Monday, November 28, 2011

How does water get to the top of a redwood tree?


The upward movement of water more than 100 meters in a redwood or eucalyptus (there is  a traditional dispute between Americans and Australians over who has the tallest trees!) seems to be a gravity-defying task of epic proportions.  Gravity is certainly a factor, and ultimately limits how tall a tree can get, but there are other forces at work that can meet that challenge.  The amazing thing about plants is that the process is largely passive, in the sense that plants expend practically no energy to accomplish it.  There are no muscles and no heart in a tree to pump water upward.  What there is, is basically a gigantic paper towel.

You may recall the ads for the “Bounty Quicker-Picker-Upper”  a decade or two ago.  In the ads, these paper towels quickly absorbed any spilled liquid.  You can take a paper towel (even a cheap slower-picker-upper), roll it into a tube, and stick the end in water. You’ll quickly see the water begin creeping up the paper towel.  We see the same gravity-defying process in plants.  What is a paper towel made of?  Wood!  And the magic ingredient in both a paper towel and a living plant is cellulose. 

Every plant cell is wrapped in a layer of cellulose – the cell wall.   In wood, the cells of the xylem die after laying down strong cellulose walls, leaving the latter as narrow, empty conducting tubes.  These tubes line up so as create masses of interconnected passageways through which water can move freely.  So a tree trunk is essentially a massive, non-living, interwoven paper towel.   

But what is the interaction between cellulose and water?  What is the force that can overcome gravity?  The short answer is magnetism.

Magnets can defy gravity by lifting nails and other iron objects.  No, neither cellulose nor water is made of iron, but the forces involved are similar.  A molecule of water is electrically charged.  Imagine each molecule as a Mickey Mouse head.  The face is an atom of oxygen, and the ears a pair of hydrogen atoms.   The two types of atoms are bound together by sharing electrons, but the oxygen atom holds onto the electrons much more tightly than the hydrogen atoms do.  the result is that the electrons hang out around the oxygen atom most of the time, giving its side of the molecule a net negative charge.  The two hydrogen atoms are left with a net positive charge as they are visited less often by the electrons.   Thus water molecules are thus like tiny magnets and tend to stick together.  This actually is what makes water liquid, rather than a gas, at room temperature, and which accounts for a lot of other properties that we don’t have time to review here.

It so happens that the complex surface of cellulose fibers also have positive and negative charges, and so attracts water molecules.  In a paper towel, water molecules are pulled into every available niche in the cellulose matrix.  Those at the top are pulled into higher niches, which pulls more water molecules from the bottom.  Narrow spaces within the matrix also fill with water molecules, which attract and pull each other in.  This happens until the towel is saturated and there is no room for any more water. 
The water molecule (A) consists of a negatively charged oxygen atom and two positively charged hydrogen atoms.  This causes them to stick together in chains (B) and to the walls of cellulose fibers (C).

If we return to our rolled up paper towel with the end sitting in the water, we can watch something else happening over time.  The paper towel only gets saturated at the base, and even remains fairly dry at the top.  But given enough time, the container of water it is standing in will completely dry up.  Of course what is happening is that the water that has moved up the column continually evaporates when exposed to the air, leaving spaces for more water molecules to move up.  The result is a steady stream of water moving upward, drawn by both evaporation and the electromagnetic attraction of water molecules to the cellulose and to each other. 

This is the process of transpiration, which is powerful enough to lift water and dissolved minerals to the top of a tall tree.   It continues as long as water is evaporating through the leaves.  In extremely humid weather, or if the leaf pores (stomata) shut down for the night, the stream is suspended in place until evaporation resumes again later. 

I should note that a tall tree, if dried out, cannot start this process from scratch.  Gravity will stop the electromagnetic movement of water long before it can reach the evaporation zone of the leaves.  The transpiration stream develops in a germinating tree seedling and is maintained and strengthened as the tree grows, but if that stream should be broken (interrupted by extensive air bubbles) in an extreme drought, it cannot be repaired and the tree will die.   Only small plants like mosses can completely dehydrate and recover when wet conditions return. 

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