美国GRE·94-10考题—（6）
2011-09-28 16:18:00   来源：作文地带整理    双击单词自动翻译

　　The defoliation of millions of acres

　　of trees by massive infestations of

　　gypsy moth caterpillars is a recurring

　　phenomenon in the northeastern United

　　(5) States. In studying these outbreaks,

　　scientists have discovered that

　　affected trees fight back by releasing

　　toxic chemicals, mainly phenols, into

　　their foliage. These noxious substances

　　(10) limit caterpillars growth and reduce

　　the number of eggs that female moths

　　lay. Phenols also make the eggs

　　smaller, which reduces the growth of

　　the following year s caterpillars.

　　(15) Because the number of eggs a female

　　moth produces is directly related to

　　her size, and because her size is deter-

　　mined entirely by her feeding success as

　　a caterpillar, the trees defensive

　　(20) mechanism has an impact on moth

　　fecundity.

　　The gypsy moth is also subject to

　　attack by the nucleo-polyhedrosis

　　virus, or wilt disease, a particularly

　　(25) important killer of the caterpillars in

　　outbreak years. Caterpillars contract

　　wilt disease when they eat a leaf to

　　which the virus, encased in a protein

　　globule, has become attached. Once

　　(30) ingested by a caterpillar, the protein

　　globule dissolves, releasing thousands

　　of viruses, or virions, that after

　　about two weeks multiply enough to fill

　　the entire body cavity. When the

　　(35) caterpillar dies, the virions are

　　released to the outside, encased in a

　　new protein globule synthesized from

　　the caterpillar s tissues and ready to

　　be picked up by other caterpillars.

　　(40) 　Knowing that phenols, including

　　tannins, often act by associating with

　　and altering the activity of proteins,

　　researchers focused on the effects on

　　caterpillars of ingesting the virus and

　　(45) leaves together. They found that on

　　tannin-rich oak leaves, the virus is

　　considerably less effective at killing

　　caterpillars than when it is on aspen

　　leaves, which are lower in phenols.

　　(50) In general, the more concentrated

　　the phenols in tree leaves, the less

　　deadly the virus. Thus, while highly

　　concentrated phenols in tree leaves

　　reduce the caterpillar population by

　　(55) limiting the

　　size of caterpillars and

　　consequently, the size of the female s

　　egg cluster, these same chemicals also

　　help caterpillars survive by disabling

　　the wilt virus. Forest stands of red

　　(60) oaks, with their tannin-rich foliage,

　　may even provide caterpillars with

　　safe havens from disease. In stands

　　dominated by trees such as aspen,

　　however, incipient gypsy moth outbreaks