Color change in leaves

Autumn leaf color is a phenomenon that affects the normally green leaves of many deciduous trees and shrubs by which they take on, during a few weeks in the autumn season, various shades of red, yellow, purple, black, orange, pink, magenta, blue, and brown. The phenomenon is commonly called autumn colours or autumn foliage in British English and fall colors,fall foliage, or simply foliage in American English.

In some areas of Canada and the United States, "leaf peeping" tourism is a major contribution to economic activity. This tourist activity occurs between the beginning of color changes and the onset of leaf fall, usually around September and October in the Northern Hemisphere and April to May in the Southern Hemisphere.

A green leaf is green because of the presence of a pigment known as chlorophyll, which is inside an organelle called a chloroplast. When they are abundant in the leaf's cells, as they are during the growing season, the chlorophyll's green color dominates and masks out the colors of any other pigments that may be present in the leaf. Thus, the leaves of summer are characteristically green.

Chlorophyll has a vital function: it captures solar rays and uses the resulting energy in the manufacture of the plant's food — simple sugars which are produced from water and carbon dioxide. These sugars are the basis of the plant's nourishment — the sole source of the carbohydrates needed for growth and development. In their food-manufacturing process, the chlorophylls breaks down, thus are being continually "used up". During the growing season, however, the plant replenishes the chlorophyll so that the supply remains high and the leaves stay green.

In late summer, as daylight hours shorten and temperatures cool, the veins that carry fluids into and out of the leaf are gradually closed off as a layer of special cork cells forms at the base of each leaf. As this cork layer develops, water and mineral intake into the leaf is reduced, slowly at first, and then more rapidly. During this time, the chlorophyll begins to decrease. Often, the veins are still green after the tissues between them have almost completely changed color.

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