【正文】 g 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(shí) (六 ) 太陽(yáng)系的行星 九大行星通常按以下幾個(gè)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。 小行星的直徑小于 13000 公里。 巨行星有時(shí)被稱為氣態(tài)行星。 根據(jù)相對(duì)地球的位置： 地內(nèi)行星：水星和金星。 地外行星：火星到冥王星。 史前即以得知 可用肉眼觀測(cè) 現(xiàn)代行星：天王星，海王星，冥王星。 托福聽力背景知識(shí) (七 ) glaciers 背景知識(shí) Glaciers exist where, over a period of years, snow remains after 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。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(dòng)物納入到其中的。具有重大的經(jīng)濟(jì)意義和科學(xué)意義。在過(guò)去，細(xì)菌被劃分為植物（僅僅因?yàn)橛屑?xì)胞壁?。?，但在在現(xiàn)代分類學(xué)中，細(xì)菌已獨(dú)立一界。如鞭毛 (動(dòng)畫示鞭毛運(yùn)動(dòng) )。 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(zhǎng) 1020 微米，直徑 微米）。 細(xì)菌的貢獻(xiàn)：由于有了細(xì)菌，自然界的有機(jī)物才能被分解，才不會(huì)到處是生物遺體，才有了食物鏈（即有機(jī)物在自然界的不斷循環(huán)）。有些種類和藻類共生 ,形成地衣 。真菌細(xì)胞內(nèi)有線粒體 ,高爾基復(fù)合體 ,內(nèi)質(zhì)網(wǎng)等細(xì)胞器 ,而細(xì)菌沒有。酵母可以用于制作面食和釀酒 (酵母在無(wú)氧的環(huán)境下可以把葡萄糖轉(zhuǎn)變?yōu)榫凭?,而面粉中或多或少都有一些葡萄糖 ,所以放久了的面團(tuán)會(huì)有酒味 ),曲霉也可以用于釀造業(yè) 。但是它們的起源不詳。我們衡 量一個(gè)原子的直徑用埃來(lái)表示。二級(jí)分類：動(dòng)物病毒；植物病毒；細(xì)菌病毒。病毒的蛋白質(zhì)外殼 衣殼組成成分較復(fù)雜，上面有各種不 同類（如圖 :HIV 病毒 艾滋病病毒）型的受體，多糖等。概括起來(lái)分為：吸附，侵入，脫殼，生物合成，裝配與釋放等 5個(gè)步驟。但有些 種類的病毒只是把它的核酸送入細(xì)胞，而把衣殼留在細(xì)胞外面。有的是使宿主細(xì)胞破裂死亡，使病毒粒子釋放出來(lái)；有的以出芽的方式從細(xì)胞上“長(zhǎng)”出來(lái)，比如流感病毒 病毒的用處：病毒雖然可以使人得病，但它們卻大有用處：它們是遺傳學(xué)研究的主要材料；細(xì)菌病毒可以使病原細(xì)菌死亡；昆蟲病毒可以殺死害蟲，而不會(huì)對(duì)環(huán)境造成破壞，害蟲也不容易產(chǎn)生抗藥性 . 托福聽力背 景知識(shí) (九 ) Dust Bowl Dust Bowl, mon name applied to a large area in the southern part of the Great Plains region of the United States, much of which suffered extensively from wind erosion during the 1930s. The area included parts of Kansas, Oklahoma, Texas, New Mexico, and Colorado. In its original state, the region was covered with hardy grasses that held the finegrained soil in place in spite of the long recurrent droughts and occasional torrential rains characteristic of the area. A large number of homesteaders settled in the region in the 30 years before World War I, planting wheat and row crops and raising cattle. Both of these land uses left the soil exposed to the danger of erosion by the winds that constantly sweep over the gently rolling land. Beginning in the early 1930s, the region suffered a period of severe droughts, and the soil began to blow away. The anic matter, clay, and silt in the soil were carried great distances by the winds, in some cases darkening the sky as far as the Atlantic coast, and sand and heavier materials drifted against houses, fences, and barns. In many places 8 to 10 cm (3 to 4 in) of topsoil were blown away. Many thousands of families, their farms ruined, migrated westward。 and, in areas of greater rainfall, the planting of long “shelter belts” of trees to break the force of the wind. Dry spells in the 1950s, 39。t dealt with the chemical reactions that can occur, Benedetti said. The interior of these plas may be much more plicated that our current picture. A simple calculation, for example, shows that the energy released by diamonds settling to the pla39。s really cool to watch, said Benedetti. When you turn on the laser the methane turns black because of all the diamonds created. The black diamond specks float in a clear hydrocarbon liquid melted by the laser. Raman spectroscopy confirmed the identity of the suspended specks, as did subsequent analysis with Xray crystallography. The flecks were diamonds interspersed with hydrocarbons. Jeanloz said that the high temperature breaks up methane (CH4) into carbon and hydrogen, while high pressure condenses the carbon to diamond. Other types of hydrocarbons doubly and tr