You've probably heard that leaves of deciduous trees change color in autumn because they lose their green chlorophyll molecules,allowing the underlying yellow-orange pigments to shine through.However,this doesn't explain why the leaves change color in the first place.
Deciduous trees drop their leaves each year to avoid the high cost of winterizing them,but this seasonal shedding would be too wasteful to be worth doing if the tees lost all of the valuable nutrients they laboriously extracted from the soil to build their leaves earlier in the year.So each fall,deciduous trees recycle their leaves before dropping then.
That means taking apart cells and photosynthetic apparatus from the inside out in order to recoup their nitrogen and phosphorus and store it in twigs and branches until next spring.This is actually really tricky.because as the disassembly starts,the chlorophyll molecules that absorb sunlight for photosynthesis are still absorbing the sun's energy.
因此 每次落叶前落叶树会对树叶进行营养回收。也就是说会对树叶的细胞和光合器进行拆解回收氮和磷，并将它们储在树枝中等待来年春天。这过程非常巧妙 因为随着分解开始，叶绿素为了光合作用 吸收太阳能的过程依然在继续。
But with no photosynthesis happening,they end up passing the unused energy along to oxygen molecules,which become dangerously reactive when energized.These molecules wreak havoc,damaging the parts of the leaf that are in the process of recovering and transporting nutrients back to the rest of the tree.To keep this destruction to a minimum,leaves break down their chlorophyll into less dangerous molecules that are typically transparent,though sometimes yellow.
但是因为不再发生光合作用，它们将多余的能量给了氧分子，此时氧分子一旦被激发会发生危险反应。这些氧分子横行肆虐，破坏了树叶中正在进行回收将剩余营养盐输送回树木的那部分组织。为了将破坏将至最低，树叶关闭叶绿素功能 使之变成低危险的分子，这些分子通常是透明的 也有些是黄色的。
Either way,with the bright green molecules gone,yellow and orange pigments that were there all along (helping with photosynthesis),are no longer overshadowed,and ta-ta!Yellow and orange leaves.Some trees take an extra autumnal precaution against chlorophyll-induced-destruction:as the leaf dismantling starts they build newspecial pigments to shade chlorophyll from sunlight until it can be broken down.
无论如何 随着绿色分子的消失，黄色和橙色的色素却一直都在（协助进行光合作用）叶子不再褪色 当当当当！黄色和橙色的树叶。有些树采取了额外的秋季防御措施 避免叶绿素造成的危害：随着树叶开始分解 它们会形成新的特殊色素 遮挡叶绿素 防止其吸收阳光 直至叶绿素被分解。
These new pigments tend to be red or purple in color,so trees that use them have red leaves in the fall-sometimes insanely bright red.In the end,these exquisite gold and russet displays help deciduous trees recover as much as 50% of the nitrogen and phosphorus from their old leaves to help grow fresh new green ones in the spring.They are perhaps the world's prettiest recycling plants.
这类新色素通常是红色或紫色的，因此使用它们的树木 落叶是红色的——有时会是鲜红色。最后 这些漂亮的金色和黄褐色叶片 帮助树木从旧叶中回收50%的氮和磷令大树可以在新春焕发新绿。它们大概是世界上最漂亮的回收工。