【英语时差8,16】秋季落叶的奥秘-下（在线收听）

 Because chlorophyll is the chemical responsible for making leaves green, its absence allows chemicals such as carotinoids, xanthophylls, and others to give leaves their orange, yellow, and various other colors. As trees stop producing chlorophyll, they start producing sugars and amino acids that act as a sort of antifreeze to protect plants from the winter cold. Meanwhile, nutrients drain from the leaves, through the stems, and eventually into the roots, where they remain for the winter. When the nutrients have left the leaves, all that remain are cell walls and nutrient-depleted protoplasm. Since all that now holds leaves to a tree are brittle, dry, and easily broken veins, a gust of wind is enough to tear leaves from the tree and send them fluttering to the ground. When dead leaves break away, they leave behind waxy cells that form a small bud called a bundle scar. When warm weather returns, each scar will give rise to a newly born leaf.