劍橋雅思10Test3閱讀Passage2原文翻譯 Autumn leaves

劍橋雅思10Test3閱讀Passage2原文翻譯 Autumn leaves

劍橋雅思10 Test3 Passage2閱讀原文翻譯

A部分

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. 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.

在北美許多地區(qū)，一年之中最引人注目的自然現(xiàn)象之一就是秋天里樹葉的變化。其顏色絢麗奪目。但究竟為什么一些樹木變成黃色或橙色，而其他卻變成紅色或紫色？這一問題長期以來都困擾著科學(xué)家。

B部分

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. As fall approaches in the northern hemisphere, the amount of solar energy available declines considerably. For many trees —- evergreen conifers being an exception -the best strategy is to abandon photosynthesis* until the spring. So rather than maintaining the now redundant leaves throughout the winter, the tree saves its precious resources and discards them. But before letting its leaves go, the tree dismantles their chlorophyll molecules and ships their valuable nitrogen back into the twigs. As chlorophyll is depleted, other colours that have been dominated by it throughout the summer begin to be revealed. 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.

夏天的葉子是綠色的，因為它們充滿了葉綠素。葉綠素是一種捕獲陽光并將該能量轉(zhuǎn)化為新的樹木增長材料的分子。隨著北半球秋天的到來，可利用的太陽能數(shù)量大大減少。對于許多樹木來說- 常綠針葉樹除外-最佳策略是放棄光合作用直到春天來臨。因此，與其保留冬季里十分多余的葉子，樹木選擇節(jié)省其寶貴的資源而拋棄它們。但在棄置這些葉子之前，樹木會分解其中的葉綠素分子，并將里面含有的寶貴的氮元素運回枝干。隨著葉綠素的枯竭，夏季里被它主導(dǎo)的顏色開始顯現(xiàn)出來。這一顯露過程解釋了秋天里出現(xiàn)的黃色和橙色，但卻無法解釋諸如楓樹或漆樹之類的樹木所呈現(xiàn)的艷麗的紅色和紫色。

C部分

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. They belong to a class of sugar-based chemical compounds also known as flavonoids. 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. 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?

紅色的來源廣為人知：它是由花青素（一種可反射可見光譜中紅色到藍色范圍的水溶性植物色素）產(chǎn)生的。它們屬于一類糖基化合物，也被稱為類黃酮。令人費解的是，花青素實際上是新合成的，是在樹木準備將樹葉丟棄的同時在里面產(chǎn)生的。但是，很難理解合成花青素的意義-為什么樹木在急于收回并保存原本存在于樹葉中的化學(xué)物質(zhì)時，還要在里面再制造出新的來呢？

D部分

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. 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.

一些有關(guān)花青素的理論認為，它們可能是作為抵御昆蟲或真菌攻擊的化學(xué)防御，或者說，它們可能會吸引以果實維生的鳥類或者提升樹葉對寒冷的耐受性。但是，這些理論都存在問題，例如樹葉變紅的時間如此之短，以至于制造花青素所需要耗費的能量會超過任何抗真菌或防食草動物行為所取得的收益。

E部分

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. If insects paid attention to such advertisements, they might be prompted to lay their eggs on a duller, and presumably less resistant host. The flaw in this theory lies in the lack of proof to support it. No one has as yet ascertained whether more robust trees sport the brightest leaves, or whether insects make choices according to colour intensity.

還有一種理論認為，樹木產(chǎn)生鮮艷的紅色是為了讓那些食草昆蟲相信，它們十分健康強壯，能夠輕松的發(fā)起抵御侵擾的化學(xué)防御。如果昆蟲會注意到這些現(xiàn)象，它們可能會將卵產(chǎn)在顏色更為暗淡，不太可能進行抵抗的宿主上。這一理論的缺陷在于缺乏證據(jù)支持。尚無人能夠確定，更健壯的樹木是否會產(chǎn)生最鮮艷的葉子，又或者昆蟲是否根據(jù)顏色強度來進行選擇。

F部分

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. 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. Why does chlorophyll need protection when it is the natural world’s supreme light absorber? Why protect chlorophyll at a time when the tree is breaking it down to salvage as much of it as possible?

樹葉為什么會在忙于為冬季做準備的時候還不嫌麻煩的生產(chǎn)葉綠素。這一現(xiàn)象最合理的解釋是一種被稱為“光屏障”的假說。它聽起來有些自相矛盾。因為這種假說背后的想法是，秋天里樹葉產(chǎn)生的紅色素是為了防止葉綠素這種吸光物質(zhì)吸收太多的陽光。為什么葉綠素作為自然界的吸光能手還需要保護呢？為什么要在樹木分解葉綠素以拯救盡可能多的營養(yǎng)物質(zhì)時去保護它呢？

G部分

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. 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. 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. The weakened fall leaf then becomes vulnerable to the highly destructive effects of the oxygen created by the excited chlorophyll molecules.

雖然葉綠素經(jīng)過精巧的演變來捕捉太陽光中的能量，但它有時也會被過多的陽光所傷害，尤其是在干旱、低溫、或者營養(yǎng)缺乏的時候。此外，秋天里，當樹葉正忙于拆解其內(nèi)部結(jié)構(gòu)為冬天做準備的時候，對陽光過度敏感的問題會更加嚴重。不穩(wěn)定的秋葉中，葉綠素分子所吸收的能量并不會被立刻輸送到有用的產(chǎn)品或過程中，就像夏天里完好無損的葉子里所發(fā)生的那樣。這時候，衰弱的落葉十分容易受到活躍的葉綠素分子所產(chǎn)生的氧氣的破壞。

H部分

Even if you had never suspected that this is what was going on when leaves turn red, there are clues out there. One is straightforward: on many trees, the leaves that are the reddest are those on the side of the tree which gets most sun. Not only that, but the red is brighter on the upper side of the leaf. 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. And finally, trees such as maples usually get much redder the more north you travel in the northern hemisphere. It’s colder there, they’re more stressed, their chlorophyll is more sensitive and it needs more sunblock.

即使你從未懷疑過這就是樹葉變紅時所發(fā)生的事情，也仍然存在許多提示線索。最簡單明了的一條是：在許多樹木上，最紅的葉子出現(xiàn)在光照最多的那一側(cè)。不僅如此，葉子上部的紅色會更加鮮亮。幾十年來，如下事實都廣為人知，產(chǎn)生鮮艷紅色的最佳條件為干燥、陽光充足的白天和寒冷的夜晚。這些條件恰好與那些使葉子容易遭受過度陽光損害的條件相匹配。最后，在北半球越往北去，楓樹之類的樹木就會變得越紅。那里更加寒冷，它們要承受更多的壓力，它們的葉綠素更加敏感，需要更多的光屏障。

I部分

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. Do these trees have other means at their disposal to prevent overexposure to light in autumn? Their story, though not as spectacular to the eye, will surely turn out to be as subtle and as complex.

然而，至今仍未被完全理解的是，為什么一些樹木會選擇產(chǎn)生紅色素，而其他的則嫌麻煩，只是顯露出其橙色或者黃色而已。難道這些樹木在拋棄葉子的時候擁有其他手段來防止過度暴露于秋天的陽光之下嗎？它們的故事雖然不那么引人注目，但肯定也同樣微妙和復(fù)雜。

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