托福閱讀真題Official 44 Passage 3（七）

托福閱讀真題Official 44 Passage 3（七）

Seagrasses

Many areas of the shallow sea bottom are covered with a lush growth of aquatic flowering plants adapted to live submerged in seawater.These plants are collectively called seagrasses.Seagrass beds are strongly influenced by several physical factors.The most significant is water motion:currents and waves.Since seagrass systems exist in both sheltered and relatively open areas,they are subject to differing amounts of water motion.For any given seagrass system,however,the water motion is relatively constant.Seagrass meadows in relatively turbulent waters tend to form a mosaic of individual mounds,whereas meadows in relatively calm waters tend to form flat,extensive carpets.The seagrass beds,in turn,dampen wave action,particularly if the blades reach the water surface.This damping effect can be significant to the point where just one meter into a seagrass bed the wave motion can be reduced to zero.Currents are also slowed as they move into the bed.

The slowing of wave action and currents means that seagrass beds tend to accumulate sediment.However,this is not universal and depends on the currents under which the bed exists.Seagrass beds under the influence of strong currents tend to have many of the lighter particles,including seagrass debris,moved out,whereas beds in weak current areas accumulate lighter detrital material.It is interesting that temperate seagrass beds accumulate sediments from sources outside the beds,whereas tropical seagrass beds derive most of their sediments from within.

Since most seagrass systems are depositional environments,they eventually accumulate organic material that leads to the creation of fine-grained sediments with a much higher organic content than that of the surrounding unvegetated areas.This accumulation,in turn,reduces the water movement and the oxygen supply.The high rate of metabolism(the processing of energy for survival)of the microorganisms in the sediments causes sediments to be anaerobic(without oxygen)below the first few millimeters.According to ecologist J.W.Kenworthy,anaerobic processes of the microorganisms in the sediment are an important mechanism for regenerating and recycling nutrients and carbon,ensuring the high rates of productivity—that is,the amount of organic material produced—that are measured in those beds.In contrast to other productivity in the ocean,which is confined to various species of algae and bacteria dependent on nutrient concentrations in the water column,seagrasses are rooted plants that absorb nutrients from the sediment or substrate.They are,therefore,capable of recycling nutrients into the ecosystem that would otherwise be trapped in the bottom and rendered unavailable.

Other physical factors that have an effect on seagrass beds include light,temperature,and desiccation(drying out).For example,water depth and turbidity(density of particles in the water)together or separately control the amount of light available to the plants and the depth to which the seagrasses may extend.Although marine botanist W.A.Setchell suggested early on that temperature was critical to the growth and reproduction of eelgrass,it has since been shown that this particularly widespread seagrass grows and reproduces at temperatures between 2 and 4 degrees Celsius in the Arctic and at temperatures up to 28 degrees Celsius on the northeastern coast of the United States.Still,extreme temperatures,in combination with other factors,may have dramatic detrimental effects.For example,in areas of the cold North Atlantic,ice may form in winter.Researchers Robertson and Mann note that when the ice begins to break up,the wind and tides may move the ice around,scouring the bottom and uprooting the eelgrass.In contrast,at the southern end of the eelgrass range,on the southeastern coast of the United States,temperatures over 30 degrees Celsius in summer cause excessive mortality.Seagrass beds also decline if they are subjected to too much exposure to the air.The effect of desiccation is often difficult to separate from the effect of temperature.Most seagrass beds seem tolerant of considerable changes in salinity(salt levels)and can be found in brackish(somewhat salty)waters as well as in full-strength seawater

Question 13 of 14

Look at the four squares[■]that indicate where the following sentence could be added to the passage.Seagrasses grow together in dense patches,or beds,with as many as 4,000 blades per square meter.Where would the sentence best fit?Click on a square[■]to add the sentence to the passage.

正確答案：B

題目詳解

題型分類：插入句子題

題干分析：考察句子和句子之間的銜接關(guān)系。插入句主要表達(dá)的信息是海草的生長狀態(tài)。

選項(xiàng)分析：

B選項(xiàng)此空前面引出seagrass這一名詞These plants are collectively called seagrasses此空后面都是對seagrass在不同水域中生長狀態(tài)的具體描述。插入句的信息是對seagrass生長狀態(tài)的整體概括，按照從總到分的文章段落思路，放在此處是合適的。

A選項(xiàng)此空后面才提到seagrass這一名詞，而插入句中已經(jīng)有seagrass這個(gè)詞了，所以放在此空處不合適。

C選項(xiàng)此空前總說影響海草床有幾個(gè)因素，此空后說其中最重要的一項(xiàng)，邏輯和內(nèi)容銜接緊密，插不進(jìn)其他信息。

D選項(xiàng)此空前提到水流運(yùn)動(dòng)water motion，此空后具體展開說在turbulent waters和calm waters中的不同，銜接緊密，無法插入其他信息。

Question 14 of 14

Directions:An introductory sentence for a brief summary of the passage is provided below.Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage.Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.This question is worth 2 points.

Seagrasses are aquatic flowering plants that grow in either sheltered or open areas of the sea.

Answer Choices:

A.

Seagrass beds are influenced by several physical factors,the most significant being the stability of the sea bottom,which must anchor them against the currents.

B.

Sediments in seagrass beds vary by region,with temperate beds accumulating sediments from within,and tropical beds collecting sediments from without.

C.

Because they slow currents and waves,seagrass beds collect deposits of rich organic sediments,which are home to many anaerobic microorganisms.

D.

Seagrasses under weak currents tend to have higher rates of metabolism than those under strong currents,perhaps because of differences in oxygen levels.

E.

Unlike sea organisms that depend on the water column for their productivity,seagrasses ensure high rates of productivity by taking nutrients from ocean floor sediment.

F.

Although seagrasses survive in temperatures ranging from 2 to 28 degrees Celsius,more extreme temperatures can damage them,as can desiccation and lack of light.

正確答案：CEF

題目詳解

題型分類：文章總結(jié)題

題干分析：選擇概括性的正確選項(xiàng)。

選項(xiàng)分析：

C選項(xiàng)是第二段和第三段的重要信息。they slow currents and waves,seagrass beds collect deposits of rich organic sediments對應(yīng)第二段的第一句The slowing of wave action and currents means that seagrass beds tend to accumulate sediment也是本段的主旨句。are home to many anaerobic microorganisms對應(yīng)第三段的第一句Since most seagrass systems are depositional environments,they eventually accumulate organic material that leads to the creation of...也是本段的主旨句。

E選項(xiàng)是第三段的重要信息，Unlike sea organisms that depend on the water column for their productivity對應(yīng)第三段的倒數(shù)第二句In contrast to other productivity in the ocean...seagrasses are rooted plants that absorb nutrients from the sediment or substrate，seagrasses ensure high rates of productivity by taking nutrients from ocean floor sediment對應(yīng)最后一句They are,therefore,capable of recycling nutrients into the ecosystem that would otherwise be trapped in the bottom and rendered unavailable。

F選項(xiàng)是第四段的重要信息。more extreme temperatures can damage them,as can desiccation and lack of light對應(yīng)第四段的第一句Other physical factors that have an effect on seagrass beds include light,temperature,and desiccation(drying out)。

A選項(xiàng)說the most significant being the stability of the sea bottom，文中無此信息，無中生有。

B選項(xiàng)說temperate beds accumulating sediments from within,and tropical beds collecting sediments from without，而第二段的第四句說的是It is interesting that temperate seagrass beds accumulate sediments from sources outside the beds,whereas tropical seagrass beds derive most of their sediments from within因此該選項(xiàng)與原文描述矛盾。

D選項(xiàng)說Seagrasses under weak currents tend to have higher rates of metabolism than those under strong currents，文章第三段沒有此方面的比較，同時(shí)選項(xiàng)中說perhaps because of differences in oxygen levels也是文中未提及的信息。無中生有。

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