英语时差：枫糖浆到底是什么？（上）

If you take a spring walk through the woods in many parts of the Northeast, you’ll find buckets hanging from the sides of maple trees. Drop by drop, the buckets are filling up with the slightly sweet sap from the trees. When that sweet liquid is boiled down, it becomes maple syrup, all ready for a plate of pancakes. Colonists arriving from Europe learned from the American Indians how to tap maple trees and boil the sap into syrup. For a long time, maple syrup was the only concentrated sweetener available on this continent. Today, little is known for certain about why a sugar maple produces such sweet sap. The sugar in the sap is made in the leaves by photosynthesis — the same process that all green plants use to convert water, sunlight, and carbon dioxide into food. So the best year for maple syrup is one with the best conditions for photosynthesis — a sunny summer and fall with a late frost. A cold winter and a heavy snow fall to keep the roots cool in the spring also improve the syrup harvest.