Do plants excrete?

April 19, 2002

Dear Cecil:

Does fresh produce eliminate? In other words, does your lettuce continue to breathe, process oxygen, and produce waste products? I have often noticed a bitter, for lack of a better word, organic chemical taste on lettuce, apples, and other produce. The appearance and relative strength of this taste appears to correspond with the length of storage time, etc. I've always assumed that this is the result of (relatively--can't be good for you) harmless waste produced by the living plant and that the only solution (which always works) is to rinse the produce thoroughly. This debate was touched off by a visit to my Mum's house--bitter lettuce in a salad, which I proceeded to wash, was asked why, gave explanation as per above, was given lecture on the One True Virtue of Iceberg Lettuce, that it doesn't have to be washed as it doesn't contain any sand. Am I hallucinating here?

Once again you're asking yourself: Why is Cecil answering this question, the lameness of which ranks up there with . . . well, frankly, I think we have to exit the Cecilian oeuvre altogether and delve into "Hints From Heloise." Official answer: Because there are scientifically interesting aspects to this question. Real answer: Because I can't wait to see Slug's illustration.

All right, Steve, back to you. The question you think you're asking is whether vegetables eliminate. The question you're actually asking is whether dead vegetables eliminate. Answer: no. Dead vegetables rot. Granted, neither process makes for the best visual, but there's a fundamental philosophical difference between "Socrates is in the next room going to the bathroom" and "Socrates is in the next room decomposing."

Some metabolic processes continue in dead vegetables, such as respiration, conversion of starch to sugar (in potatoes), etc. However, these processes are pretty much a yawn. Let's stick with elimination. Do living vegetables or, to put it more broadly, living plants excrete? Not necessarily in the sense that animals do. But like all other organisms they process certain inputs and produce certain outputs, some of which we may characterize as waste. Figuring out where the waste went puzzled some early naturalists. In the late 18th century, the pioneering naturalist Erasmus Darwin (grandfather of Charles) wrote, "Others have believed [the leaves to be] excretory organs of excrementitious juices, but as the vapor exhaled from vegetables has no taste, this idea is [not] probable." But there may be something to the notion that leaves perform an excretory function, as we shall see.

Eventually biologists learned that plant cells have internal sacs called vacuoles that are used for storage, and that a lot of what gets stored is waste--vacuoles can account for as much as 90 percent of cell volume in a mature plant. But the waste is being stored on site, so to speak. Some scientists claim that heartwood formation enables trees and other woody plants to eliminate toxic byproducts, and that the great trunk of an oak should properly be understood as a monumental form of defecation. Heartwood also helps hold the tree up, so you'll get some argument about whether it truly constitutes waste. Similarly, some plants deposit toxic compounds in the soil, but the counterargument is that they do this to ward off pests.

An even more piquant proposition was advanced in a 1986 letter in Nature by British biologist Brian Ford: "I postulate that leaf fall, abscission, is the mechanism of excretion in vascular plants. . . . The classical reason given for leaf fall is that it obviates damage to a plant during winter. The evergreen species demonstrate that this is not obligatory" (see www.sciences.demon.co.uk/wleaf03.htm). Evergreens do drop their needles; they just do so continuously rather than all at once like deciduous plants. In Ford's view, the bright colors of autumn are merely the outward signs of a process in which the leaf "is systematically stripped of its vital constituents and charged with metabolic waste materials." I don't know that this notion is universally accepted, but it puts a fresh perspective on raking the leaves (or "excretophores," as Ford calls them). Trees provide us with shade, fruit, wood, and other good things; isn't it only fair that we clean up after them when they take a dump?

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