By John Palka — Posted February 21, 2016
The majesty of the great trees of the North American coastal rainforest is awe-inspiring. Many a visitor is drawn to touch the massive trunks, sensing that this brings contact with life. My own experience is that my connection with living beings feels even stronger when I visualize some of the life processes that are going on inside them, underneath the surface that I can see. Let me guide you inside a tree to explore one such life-sustaining process, the movement of the water that is necessary for photosynthesis and, indeed, for life itself.
Water enters a plant, including any tree, not through its leaves but through its roots. This fact immediately presents us with a conundrum—all of the plant’s living cells require water, so this vital substance must be distributed throughout the plant from the point where it enters, at the roots. What’s more, the plant’s fundamental process of photosynthesis, which requires water, takes place not in the roots or stems, but in the leaves. The leaves (or in conifers, the needles) can be in a canopy that is easily 200, and sometimes more than 300, feet above the ground. Here is a view of the canopy of the Wind River Experimental Forest, an old-growth forest in the Washington Cascades. It was taken from a special-purpose crane rising even higher than the trees, so that we could look down on them from a gondola. From this perspective, it’s
easy to see that delivering the water needed for life and for photosynthesis to leaves or needles so high off the ground is an enormous challenge. How is it even possible? How do you move water twenty-plus stories into the air without even having a pump?
The first clue comes from the structure of wood. We can get a hint of this structure by examining a common sight in Northwestern forests, the cut surface of a tree that has fallen to the forest floor and blocked a trail. Here is an example: a young Douglas fir that fell in the woods near our home and was cut into rounds by a neighbor who maintains the trail for the public. It
is lying next to a red alder that met a similar fate. The cut surface shows us the tree’s older, darker wood (the heartwood) in the center; the younger, lighter wood (the sapwood) around it; and the growth rings that allow us to determine exactly how old the tree was when it fell. All the wood we see—heartwood, sapwood, rings—is made of elongated cells that secrete around themselves stiff walls built primarily of the complex molecules cellulose and lignin. These molecules are what makes wood tough. Remarkably, however, most of the cells soon die, leaving behind only their walls—stiff, hollow cylinders.
So, wood is made up primarily of dead cells, but in death these cells serve an essential function. They are arranged to form water-conducting tubes that extend the full length of the tree, starting in the roots and ultimately branching out within the leaves or needles. Collectively these tubes are called the xylem. In the leaves the xylem tubes are organized within veins, as demonstrated in this photograph of the underside of a leaf of a garden cultivar of ninebark, one of the common shrubs of Northwest forests.
Water from the soil is carried within the conducting tubes of the xylem all the way into this dense network. The veins are in close proximity to the cells that contain chlorophyll and conduct photosynthesis. Along the way, side pathways are available to deliver water to all of the tree’s other living cells.
So, water travels through cellular tubes within the wood, but what makes this water actually rise within a tree, sometimes for hundreds of feet? We know that in animals like ourselves, blood is circulated around the body by a mechanical pump, the heart. A tree has no pump of any kind. What, then, makes the water move?
Several processes contribute, but here is the most important. Look at this photograph of the underside of a needle of a western hemlock, one of the dominant trees of the Pacific Northwest. We are working here at the edge of what a camera lens can resolve, but, still, the image is revealing. You can
see rows of lighter and darker green structures along the edges of the needle. These are internal constituents of single cells, colored green because the cells contain chlorophyll. In addition you can see rows of bright ovals closer to the mid-rib (central vein) of the needle. These are stomata, pores that pass through the waxy undersurface of the needle. The job of the stomata is to allow gases to pass across the surface of a leaf whose outer waxy layer is impermeable to both water and gases. The stomata allow carbon dioxide to flow in and oxygen to flow out, two processes that are necessary for photosynthesis. In addition, these pores allow water vapor to escape.
The escape of water can be a serious problem for a plant, especially if it is living under hot, dry conditions. This is, however, also the answer to our puzzle about how water can rise 200 feet and more up the trunk of a tree. The roots live in an environment relatively rich in water. In contrast, the water content of the leaves is constantly depleted through evaporation (except when the humidity is 100%). This loss of water from the leaves exerts a pull on the water remaining within the tree’s conducting system. Because the water-conducting tubes that make up xylem are continuous from leaf to root, the pull from the leaves is felt all the way down to the roots. So, rather than being pushed (pumped) up the trunk of a tree, water is pulled up by evaporation from the leaves.
The energy driving evaporation from the leaves comes from the heat generated by the Sun. Thus, the energy required for pulling water up against the force of gravity comes not from the metabolic activity of the tree but directly from the Sun. Water is heavy, so there is a limit to the height to which this mechanism can draw it. This is currently calculated to be slightly over 400 feet. The tallest known redwood tree measures 379 feet, within about 30 feet of the theoretical limit.
The tree’s whole system for water delivery depends on the continuity of the water column within each conducting tube. This continuity is possible only because water molecules are strongly attracted to each other by electrical forces. Chemically speaking, water, H2O, is composed of two atoms of hydrogen and one of oxygen. These atoms arrange themselves into a V, with the single oxygen at the apex and the two hydrogens forming the tails. Within this molecule, the oxygen apex is slightly negatively charged, while the two hydrogen tails are slightly positively charged. Since opposite charges attract, adjacent water molecules attract each other, oxygen apex to hydrogen tail. This attraction is very strong, so water is said to be highly cohesive. The evaporation of water from leaves is called transpiration, and the whole hypothesis about how water moves up a tree is called the transpiration-cohesion theory of water transport.
We can see cohesion at work in this photograph of a leaf of the common garden plant lady’s mantle. The water stays together in spherical drops because the
cohesion of water molecules is so strong. The drops stay in place because they are also attracted to the leaf surface—this is called adhesion. Some of the drops have been pulled out into ovals, revealing the simultaneous operation of cohesion and adhesion.
A critical factor in drawing water up a tree trunk by transpiration is the continuity of the water column. The cohesion of the water can be disrupted. In such a case, the column breaks to form a bubble, and sensitive instruments can actually pick up the resulting popping sound within the tree’s wood. When the water column within a tube breaks, the tube ceases to function in water transport. The system is dynamic, however, and tubes can be refilled through several different mechanisms. At some point breaks in the water column become permanent, so that any tree contains a mix of conducting tubes: those that function are usually younger and closer to the bark, and those that have failed permanently are usually older and closer to the tree’s core. Fully formed heartwood is considered dead and non-conducting.
Next time you approach a tree and perhaps touch it, remember how its life-sustaining water is drawn up from the tips of the roots to the topmost leaves or needles in a way that depends on the tree’s internal structure, the properties of water, and the energy provided by the Sun.