It is autumn in the northern hemisphere and nowhere are the leaves more beautiful than in the deciduous forests of the northeastern United States. A whole travel and tourism industry is based on viewing the colorful leaves of New England. Just type in “fall foliage tours” into Google and you will see what I mean.
However, many people do not know how and why leaves fall and change colors. The story behind fall foliage is one of the most interesting wonders of the natural world. Let’s start with falling leaves. In the fall when the days get shorter and colder, a hormone is release that tells the plant to begin cutting the leaf where it joins the branch. This area is called the abscission zone. Abscission cells appear in this area and begin to literally cut the leaf off from the tree. The word abscission (from the Latin ab meaning away and scindere meaning to cut) has the same root word as the word scissors, and the main job of the abscission cells is to cut (through the stem of the leaf). The wind is just a helper that finishes the job of the making the leaves fall. The tree has to shed its leaves, because if it does not, the water in the leaves will freeze and the tree will die during the winter. To read and hear more about this phenomena, listen to an October 30, 2009 NPR interview on “All Things Considered.” http://www.npr.org/templates/story/story.php?storyId=114288700 As for the colors, when the days get shorter and shorter, a trigger is send throughout the tree to stop the production of glucose (the food produced by the tree). Since glucose production has stopped, the chlorophyll—the green pigment in the leaves—stops being produced. The tree does not produce food during the winter because there is not enough light and water for photosynthesis. When this happens the green color fades away, leaving yellow, orange colors of the leaves. These colors were always there in the leaf, they were just masked by the green in the leaf. The purple and red colors we see are a result of leftover glucose responding to cooler temperatures at night. The browns we see are the result of waste materials left in the leaves. Click here to listen to Edith Piaf singing Autum Leaves