[新gre阅读训练材料]GRE阅读材料练习：树木的年轮判断其年龄

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　　Not always the ring of truth.The use of wood to assess past environments needs care.TREE rings are Mother Nature's history books.Researchers often look at chemicals stored in them when trying to reconstruct past environments.Like all history books, though, they can be unreliable.And in the case of one particular chemical of interest, mercury, Jose Antonio Rodriguez Martin of the National Institute for Agricultural and Food Research and Technology, in Madrid, and his colleagues have just shown how unreliable they are.

　　年轮不一定反映事实通过树木判断过去的环境时要小心了年轮是大自然的史书。在试图重现过去的环境时，研究人员经常会从年轮中所含的化学物质入手。然而就和所有的史书一样，年轮也并不怎么靠谱。通过其关注的一个特别的化学物质-汞，西班牙国家农业与食物研究及技术学会的何塞·安东尼奥·罗德里格斯·马丁及其同事便刚刚证明了年轮多么不靠谱。

　　Dr Rodriguez Martin was studying pines on La Palma, in the Canary Islands.He knew that Hoyo Negro, one of the island's volcanoes, had erupted violently in 1949, and believed that the clouds of ash and explosive ejections of rock from the eruption had destroyed all of the pines nearby.However, as he describes in Naturwissenschaften, when he and his team examined the area, they were stunned to discover a revenant from this cataclysm just 50 metres from the crater.

　　罗德里格斯·马丁博士一直在研究拉帕尔马岛上的松树。他知道该岛上的霍约内格罗火山曾于1949年发生过剧烈喷发，并认为喷发产生的火山灰及迸散的岩石摧毁了附近所有的松树。然而，正如他在《自然科学》杂志上所说，当他和自己的团队对该区域进行调查时，在离火山口仅50米的地方发现了一棵劫后余生的松树，让他们震惊不已。

　　Like a battle-scarred soldier, this now-great pine had many half-healed wounds in its trunk.It had survived those. It did not, however, survive Dr Rodriguez Martin's axe.He reluctantly felled it and, for comparison, collected a tree of similar age that was growing 400 metres from the eruption site, and another that was 16km away.

　　One unpleasant element often ejected in volcanic eruptions is mercury.That was certainly the case in 1949, as soil-sampling showed.

　　就像一名身经百战的士兵，这棵现已长成参天大树的松树的树干上疤痕累累。这些伤口没让它倒下，但却没有逃过罗德里格斯·马丁博士的斧头。罗德里格斯·马丁博士忍痛将它放倒，并分别从离喷发点400米及16千米处砍倒两棵与其树龄相同的松树进行比较。火山喷发时经常会喷 射出一种不怎么好的元素-汞。正如土壤采样所示，1949年的火山喷发确实释放出了汞元素。

　　The question was, how much of it had got into the trees·Mercury falling.When he analysed the survivor's scar tissue Dr Rodriguez Martin found high levels of the metal in it:more than 22 micrograms per kilogram of wood.That was not surprising, given that this tissue would have formed soon after the eruption.What was surprising was that the wood of the survivor tree's rings had only 2.3 micrograms of mercury per kilogram in it.

　　但问题是，有多少进入了树体中?脱落的汞当罗德里格斯·马丁博士分析了这棵幸存的树上的疤痕组织后，他发现其中的汞元素含量很高：每公斤木材中超过22毫克。这并不出人意料，因为该组织是在火山喷发不久后形成的。出人意料的是这棵幸存的树中年轮的木质中每公斤仅含2.3毫克汞。

　　More surprising still, the rings of the other two trees were mercury-free, despite there being mercury in nearby soil.Tree rings are made of tissue called xylem.This is a set of channels that carry water and minerals from the soil to the leaves, so its composition might be expected to reflect what is in the soil.A second set of channels, the phloem, found just under a tree's bark, carries the products of photosynthesis to the roots.

　　更让人大跌眼镜的是，其他两棵书的年轮中汞含量为零，尽管其附近的土壤中含有汞。构成年轮的组织被称为木质部。这是从土壤中将水合矿物质输送至叶片的一组通道，所以其组分应该能够反映出土壤中的组分。第二组通道叫韧皮部，就长在树皮之下，它将光合作用的产物输送到树的根部。

　　This tissue, however, is not preserved from year to year.When Dr Rodriguez Martin sampled the phloem of the trees he had chopped down, he found it much richer in mercury than their xylem.Phloem from the tree at the eruption site had 9.5 micrograms of the metal per kilogramThe two more distant trees had 2.9 and 2.0 respectively.The explanation for the discrepancy lay in the trees' bark.

　　然而该组织每年都会更新。罗德里格斯·马丁博士对被他砍倒的树的韧皮部采样后，他发现韧皮部的汞含量比木质部的要高。位于喷发点的树上的韧皮部中每千克含有9.5毫克汞。另外两棵较远的树的韧皮部每千克汞含量分别为2.9及2.0。产生这种差异的原因就藏在树皮之中。

　　This had similar mercury levels to the phloem.What Dr Rodriguez Martin reckons is going on is that trees are using their bark, which they shed continually, as a waste-disposal system.Mercury is carried there by the phloem and selectively deposited in it, rather than in the plants' more permanent parts.The metal is then got rid of when the bark is sloughed off.

　　树皮的汞含量与韧皮部相近。罗德里格斯·马丁博士认为是这么回事：树木将其不断脱落树皮当做一种废物处理系统。汞通过韧皮部被转运到树皮中，有选择性地在其中沉积，而非在自己不易更新的部位沉积。然后当树皮脱落时，其中的汞也随之被除去。

　　That makes it invisible to researchers who come along later. What happens to mercury, moreover, might well happen to other toxic materials.That could throw the calculations of palaeoecologists severely out of kilter.All of which is a caution to those who read the Book of Tree Rings for clues to what happened in the past.Trees, it seems, are as good as rewriting history as people are.

　　这使随后而来的研究人员无法发现汞的存在。而且，在汞身上出现的情况很可能也会出现在其他有毒物质上。这会让古生态学家的推测严重失准。对于那些在年轮史书中寻找线索来了解过去发生了什么的人来说，所有这些都是一个警示。树木，似乎和人一样精于改写历史。

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