Other than in April, when landscapes parade dozens of shades of green before us, it is September when we most notice plant pigments. The fact is these two conditions, spring's greens and autumn's golds, are related to each other.

Looking through a microscope at a plant leaf cell, we can see the tiny, green bodies that make the entire leaf green. If we were to look at a similar cell now, in September, we would see little yellow, orange and red bodies, but no green ones. The green structures, we call them plastids, are gone and have been replaced by more colorful plastids. Actually, the green...