How did the theory evolve? We have always known that plants must excrete (this is one of the cardinal properties of all living things) yet the term excretion is virtually absent from the index of standard works on plant physiology. Leaves are being revealed as having a third function.
The shedding of the autumn leaves of deciduous trees has a convenient explanation - it is due to the damage that the leaves would suffer in the harsh winter weather. This simple answer, which everyone is taught in school, turns out to be misleading.
The immediate objection is that many plants do not lose their leaves at all. These, the evergreens, have leaves that are anatomically adapted to survive the extremes of winter.Yet evergreens do lose their leaves: it is just that they drop them at different times of the year. Stand under a tree of an evergreen species, and you will still find yourself up to the ankles in dead, brown leaves. Some other theories say that plants lose their leaves because of competition for nutriment. Study of abscission in plants of tropical rain forest disproves this idea, for we have found that their leaves are regularly dropped from the lush growth even in the richest soil. Another popular view is that the leaves are shed because they are damaged, or to prevent shading of those lower down. But just look at a palm-tree. Instead of keeping a luxurious growth of photosynthetic tissue from ground to crown, the typical palm sheds leaves as it grows, retaining only a tuft of greenery at the very top. Even submerged aquatic angiosperms shed their leaves!
The Key Question
Why must this be? The answer is now emerging - a plant sheds its leaves as its excretory mechanism. We know that all living things must undergo excretion. It is one of the essential properties of all life. Yet the term excretion is missing from the index of most books on plant physiology.
This theory now provides the answer. The yellow leaves of the Fall are the plants seasonal chance to relieve itself of accumulated wastes: the leaf is an excretophore, as well as an energy trap. The colours are not merely due to the removal of chlorophyll, as the standard textbooks state; microscopical examination of autumnal leaves shows the extent to which the leaf cells are loaded with pigmented compounds prior to abscission. Levels of many toxic materials - like heavy metals - increase sharply as the yellowing proceeds. And metabolic rates rise, rather than fall, during this phase.
Not only does this concept explain one of the last great mysteries of plant life, but it offers a new way to purify contaminated soil. Specially planted metal-tolerant deciduous vegetation could lift poisons from damaged ground, and offer valuable metals for recycling as a result. An interview giving some of the background was published by Hart (infra) in 1998.
BJF, 1986, [preprint lecture summary] Excretion in Higher Plants, A Physiological Purpose For Abscission , Inter Micro 86, McCormick Center, Chicago, Illinois, July.
BJF, 1986, Even Plants Excrete, Nature, 323: 763, 30 October - 5 November.
Note also: letters re plant excretion theory in Nature:-
[Technology Correspondent, 1986, (telephoned interview) re Plant Excretion theory, Daily Telegraph, 1 November].
Stevenson, Nigel, 1986, Welsh Boffins Theory could rewrite Biology, Western Mail: 2, 1 November.
BJF, 1986, How Plants Excrete [abstract], The Microscope, 34 (3): 261.
BJF, 1986, A Physiological Purpose for Abscission, The Microscope, 34 (4): 329-330.
Simons, Paul, 1987, A Message in the Yellowing Leaves, Futures Column, The Guardian, 9 October.
Hart, Vanessa, 1998, My story [of the theory], Peterborough Evening Telegraph: 4, 28 November.
Report, 2006, Brian J Ford and asbscission on the curezone.com website, 11 July.
BJF, 2006, The Autumn Revolution, Biologist 53 (6): 300-304
Note: Many international centers are now utilizing
this process to bioremediate contaminated soils.
Search for PHYTOREMEDIATION to call up the latest web-sites in this growing field.