A One of the most captivating natural events of the year in many areas throughout North America is the turning of the leaves in the fall. A 在北美的大部分地区，最美的自然景色之一就是秋天的树叶换装之景。 The colours are magnificent, but the question of exactly why some trees turn yellow or orange, and others red or purple, is something which has long puzzled scientists. 颜色之壮观令人叹服，但是，为什么有些树是变成黄色或者橙色，而有些树则是变成红色或者紫色这是长期困扰科学家的一个问题。

B Summer leaves are green because they are full of chlorophyll, the molecule that captures sunlight and converts that energy into new building materials for the tree. B夏天，树叶是绿色的，因为里面富含叶绿素。叶绿素是吸收阳光并把它的能量转化为树木需要的其他物质的分子。 As fall approaches in the northern hemisphere, the amount of solar energy available declines considerably. 在北半球，随着秋天的临近，能获得的太阳能总量也大大降低。 For many trees —­ evergreen conifers being an exception -the best strategy is to abandon photosynthesis* until the spring. 对于很多树来说——长青的松树是个例外——最好的策略是在春天来临之前放弃光合作用※ 。 So rather than maintaining the now redundant leaves throughout the winter, the tree saves its precious resources and discards them. 所以，树木为了保存它们宝贵的资源就放弃了树叶，而不是整个冬天都保留着茂盛的叶子。 But before letting its leaves go, the tree dismantles their chlorophyll molecules and ships their valuable nitrogen back into the twigs. 但是在树叶掉落之前，树木先分散他们的叶绿素成分，并把它们中重要的氮物质返还到树枝上。 As chlorophyll is depleted, other colours that have been dominated by it throughout the summer begin to be revealed. 随着叶绿素的消失，主导整个夏天的绿色也就跟着消失了，而取而代之的是其他的颜色。 This unmasking explains the autumn colours of yellow and orange, but not the brilliant reds and purples of trees such as the maple or sumac. 这一结果解释了秋天黄色和橙色的原因，但没有解释像枫叶或漆树变成亮红色或紫色的原因。 * photosynthesis: the production of new material from sunlight, water and carbon dioxide ※ 光合作用：阳光、水和二氧化碳产生新物质的作用

C The source of the red is widely known: it is created by anthocyanins, water-soluble plant pigments reflecting the red to blue range of the visible spectrum. C红色的来源众所周知：它是源自花青素，即呈现从红色到蓝色可见光谱的水溶解植物色素。 They belong to a class of sugar-based chemical compounds also known as flavonoids. 它们属于糖基化合物的一种，又叫黄酮类化学物。 What’s puzzling is that anthocyanins are actually newly minted, made in the leaves at the same time as the tree is preparing to drop them. 让人疑惑的是，花青素实际上树里原来没有，是在树木想要落叶时才产生的物质。 But it is hard to make sense of the manufacture of anthocyanins—why should a tree bother making new chemicals in its leaves when it's already scrambling to withdraw and preserve the ones already there? 但是，花青素是怎么形成的还很难说清楚——为什么树木在赶着落叶并保存自己的能量的时候还费劲制造一种新物质呢？

D Some theories about anthocyanins have argued that they might act as a chemical defence against attacks by insects or fungi, or that they might attract fruit-eating birds or increase a leaf’s tolerance to freezing. D 有些关于花青素的理论认为花青素是用来抵抗昆虫或菌类物质侵袭的化学物质，或者认为它们可能是用来吸引吃果实的小鸟或者提升树木抗寒能力的物质。 However there are problems with each of these theories, including the fact that leaves are red for such a relatively short period that the expense of energy needed to manufacture the anthocyanins would outweigh any anti-fungal or anti-herbivore activity achieved. 然而，这些理论都有些问题，其中，在这样短的时间里树叶变成红色所需的花青素所耗费的能量超出了抗菌或抗虫运动了。

E It has also been proposed that trees may produce vivid red colours to convince herbivorous insects that they are healthy and robust and would be easily able to mount chemical defences against infestation. E也有些研究认为树木生产鲜活的红色是想告诉食草昆虫：我们是很健康和强壮的，会很容易积聚化学制剂抵御你们的侵袭的。 If insects paid attention lo such advertisements, they might be prompted to lay their eggs on a duller, and presumably less resistant host. 如果昆虫注意到这类警示，它们可能会转到颜色更暗淡的树上下蛋，进而就不会那么骚扰红色树木了。 The flaw in this theory lies in the lack of proof to support it. 这个理论的缺陷是缺少证据证明它的可靠性， No one has as yet ascertained whether more robust trees sport the brightest leaves, or whether insects make choices according to colour intensity. 目前还没有人确信更健康的树就能生出更亮的树叶或者昆虫就喜欢颜色更深的树。

F Perhaps the most plausible suggestion as to why leaves would go to the trouble of making anthocyanins when they’re busy packing up for the winter is the theory known as the ‘light screen’ hypothesis. F这些理论中，貌似最说得通的就是“遮光板”假说，它很好地解释了它们为什么在忙着准备过冬时还不嫌麻烦地生产花青素。 It sounds paradoxical, because the idea behind this hypothesis is that the red pigment is made in autumn leaves to protect chlorophyll, the light-absorbing chemical, from too much light. 这个理论听起来自相矛盾，因为这个假说后面的理论是秋天树叶里生产红色素是为了保护叶绿素，使这一吸光的化学物质不受过多光的照射。 Why does chlorophyll need protection when it is the natural world’s supreme light absorber? 那么既然叶绿素是自然界超强的吸光器，为什么还需要保护它呢？ Why protect chlorophyll at a time when the tree is breaking it down to salvage as much of it as possible? 为什么树在一方面放弃叶绿素的时候又一方面在尽力保护、抢救叶绿素呢？

G Chlorophyll, although exquisitely evolved to capture the energy of sunlight, can sometimes be overwhelmed by it, especially in situations of drought, low temperatures, or nutrient deficiency. G 叶绿素虽然吸收光线的能量特别灵敏，但是它有时会被光线控制，尤其是在干旱、低温或者营养不足的情况下。 Moreover, the problem of oversensitivity to light is even more acute in the fall, when the leaf is busy preparing for winter by dismantling its internal machinery. 而且，在秋季，当树叶忙着打散内部机制为过冬准备时，叶绿素对光线过度敏感的问题更加严重。 The energy absorbed by the chlorophyll molecules of the unstable autumn leaf is not immediately channeled into useful products and processes, as it would be in an intact summer leaf. 秋天不稳定的叶子的叶绿素分子吸收的能量没有马上运送至有用的物质或者加工过程中，因为它可能是在夏天有缺陷的树叶里。 The weakened fall leaf then becomes vulnerable to the highly destructive effects of the oxygen created by the excited chlorophyll molecules. 秋天变弱的树叶然后变得对活跃的叶绿素分子创造的强破坏性的氧化作用非常脆弱。

H Even if you had never suspected that this is what was going on when leaves turn red, there are clues out there. H即使你从来没想过这是叶子变红时的内部变化，线索就在那里。 One is straightforward: on many trees, the leaves that are the reddest are those on the side of the tree which gets most sun. 一个原因是直接的：对于多数树木来说，叶子最红的那面是接触阳光最多的那面。 Not only that, but the red is brighter on the upper side of the leaf. 不仅如此，叶子的上部的红色要更亮一些。 It has also been recognised for decades that the best conditions for intense red colours are dry, sunny days and cool nights, conditions that nicely match those that make leaves susceptible to excess light. 还有一点被公认是这样的：干燥、有阳光的白天和凉爽的夜晚是最利于红色养成的条件，这种条件使叶子很好地适应光过多的情况。 And finally, trees such as maples usually get much redder the more north you travel in the northern hemisphere. 实际上，像枫树这类树，你越往北半球走它越红。 It’s colder there, they're more stressed, their chlorophyll is more sensitive and it needs more sunblock. 那边越冷，它们越有压力，它们的叶绿素就越敏感，就越需要遮挡太阳的物质。

I What is still not fully understood, however, is why some trees resort to producing red pigments while others don't bother, and simply reveal their orange or yellow hues. I然而，到目前为止还没有被充分理解的是为什么有些树偏向于创造红色素，而其他树就不是，只是产生橙色或黄色。 Do these trees have other means at their disposal to prevent overexposure to light in autumn? 这些树还有别的方式可以让自己在秋天避免过度接触光吗？ Their story, though not as spectacular to the eye, will surely turn out to be as subtle and as complex. 它们的故事虽然不像它们的颜色变化那样壮观，但是必定会呈现同样的微妙和同样的复杂。