本文是这个系列的第四部分。
31. Studies by Hargrave and Geen estimated natural community grazing rates by measuring feeding rates of individual zooplankton species in the laboratory and then computing community grazing rates for field conditions using the known population density of grazers. (5)
由哈格雷夫(Hargrave)和吉恩(Geen)所进行的研究，对自然条件下的群落捕食速率进行了估计，其手段是通过测量出实验室内单独的浮游动物种类的捕食速率，然后利用已知的食草动物种群密度，计算出实地状况下的群落捕食速率。
难句类型：复杂修饰
解释：本句对阅读者的阅读能力提出了更高的要求，不但要求读者读清楚句子的复杂的结构，而且对读者的词汇能力也提出了更高的要求。
By以前结构无需解释，by以后有两个作介词宾语短语的动名词结构，measuring和后面表示并列和顺接的and then之后的computing，说的是计算方法;前一个measuring说的是先算出实验室中浮游动物品种的单位捕食速率，后面的computing前其实省略了一个by，而且在这个动名词结构中还包括一个分词结构using the known population density of grazers，用已知的捕食者的种群密度计算出其野外的群体捕食速率。整个的by以后的计算过程简单了就是：实验室单位速率×野外密度=野外群体速率
另外本句中的词汇也颇有迷惑性;虽然没有太难的单词，但是很多单词都是同义词和反义词，容易让人读混;(斜线分隔同义词，冒号分隔反义词)
estimate/measure/compute; feeding rates/grazing rates; zooplankton/grazers; natureal/field; laboratory community; individuall
意群训练：Studies by Hargrave and Geen estimated natural community graz-ing rates by measuring feeding rates of individual zooplankton species in the laboratory and then computing community grazing rates for field conditions using the known population density of grazers.
32. In the periods of peak zooplankton abundance，that is，in the late spring and in the summer，Haney recorded maximum daily community grazing rates，for nutrient-poor lakes and bog lakes，respectively，of 6.6 percent and 114 percent of daily phytoplankton production. (3+)
在浮游动物数量激增的高峰期，亦即在春季后期以及夏季，哈尼记录了最大程度上的每日群落食草比率，对于营养物不充足的湖和沼泽湖而言，分别为每日浮游植物繁殖量的6.6%和114%.
难句类型：插入语
解释：句中有两组插入语，每组两个插入语，尤其是第二组，把rate of从中间劈开。不过因为句义尚好理解，所以本句只是有一点别扭，但不算太难。
意群训练：In the periods of peak zooplankton abundance，that is，in the late spring and in the summer，Haney recorded maximum daily community grazing rates，for nutrient-poor lakes and bog lakes，respectively，of 6.6 percent and 114 percent of daily phytoplankton production.
33. The hydrologic cycle，a major topic in this science，is the complete cycle of phenomena through which water passes，beginning as atmospheric water vapor，passing into liquid and solid form as precipitation，thence along and into the ground surface，and finally again returning to the form of atmospheric water vapor by means of evaporation and transpiration. (4)
水文循环(hydrologic cycle)，作为该学科中的一个主要课题，指的是水所经过的诸现象的整个循环过程，开始时是作为大气中的水蒸气，转而作为雨、雪、露、雹一类的降水量经过液体和固体形态，由此而沿着地层表面分布或进入地层表面，最终通过蒸发和散发作用再度回复到大气水蒸气的形态。
难句类型：复杂修饰、插入语、省略
水文循环(hydrologic cycle)，作为该学科中的一个主要课题，指的是水所经过的诸现象的整个循环过程，开始时是作为大气中的水蒸气，转而作为雨、雪、露、雹一类的降水量经过液体和固体形态，由此而沿着地层表面分布或进入地层表面，最终通过蒸发和散发作用再度回复到大气水蒸气的形态。
解释：beginning开始的一系列分词都是修饰主语The hydrologic cycle的，分别是beginning/passing/passing/returning。值得注意的是，thence along and into the grund surfacee 是thence passing along and entering into the ground surfacee的省略形式，但是看上去意思却很清楚。类似这样的省略，是出题者合理地利用了省略来达到改编学术论文时压缩文章篇幅的目的。
意群训练：The hydrologic cycle，a major topic in this science，is the complete cycle of phenomena through which water passes，beginning as atmospheric water vapor，passing into liquid and solid form as precipitation，thence along and into the ground surface，and finally again returning to the form of atmospheric water vapor by means of evaporation and transpiration.
34. Only when a system possesses natural or artificial boundaries that associate the water within it the hydrologic cycle may the entire system properly be termed hydrogeologic.(4)
只有当一个系统拥有自然的或人工形成的边界来把边界中的水与水文循环联系起来的时候，才有可能把整个的系统恰当的成为与水文地质学有关。
难句类型：复杂修饰、倒装
解释：only放在句首时，主句的情态动词may倒装，这是我们高中就已经学过的东西。然而句首的only与倒装的may距离如此之远，这在以前却没有见过，所以读上去让人不习惯。另外一个难点在于，前面的由when所引导的时间状语从句中，宾语boundaries后面跟了一个定语从句，that associate the water within it with the hydrologic cycle,而且从句中的associate with又被water within it所分开，读者还要分神去考虑it所指的是什么，所以读到后面倒装的may的时候，难免不知所云。
意群训练：Only when a system possesses natural or artificial boundaries that associate the water within it with the hydrologic cycle may the entire system properly be termed hydrogeologic.
35. The historian Frederick J. Turner wrote in the 1890’s that the agrarian discontent that had been developing steadily in the United States since about 1870 had been precipitated by theclosing of the internal frontier——that is，the depletion of available new land needed for further expansion of the American farming system. (4)
史学家弗雷德里克.杰.特纳(Frederick J.Turner)于十九世纪九十年代著述道，美国约自18世纪70年代以来一直在持续不断发展的农民不满，由于国内边远地区(internal frontier)的封闭而更趋加剧——亦即是说，美国农业系统进一步扩展所必需的可资利用的新土地几近耗竭。
难句类型：复杂修饰
解释：典型的句子套句子。主句是F.J.T. wrote that，宾语从句中主干是the agrarian discontent had been precipitated by the closing of the internal frontier，在从句的主语the agrarian discontent后面又来了一个定语从句。
意群训练：The historian Frederick J. Turner wrote in the 1890’s that the agrarian discontent that had been developing steadily in the United States since about 1870 had been precipitated by theclosing of the internal frontier——that is，the depletion of available new land needed for further expansion of the American farming system.
36. In the early 1950’s，historians who studied preindustrial Europe (which we may define here as Europe in the period from roughly 1300 to 1800) began， for the first time in large numbers，toinvestigate more of the preindustrial European population than the 2 or 3 percent who comprised the political and social elite：the kings，generals，judges，nobles，bishops，and local magnates who had hitherto usually filled history books. (4)
难句类型：复杂修饰、插入语
译文：二十世纪五十年代早期，研究前工业化时代欧洲(此处我们可将其界定为约自1300年至1800年这一时期的欧洲)的史学家，首次以众多的人数(杨鹏的书中：第一次以大量的数据)，开始调查前工业化时代欧洲人口中的大多数，而非那些构成了政治与社会精英阶层的百分之二或三的人口，即国王、将军、法官、贵族、主教、以及地方上的达官显贵，而正是这部分人一直到那时为止普遍充斥于史学著作。
解释：句子中有两个插入语，一个是跟在preindustrial Europe之后的which we may define here as Europe in the period from roughly 1300 to 1800,这个插入语的直接作用是解释前面的preindustrial Europe的年代，而起到的客观作用则是把主句中的主语和谓语分割得很远;第二个插入语是在began，for the first time in large numbers, to investingate中间，把一个好好的began to斩为两段。
这句话另外一个难以理解的地方是more of the preindustrial European population than the 2 or 3 percent，表示的是在工业化以前的欧洲人口中超过了那2%到3%的人，亦即占97%到98%的人民群众。
意群训练：In the early 1950’s，historians who studied preindustrial Europe (which we may define here as Europe in the period from roughly 1300 to 1800) began， for the first time in large numbers，toinvestigate more of the preindustrial European population than the 2 or 3 percent who comprised the political and social elite：the kings，generals，judges，nobles，bishops，and local magnates who had hitherto usually filled history books.
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