It's a sure sign that summer is over and winter is on the way -- leaves that were once deep green turn red, yellow and bright orange. It happens every autumn. But did you ever wonder why? The leaves start changing color when the nights start getting longer and the temperature starts getting cooler. Some scientists think that as the number of daylight hours _ the leaves stop making chlorophyll which makes plants green. The reason is that the process of making chlorophyll requires sunlight. But according to horticulture   educator Susan Rose from Colorado State University, it turns out leaves don't really change color at all. "The fall colors are actually there all along," she said. "But they are covered by the green chlorophyll. As the chlorophyll stops being produced, the other colors can shine through." The kind of color that the leaves are going to change to is determined by the plant's genetic   background. In some places, the leaves start changing color in September -- before autumn even officially begins. In other places, they don't change until late October or even November. Some scientists say the leaves have been changing color later than usual in recent years. And they think global warming has something to do with that. In one study, researchers found that the growing season in the Northern Hemisphere was nearly a week longer, on average, in 2008 than it was in 1982. And a professor from Harvard University found that the leaves, west of Boston, Massachusetts, are changing color about three days later than they used to. However, other scientists disagree with the global warming theory. Susan Rose said the leaves in her part of Colorado are changing "right on schedule." But the leaves in other parts of that state are running a little late. "The lateness in the mountains may have something to do with the really warm late summer and early fall," she said. "But I'm afraid that's just a guess." Scientists guess leaves change color later than usual probably as a result of   _  .
genetic background dry weather global warming low temperature