【英语时差8,16】落叶美景从何而来（上）（在线收听）

As the dog days of summer trot into autumn, you might start thinking about one of nature’s most spectacular shows–the annual fall foliage display that can break out like brilliant slow-motion fireworks in woodlands around the country. Let’s face it though, not all fall leaf displays are created equal. While some autumns explode like the Fourth of July, others fizzle like a wet roman candle. What Makes Some Year’s Leaf Displays Better Than Others? Actually, a few different processes work together to make an autumn leaf display. As daylight shortens at the end of summer, a layer of cells grows at the base of each leaf stem, cutting off the water supply like a cork. Along with other metabolic changes, this stops production of green chlorophyll in the leaf. When the chlorophyll breaks down, it unmasks yellow or orange pigments that have been present in the leaf all summer, but only now become visible.