UCSB Science Line
 I know that leaves are green, but they turn yellow and red in fall. I think they are drying up, but I don't know why. What happens to leaves ? Question Date: 2001-11-06 Answer 1:Leaves actually have several different colors in them. We call them pigments. The green pigment is called chlorophyll. It allows plants to turn the sun's energy into a type of energy they can use. Other pigments can do this too, but chlorophyll is the main one. Chlorophyll is extremely important to a tree, but it is also very expensive (in terms of energy) for the tree to make. So when the tree is about to drop its leaves, it stops making new chlorophyll. As the old chlorophyll is broken down, the green fades. Other pigments are left in the leaves. Of course in Santa Barbara, not all plants lose their leaves in the Fall. Even here in Wisconsin, pines, firs, and other "evergreens" don't. Does this tell you why there are pine trees high up on mountains where you would never see a eucalyptus or an oak? For some great pictures of Autumn colors here in Wisconsin, check out this site. This site also explains the chemistry of changing leaf color. Answer 2:Do you know what makes leaves green? What makes them yellow or red? What makes paint or clothing green or yellow or red? I wonder what you would find if you weighed 10 green leaves, one at a time, and 10 yellow or red leaves and 10 brown leaves from the same tree. If you calculated the average weight of a green leaf, a red or yellow leaf, and a brown leaf, do you think they would be the same? If I were doing it, I'd want to try to find 30 leaves (10 of each color) that were all about the same size. Or, I guess, I could try to find 3 leaves, 1 of each color, where all 3 leaves were the same size, and weigh each one, and then find 3 more leaves and so on, and see which color leaf was the heaviest and the lightest for each group of three. That would make a nice chart. I've heard that leaves turn yellow or red when their green pigment disappears - so they always have red and yellow pigments, but it's just hidden by the green. I don't know if that's true. Trees must have evolved to lose their leaves in winter because then they were able to live through the cold weather without having their delicate leaves freeze. And the water in the tree moves down towards the trunk and roots to protect the small branches from freezing, too.Click Here to return to the search form.