Trees' Daily Burden

by The Old Scot

Bring to mind, please, a tall tree which you admire-a monarch of the forest, perhaps. There it stands, a magnificent specimen, towering against the sky.

Now, if you please, image that you must climb that tree to the very top, with a 60-pound pack on your back. As soon as you reach the top, you must leave your burden and descend, only to start up again with a fresh load. And you must repeat this cycle a total of 12 times during the day-and every day in the future.

Awesome to contemplate as our fantasy assignment may be, it is no more than the tree itself must perform daily, if it is to live and thrive. The weight which every tree must lift is water, and it can amount to quite a lot. A large birch tree, for instance, requires about 90 gallons of water per day during the growing season. That means lifting 720 pounds up to 100 feet or more-as much as 300 feet or more, in the case of the really tall trees.

Trees manage this task very efficiently and without fuss. But how do they do it? Scientists have pondered this question for years, and still face mysteries.

Four possible solutions have been investigated: First, the water might be pushed up from below, either by "root pressure" or by capillarity (the tendency of water to rise in very thin tubes, or capillaries. But neither root pressure nor capillarity can provide anything like enough force to push a water column to the top of tall trees.

The second possibility is that the leaves at the top of the tree suck up the water from the roots. But suction pressure is limited by atmospheric pressure, and can only lift a column of water about 33 feet.

A third possibility might be that trees raise the water by stages, with cells acting as miniature pumping stations. But this was found to be just an empty idea when one experimenter introduced picric acid into test trees. This poisonous substance killed the cells as it passed through them, and thus would have shut down their supposed pumping activity. But the tainted water kept rising anyway.

Scientists studying the problem know of only one other possible mechanism: molecular attraction-the force with which water molecules cling to each other. This cohesive force has been measured, and was found to be more than enough to lift water to the top of the tallest trees. The theory is that as water is evacuated from leaves into the air as vapor, it exerts a cohesive tug on the column of water stretching down the twig and branch to the trunk, and thence to the root.

There are, however, grave problems. For one thing, it depends on the water column being continuous from leaf to root. But very often gaps have been found in the water columns.

This difficulty was high-lighted in an experiment in which the tree trunk was sawed more than halfway through on first one side and then, a short distance above, from the other side, so that the cuts overlapped, and no cellular channel could continue without interruption. But the trees so abused continued to live, though they needed structural support.

So, after many decades of study, the precise system trees employ to meet their need for water eludes our deepest inquiry. Yet trees have no mind at all!

Can it be that non-intelligence can create such living wonders as we see trees to be? Impossible! They were obviously designed by a great intelligence. We call that intelligence God.

The poet Joyce Kilmer has it right: "Only God can make a tree."


Evolution and Design in the Plant Kingdom, C. L. Duddington, Thomas Y. Crowell Co., N.Y., 1969, p. 64.

Plants and Water, J. Sutcliffe, Edward Arnold Ltd., London, 1968, p. 75.

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