A touch of color is starting to show on top the cottonwoods, and like sand in an hour glass, it will drift slowly down through the branches in the coming weeks until the trees are engulfed in a cascade of shimmering yellow.

Nothing speaks more eloquently of autumn than painted leaves. This striking display of color is brought about every year, not by cold weather, but by shrinking day-length. The chief cause of the change is time itself.

Sometime around the autumnal equinox, sap flow to the leaf shuts down and chlorophyll begins to dissipate. As the green chlorophyll fades, pigments in the leaf that were masked by the chlorophyll begin to show through. Carotenoids paint the leaves gold and yellow, anthocyanin red and purple.

Every species is genetically tuned to respond to a certain length of daylight, causing it to turn color around the same time every year. Sumac starts as early as August, while some oak trees stay green through the end of October.

But not all trees of the same species turn color at the same time. We have three rebellious burr oaks in our yard that insist on doing things their own way, the first one will be losing its leaves before the last one even begins to turn color.

The burr oaks stick to the same schedule when closing up shop in the fall, but strangely enough, the order is not reversed in spring with the budding of leaves, as one might expect.

No two autumns are alike, because the brilliance of the foliage is affected by drought, temperature and soil conditions. The best color seems to occur after a succession of warm sunny days and cool crisp nights. Under these conditions, more sugar gets trapped in the leaf and it spurs the production of anthocyanins, creating brighter reds and purples.

Color plays an important role in nature. Carotene and xanthophyll produce the brilliant oranges and yellows in flowers, which attracts a host of different pollinators. Though these substances have been put to work in aiding photosynthesis, trees have apparently found little use for their color.

Anthocyanin, made by “aging” sap, is a known repellant of herbivores and parasites; it reduces competition by stunting the growth of nearby saplings. But what use the sudden and brief burst of color serves in the dying leaf is uncertain.

It is characteristic of nature to eventually find some purpose for the useless, and with respect to the painted leaves, she seems to have achieved that goal with a little help from us. Fall foliage is now worth billions to America’s economy, particularly in New England, and every autumn people gravitate to the color like bees to a flower, for no other reason than to enjoy its beauty.

Nature benefits from her attractiveness in that people are more inclined to value, and to plant, trees in their yard that have beautiful fall colors, thereby promoting their conservation.

“Painted leaves” is an epithet of autumn found in the works of many poets, from Bryant to Longfellow and Thoreau. The expression most fittingly describes the fall foliage in America, where the deciduous species far outnumber those in Europe and the color is unmatched in splendor and magnificence.

A few years ago, we made a road trip back East to observe the fall foliage.

Driving north from Connecticut to the White Mountains of New Hampshire, we found the trees painted in a riot of color, the bright scarlet of the red oaks and the orange-red of the sugar maples standing out like exclamation marks in the rolling panoply of autumn tints.

The trees are less abundant here on the plains, but the colors are just as bright and intense. You can see it in the crimson and purple-red of sumac, the blazing red and orange of Virginia creeper, the bronze and gold of ash trees, the brilliant yellow of cottonwood and the deep orange of honey locust.

The color reaches its peak in October, often around the time of Indian Summer, that pleasant stretch of days when the winds calm down, the sky turns pristine blue and the earth itself seems suspended in a breathless pause.

It’s a good time for us to pause as well, in response to nature’s cue, and to go out and take a quiet stroll through the world of painted leaves.

 Richard Weber is a nature enthusiast living in Ellis County.