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　　1. It is one of nature’s great ironies that the availability of nitrogen in the soil frequently sets an upper limit on plant growth even though the plants’ leaves are bathed in a sea of nitrogen gas.(3+)下述情形真可谓是自然界的一个莫大讽刺：土壤中所能获得的氮肥量往往对植物的生长构成了一个上限，虽然植物的叶子被沐浴在一片氮气的海洋中。

　　难句类型：复杂修饰

　　解释：It是形式主语，其真正的内容是that之后的由even though连接的两个句子。其实本句的真正难度倒不在于句子的结构，而是对于其意思的理解：尤其是文科同学，可能缺乏必要的背景知识，就更不容易读懂。句中的set an upper limit on plant growth直译为”给植物的生成设立一个上限”，其真正的意思是“限制了植物的生长”;因此the availability of nitrogen in the soil frequently sets an upper limit on plant growth之中的availability一定指的是氮的供给之少。整句话的意思是，土壤中的氮植物能够利用，却太少;而空气中虽有大量的氮，植物却不能利用，这岂不是自然界开的一个大玩笑。

　　It is one of nature’s great ironies that the availability of nitrogen in the soil frequently sets an upper limit on plant growth even though the plants’ leaves are bathed in a sea of nitrogen gas.

　　2. Unless they succeed，the yield gains of the Green Revolution will be largely lost even if the genes in legumes that equip those plants to enter into a symbiosis with nitrogen fixers are identified and isolated，and even if the transfer of those gene complexes，once they are found， becomes possible. (4)

　　除非他们能取得成功，不然的话，绿色革命的产量收益将在很大程度上损失殆尽，即使豆科植物中使这些植物有条件进入到与固氮细菌共生关系的基因可被辨识出来和分离开来的话，且即使这些基因综合体(gene complex)，一旦被发现之后，其移植得以成为可能的话。

　　难句类型：复杂修饰、插入语

　　解释：主句比较简单，而后面跟着的由and连接的两个表示让步的条件状语从句就相对复杂，第一个从句是用一个修饰主语的定语从句that equip those plants to enter into a symbiosis with nitrogen fixers把主谓隔开;第二个从句则运用了插入语once they are found把主谓隔开。

　　意群训练：Unless they succeed，the yield gains of the Green Revolution will be largely lost even if the genes in legumes that equip those plants to enter into a symbiosis with nitrogen fixers are identified and isolated，and even if the transfer of those gene complexes，once they are found， becomes possible.
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