Forest Preserve District of Cook County, Illinois
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Jack Frost
Nature Bulletin No. 124   September 27, 1947
Forest Preserve District of Cook County
William N. Erickson, President
Roberts Mann, Supt. of Conservation

Jack Frost is here. Late in August the sumacs began to change from green to crimson. Some years, in October the woods are painted red, yellow, purple and brown, each kind of tree and shrub having its distinctive colors. Other years the foliage is dull. In Chicago itself, as in Europe, there is never much autumn color. Why .

Temperature is one factor. All summer, new chlorophyll -- green pigment -- is being formed in leaves. With cooler weather all the vital processes in plants slow down. The green quickly fades and then the other substances in leaves begin to show their own colors. Three substances are chiefly responsible for autumn coloration: the carotenes are pigments which vary from pale yellow to orange red: the xantholphylls are yellow; and the anthocyanins vary from red to blue.

Sunlight is another factor. Its intensity is reduced in cities by smoke and dust particles in the air. In the country, the brilliant sunlight of clear fall days causes the green chlorophyll to quickly fade and the reds and yellows to become more brilliant. Moisture is a third factor. In very dry autumns there is little red, the yellows are subdued, and the browns predominate. A cool moist autumn produces brighter foliage than a warm dry season.

The fourth factor is the nature of the plant itself. Elm, poplars, sycamores and soft maples :How little color. White oaks turn dark red or purple; red oaks to red or reddish brown; hickories and beeches to golden yellow; and sassafras to a brilliant saffron. Sour gum, sweet gum, pin oak, sumac and maple-leaved virburnum are gone of the trees and shrubs of magnificent color in fall, and the hard maples are famous for their flaming crimsons and yellows.

England and Europe simply do not have such a wide variety of trees that produce brilliant fall foliage, and there may be other factors such as milder weather and less intensity of sunlight. In our cities the shade trees are principally elms and other species which do not possess bright pigments, and the sunlight is less intense. So take a walk in the country, these sunny fall days.

And who was Kilroy? Jack Frost wants to know.

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