【正文】 r that constantly bombards the soil would leech out most of the valuable nutrients. Plants and animals in this environment have developed adaptations to deal with the constant deluge and to pete successfully for nutrients. A closer look at the rainforest reveals that it is actually posed of four layers or munities. Each layer has a unique set of environmental conditions and anisms adapted to them. Read the descriptions below and watch for these layers and the infinite variety of life forms that occupy them as you visit La Selva. The Emergent Layer The tallest trees are the emergents, towering as much as 200 feet (60 m) above the forest floor with trunks that measure up to 16 feet (5 m) around. These huge trunks are 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. 托福聽(tīng)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(méi)有光環(huán)，衛(wèi)星較少。 巨行星有時(shí)被稱為氣態(tài)行星。 地外行星：火星到冥王星。 托福聽(tīng)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。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。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。 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(jiàn)照片，一般長(zhǎng) 1020 微米，直徑 微米）。有些種類和藻類共生 ,形成地衣 。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) 。我們衡 量一個(gè)原子的直徑用埃來(lái)表示。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。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。 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。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