【英语时差8,16】落叶美景从何而来（下）（在线收听）

For leaves that turn red or purplish, a different process occurs. Reddish pigments are manufactured in the leaf as summer ends, created from sugars that are trapped in the leaf by that same cork-like growth of cells. Cool late-summer nights work best for retaining these sugars in the leaves, and sunny late- summer days are best for converting the sugar to pigment. Finally, some leaves like oaks have rich brownish pigments, due to tannins in the leaves. Since the most spectacular autumn displays will combine all these colors, you’ll want sunny days and cool nights in the late summer for the most spectacular natural fireworks in the fall.