE 17-34). The sap moves upward toward the newly forming leaf buds, which need energy to develop. Humans have long recognized that by drilling into the xylem and inserting a small tube, it is possible to collect some of the sugary sap—it simply drips out. At about 1% to 5% sugar, the sap is not sweet enough to be poured on your pancakes, so it is boiled to evaporate much of the water, concentrating the sugar. When the sap reaches about 85% sugar, it makes a tasty maple syrup. About 1 gallon of sap can be collected from a tree over the several weeks during which it flows, not enough to harm the tree. But it takes about 50 gallons of sap to make 1 gallon of syrup (FIGURE 17-35).