【文章內(nèi)容簡介】 ects 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 UnderstoryThis 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 FloorAlmost 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 organisms 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 organic 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。較大的氣態(tài)行星：木星，土星，天王星和海王星：氣態(tài)行星主要由氫和氦構(gòu)成，密度低，自轉(zhuǎn)速度快，大氣層厚，有光環(huán)和很多衛(wèi)星。 冥王星。 根據(jù)大小: 　　小行星：水星，金星，地球，火星和冥王星。　　小行星的直徑小于13000公里。 　　巨行星：木星，土星，天王星和海王星?！　【扌行堑闹睆酱笥?8000公里。 　　水星和冥王星有時被稱作次行星(lesser planets)（不要與次級行星(minor planets)－－小行星的官方命名－－相混亂）。 　　巨行星有時被稱為氣態(tài)行星。 根據(jù)相對太陽的位置：　　內(nèi)層行星：水星，金星，地球和火星。 　　外層行星：木星，土星，天王星，海王星和冥王星。 　　在火星和木星之間的小行星帶組成了區(qū)別內(nèi)層行星和外層行星的標(biāo)志。 根據(jù)相對地球的位置：　　地內(nèi)行星：水星和金星。　　離太陽與地球較近。 　　地內(nèi)行星看起來的如同地球上看有時不完整的月亮。 　　地球。 　　地外行星：火星到冥王星?！　‰x太陽與地球較遠(yuǎn)。 　　地外行星看起來通常是完整的，或近乎完整的。 根據(jù)歷史：　　古典行星：水星，金星，火星，木星和土星?！　∈非凹匆缘弥?　　可用肉眼觀測 　　現(xiàn)代行星：天王星，海王星，冥王星?！　〗F(xiàn)代所發(fā)現(xiàn) 　　用望遠(yuǎn)鏡觀測 　　地球 未知點(diǎn)：　　太陽系是怎樣起源的？一般來說是由塵粒與氣體的星云壓縮形成的，但詳情很不清楚。 　　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(yùn)動的對象的恒星的極好的證據(jù)。組成固態(tài)行星的條件是什幺？看起來地球這樣的星體并不是獨(dú)一無二的，但目前還沒有直接證據(jù)證明這個或其它。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。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 but the most dedicated snow shovelers! Several years are usually required for the snow to settle and to season into the substance we call glacier ice. We can best determine the health of a glacier by looking at its mass balance. Each year glaciers yield either a net profit of new snow, a net loss of snow and ice, or their mass may remain in equilibrium. Scientists divide each glacier into upper and lower sections termed the accumulation area, where snowfall exceeds melting during a year。 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 snow and the older snow or ice surface. Its altitude changes annually with the glacier39。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 f