Most people understand that trees require water to grow and live, and many are aware that roots play a critical role in drawing water from the ground. But this is the limit of most people’s understanding of the subject.
But by exploring the way in which trees accomplish this surprisingly tricky feat, you can learn a wealth of information about tree biology and you’ll surely appreciate the intricate engineering Mother Nature accomplishes.
Basic Tree Anatomy
Trees pack a whole lot of function into relatively featureless-looking tissues. For example, the wood is actually a complicated type of tissue called xylem. Made from a combination of living and dead tissues, the xylem provides the tree with support and serves as the pipeline for water to travel up the tree.
Xylem contains a number of specialized structures called vessel elements. Comprised almost solely of dead cell walls, the vessel elements form an unbroken tube from the roots, through the trunk and to the leaves. The vessels eventually connect to small pores in the leaf surface, called stomata (singular: stoma).
Another set of plumbing-like tubes are found immediately under the bark of trees in a tissue layer called the phloem. The phloem is tasked with the responsibility of transporting the sugars produced in the leaves to needy tissues throughout the tree. Unlike xylem, which transports water in one direction, phloem is capable of bi-directional flow.
So far, this all seems pretty intuitive. Trees have long “pipes” which carry water from the roots up through the tree. But the truth is more complicated. Consider, for example, that it usually takes energy to pull the water up from the ground. Just think about how much effort you have to apply to suck a soda through a straw. Trees accomplish similar feats, except that they are often required to draw water for much greater distances.
People often view trees as relatively inanimate objects. They may bloom in the spring and exhibit glorious color in the fall, but people rarely consider the energy required by a tree to grow, produce new tissues and repel invaders. Using energy to draw water up their trunks would increase their energy needs drastically.Fortunately, trees have devised a way around this problem.
The solution relies on a few different physical phenomena or properties.
- Osmosis begins the process. Simply put, water tends to move from areas with relatively low levels of dissolved substances to areas with relatively high levels of dissolved substances. The fine hairs on the outer surface of absorbing roots contain liquid that has very high levels of solvents. This causes the water surrounding the roots, which are relatively solvent poor, to flow across the root hair’s cell membrane and into the xylem.
- Now that the water has been drawn into the tree’s xylem, capillary action — the tendency for the water to flow into a narrow tube – becomes important. This occurs when the molecules in the water are more attracted to the vessel walls than they are to each other. This causes some of the fluid to “climb” up the sides of the vessel elements inside the xylem. However, capillary action cannot draw water very high up the tree, so it is only one part of the solution.
Now we have drawn water into the roots via osmosis, and capillary action has helped draw it up to perhaps the level of the trunk (give or take). But to understand the manner in which the water completes its journey, we need to move to the leaves at the canopy’s limit.
Moving On Up
As we said earlier, the vessels inside the tree’s xylem eventually connect to holes in the surface of the leaves called stomata. These pores serve as an exit-point for the water, which evaporates from the pores, diffusing into the air. This process is called transpiration.
But there’s a problem with this – it should be impossible to create this much negative pressure (a fancy way of saying “sucking power”) without causing the water inside the vessels to boil. Trees get around this problem by ensuring that vessel elements are very narrow and 100-percent full of water. Without air bubbles to serve as nucleation sites for the water to start boiling, the water remains in the liquid state. Accordingly, trees are able to use transpiration to achieve high negative pressures, which draws the water from the bottom of the trunk all the way to the leaves at the edge of the canopy.
As you can see, there’s a lot to the process by which trees draw water from the ground. This is a simplified version of the steps, so be sure to check out this great video if you’d like to learn a little more about the process: