【英语时差8,16】秋季落叶的奥秘-上（在线收听）

 Every autumn, citizens of the North American Midwest and Northeast enjoy a spectacularly colorful display as leaves turn various shades of red, orange, yellow, purple, and brown. But autumn also entails the backbreaking work of raking all those beautiful leaves once they fall. Why can’t leaves change colors, stay on their trees, and change back to green again when spring comes? Unfortunately, the chemical processes responsible for changing leaf color also make leaves fall. Trees prepare for their dormant winter period by undergoing senescence. Senescence involves recycling nutrients from leaves into the plant body for use in the Spring. As trees enter dormancy, they cut back on chlorophyll production.