BRIANWAVE COLUMN No 20: November 1995

Autumn Gold

Brian J Ford

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Autumn is upon us. You won’t mind my mentioning a theory on which we have been busily working. Trees seem so romantic as they stand in a deep brown leafy pile, but this alternative explanation reveals the truth. It isn’t that the colours are there to delight the eye; the trees are all having a poo.

There are several fundamental properties of all living organisms: respiration and nutrition, reproduction and response, growth and excretion ... except that, if you look through the standard works on plant physiology, the very term ‘excretion’ is missing from the index. At school I could never find out how plants excreted. Yes, they lay down some tannins and related compounds in the bark, but there was no major excretory mechanism. Worse, its absence seemed to cause no problem for one’s teachers.

A second matter, which sat in a different part of the brain, was why plants kept shedding bits and pieces. It seemed to make no sense. I know we are taught that trees lose their leaves in the fall because of the onset of winter, but plants in the tropics lose their leaves, too. Evergreen species in temperate latitudes ignore autumn - but they do shed their leaves. They may not do it all at once, and they may not favour the fall, but they still shed them.

My dear friend Fred Addicott at Davis, California, wrote the major book on leaf fall. They are shed, said Fred, to prevent competition for nutrients, to remove diseased leaves, to prevent shading ... but none of those reasons explain the ubiquity of the phenomenon.

The answer came to me one day in a tropical rain forest. There were climbers in the rich verdant areas shedding their leaves just as actively as those where nutriment might be problematical. Picture a palm tree. Were you or I palms, as conventionally construed, we would be festooned with leaves from ground to apex, all gathering light from the sun and helping us trap its energy. But palms don’t do that. They shed their leaves as they grow, so you are left with a tuft of leaves at the top of a long and wasted stem.

The reason is that the leaves have two functions, not one; as well as gathering energy, they also act as the means for the tree to shed its annual production of wastes. Heavy metals increase a thousandfold just prior to leaf-fall, just as the theory predicts. Since it appeared in Nature I have found several earlier accounts which point in this direction. But I see it as a general theory that even explains why petals are shed (to release the products of reproduction).

It also offers a way to remedy pollution. In my concept, plants are raising pollutants from the soil and shedding them in their leaves. Harvesting leaf litter - or plants themselves - offers a way to harness the energy of plant life to reclaim soils damaged by chemical contamination. Bioremedation is now a growth industry. My belief is that plants can show us how to do it simply and without effort. Once again, nature knows best.

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