The changing natural world at our doorsteps | Illustration and text by Patterson Clark
November 16, 2010
Shadows of decay
Freshly poured this past summer, concrete now serves as a printing surface for fallen leaves
As autumn progresses, deciduous trees dismantle the chlorophyll molecules in their leaves to retrieve valuable nitrogen and magnesium ions. Those precious components are ferried through leaf stalks, down through inner bark tissues and into the roots for winter storage. Other molecules, however, aren't worth a recovery effort, so they're left aboard and go down with the leaf.
Tannins are among the chemical casualties. In a living leaf, they may be present in surface wax or inside cell vacuoles, which are microscopic water
balloons that isolate tannins from the cell's protoplasm.
Tannins often serve as part of a plant's chemical arsenal. Unless an animal has neutralizing compounds in its digestive system, its attempt to chew or digest a tannin-laden leaf may be a bitter or sickening experience. (Milder tannins impart desirable flavors to fruits, nuts, tea, wine and chocolate.)
When leaves begin to decay, cells rupture and tannins flow among the wreckage. Rain plastering a
fallen leaf to the sidewalk will soak through the leaf's collapsed tissues, transporting tannins into the concrete, where they're held as a crisp, brown stain.
The sun emerges, a gust of wind carries off the leaf, a tannin print remains -- until it runs and blurs in the next autumn rain.Sources: Cornell University Department of Animal Science; David Hershey; Ross Koning, Eastern Connecticut State University; Harold McGee, "On food and cooking: the science and lore of the kitchen"