Some scientific information on how leaf prints occur: 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.
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