Guest Blogger:  Diane Ackerman from A Natural History of the Senses.  A delightfully fine book!

The stealth of autumn catches one unaware.  Was that a goldfinch perching in the late September woods, or the first turning leaf?  Early-morning frost sits heavily on the grass and turns barbed wire into a string of stars.  On a distant hill, a small square of yellow appears to be a lighted stage.  At last the truth dawns on us: Fall is staggering in, right on schedule, with its baggage of chilly nights.

Soon the leaves will start cringing on the trees, and roll up in clenched fists before they actually fall off.  Dry seedpods will rattle like tiny gourds, but first there will be weeks of gushing color so bright, so pastel, so confettilike, that people will travel up and down the East Coast just to stare at it — a whole season of leaves.

Where do the colors come from?  Sunlight rules most living things with its golden edicts.  When the days begin to shorten, soon after the summer solstice on June 21, a tree reconsiders its leaves.  All summer it feeds tham so they can process sunlight, but in the dog days of summer, the tree begins pulling nutrients back into its trunk and roots, pares down, and gradually chokes off its leaves.  A corky layer of cells forms at the leaves’ slender petioles, then scars over.  Undernourished, the leaves stop producing the pigment chlorophyll, and photosynthesis ceases.  Animals can migrate, hibernate, or store food to prepare for winter.  But where can a tree go?  It survives by dropping its leaves, and by the end of autumn only a few fragile threads of fluid-carrying xylem hold leaves to their stems.

An odd feature of the colors is that they don’t seem to have any special purpose.  We are predisposed to respond to their beauty.  They shimmer with the colors of sunset, spring flowers, the tawny buff of a colt’s pretty rump, the shuddering pink of a blush.  There is no adaptive reason for leaves to color so beautifully in the fall anymore than there is for the sky or ocean to be blue. ~