【正文】 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. 托福聽力背景知識 (六 ) 太陽系的行星 九大行星通常按以下幾個方法分類： 根據(jù)組成： 固態(tài)的由石頭構(gòu)成的行星：水星，金星，地球和火星： 固態(tài)行星主要由巖石與金屬構(gòu)成，高密度，自轉(zhuǎn)速度慢，固態(tài)表面，沒有光環(huán)，衛(wèi)星較少。 冥王星。 小行星的直徑小于 13000 公里。 巨行星的直徑大于 48000 公里。 巨行星有時被稱為氣態(tài)行星。 外層行星：木星，土星，天王星，海王星和冥王星。 根據(jù)相對地球的位置： 地內(nèi)行星：水星和金星。 地內(nèi)行 星看起來的如同地球上看有時不完整的月亮。 地外行星：火星到冥王星。 地外行星看起來通常是完整的，或近乎完整的。 史前即以得知 可用肉眼觀測 現(xiàn)代行星：天王星，海王星，冥王星。 行星系統(tǒng)如何與其它星系共處？已有了木星般大小的在附近軌道運動 的對象的恒星的極好的證據(jù)。 托福聽力背景知識 (七 ) glaciers 背景知識 Glaciers exist where, over a period of years, snow remains after summer39。 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。 and the ablation area, where melting exceeds snowfall. An equilibrium line, where mass accumulation equals mass loss, separates these areas. You can see it as the boundary between the winter39。s mass balance. To find mass balance, scientists measure the area of each region and observe amounts of accumulation and ablation relative to preset stakes. After density measurements are made they may calculate how much water has been added or lost to the glacier. After a series of positive mass balance years, the glacier may respond to the increased thickness by making a glacial advance downvalley. A series of negative years may cause a glacial retreat, meaning that the terminus is melting faster than the ice is moving downvalley. Glaciers have been likened to mighty rivers of ice. Although they move many times more slowly, glaciers have equivalent changes in flow rate and often form falls of fastmoving ice above slowmoving ice pools. Glaciers flow faster down their centers than at ice margins, and more quickly at the surface than at the bed. ... How fast a glacier moves is mostly dependent on the thickness of the ice, and on the angle of its surface slope. Glacier speeds vary when changes are made in this geometry. They respond to excessively high seasonal snow accumulations by generating bulges of thicker ice that may move downvalley many times faster than the glacier39。包括病毒、立克茨體、支原體、衣原體、細(xì)菌、放線菌、真菌中的霉菌、酵母菌和螺旋體等，也有將微植物和微動物納入到其中的。分布廣泛，繁殖快且類型多樣。具有重大的經(jīng)濟意義和科學(xué)意義。支原體沒有細(xì)胞壁，故細(xì)胞柔軟，形成分枝狀的細(xì)胞，故稱支原體。在過去，細(xì)菌被劃分為植物（僅僅因為有細(xì)胞壁?。谠诂F(xiàn)代分類學(xué)中，細(xì)菌已獨立一界。細(xì)菌微小，大多數(shù)只有 微米，但少數(shù)卻有 毫米，如納米比亞硫磺珍珠細(xì)菌。如鞭毛 (動畫示鞭毛運動 )。細(xì)菌的細(xì)胞壁由肽聚糖組成，十分堅固。 細(xì)菌分三類：種類最多的桿菌 （一般 110 微米）；數(shù)目眾多的球菌（一般 1 微米）以及纖細(xì)活潑的螺旋菌（見照片，一般長 1020 微米，直徑 微米）。還有厭氧細(xì)菌；好氧細(xì)菌和兼性細(xì)菌。 細(xì)菌的貢獻：由于有了細(xì)菌，自然