托福听力背景知识(编辑修改稿)

托福听力背景知识(编辑修改稿)内容摘要：

re usually supported by buttress roots to brace against the high winds. Most of these trees are broadleaved, hardwood evergreens. They are exposed to greater fluctuations of temperature, wind and rainfall than are their smaller panions. To hold water, leaves often have thick, waxy layers. Emergents may take advantage of the greater air movement above the canopy by developing winged seeds or fruits that are dispersed by wind to other parts of the forest. Sunlight is plentiful and animals such as eagles, monkeys, butterflies, insecteating bats and snakes inhabit this layer, some never venturing below it. The Canopy The primary layer of the rainforest, the canopy, extends beneath the emergents, rising to 150 feet (45m). Most canopy trees have smooth, oval leaves that e to a point. A possible explanation for this adaptation is that they shed rain quickly, discouraging the growth of lichens and mosses. In cloud forests such as La Selva, the canopy is lower and more dense, formed by smaller trees with twisted crowns of tiny, leathery leaves. At these higher elevations, the leaves have developed a highly reflective property that protects them from the higher levels of intense radiation. This almost solid green shield filters out 80% of the light, preventing its transmission to the forest below. Photosynthesis is everywhere. Flowers and fruits abound. Many species flower simultaneously, aiding crosspollination. In some species, flowers are produced on the trunks, making it easier for bat pollinators to find their way to the flowers. Monkeys, sloths, bats, treefrogs, ants, beetles, parrots, hummingbirds and snakes, to mention a few, can be found here, often never touching the ground during their lifetime. Epiphytes, some 28,000 species worldwide, use every tree surface as a place to live. Hollow trunks of trees and pools of water in bromeliads often are micromunities within the Canopy. The Understory This area gets only 25% of the sunlight available to the canopy. This limited light encourages the plant residents to devise unique ways to survive, such as the solarcollecting dark green leaves. Plants that survive in the understory include dwarf palms and softstalked species of families, such as the ginger family, acanthus and prayer plant or Maranta. These plants seldom grow to more than 12 feet ( m) in height. Understory plants have a more difficult time with pollination because of the lack of air movement. Most rely on insects. Some produce strong smelling flowers, others produce flowers and fruit on their trunks. This phenomenon, known as cauliflory, makes them more conspicuous to aid the process of pollination and seed dispersal. Many animals live here, including snakes, frogs, parakeets, leopards or jaguars and the largest concentration of insects. The Forest Floor Almost no plants grow in this region of 02% light and 100% humidity. The few flowering plants that live here tolerate deep shade. The floor itself is covered with a litter of rapidly deposing vegetation and anisms that break down into usable nutrients. A leaf that might take one year to depose in a temperate climate, will disappear in just six weeks on the rainforest floor. A high proportion of the nutrients in the system are locked in the large biomass (trees and other plant storage systems). There is heavy petition for these nutrients. This is why many trees are so shallowrooted. Large mammals, such as tapirs, forage for roots and tubers. Insects, including termites, cockroaches, beetles, centipedes, millipedes, scorpions and earthworms, along with the fungi, use the anic litter as a source of food. 托福听力背景知识 (六 ) 太阳系的行星 九大行星通常按以下几个方法分类： 根据组成： 固态的由石头构成的行星：水星，金星，地球和火星： 固态行星主要由岩石与金属构成，高密度，自转速度慢，固态表面，没有光环，卫星较少。 较大的气态行星：木星，土星，天王星和海王星： 气态行星主要由氢和氦构成，密度低，自转速度快， 大气层厚，有光环和很多卫星。 冥王星。 根据大小 : 小行星：水星，金星，地球，火星和冥王星。 小行星的直径小于 13000 公里。 巨行星：木星，土星，天 王星和海王星。 巨行星的直径大于 48000 公里。 水星和冥王星有时被称作次行星 (lesser plas)（不要与次级行星 (minor plas)－－小行星的官方命名－－相混乱）。 巨行星有时被称为气态行星。 根据相对太阳的位置： 内层行星：水星，金星，地球和火星。 外层行星：木星，土星，天王星，海王星和冥王星。 在火星和木星之间的小行星带组成了区别内层行星和外层行星的标志。 根据相对地球的位置： 地内行星：水星和金星。 离太阳与地球较近。 地内行 星看起来的如同地球上看有时不完整的月亮。 地球。 地外行星：火星到冥王星。 离太阳与地球较远。 地外行星看起来通常是完整的，或近乎完整的。 根据历史： 古典行星：水星，金星，火星，木星和土星。 史前即以得知 可用肉眼观测 现代行星：天王星，海王星，冥王星。 近现代所发现 用望远镜观测 地球 未知点： 太阳系是怎样起源的。 一般来说是由尘粒与气体的星云压缩形成的，但详情很不清楚。 行星系统如何与其它星系共处。 已有了木星般大小的在附近轨道运动 的对象的恒星的极好的证据。 组成固态行星的条件是什幺。 看起来地球这样的星体并不是独一无二的，但目前还没有直接证据证明这个或其它。 托福听力背景知识 (七 ) glaciers 背景知识 Glaciers exist where, over a period of years, snow remains after summer39。 s end. They exist in environments of high and low precipitation and in many temperature regimes。 they are found on all the continents except Australia and they span the globe from high altitudes in equatorial regions to the polar ice caps. There is a delicate balance between climatic factors that allows snow to remain beyond its season. Scientists and skiers alike can note that within a few days of falling, snowflakes have noticeably begun to change. ... The snowflakes are pressed under the weight of the overlying snowpack. Individual crystal near the melting point have slick liquid edges allowing them to glide along other crystal planes and to readjust the space between them. Where the crystals touch they bond together, squeezing the air between them to the surface or into bubbles. During summer we might see the crystal metamorphosis occur more rapidly because of water percolation between the crystals. By summer39。 s end the result is firn a pacted snow with the appearance of wet sugar, but with a hardness that makes it resistant to all。