托福阅读题专项练习及答案解析
阅读原文
Protection of Plants by Insects
【1】Many plants - one or more species of at least68 different families - can secrete nectar even whenthey have no blossoms, because they bear extrafloral nectaries (structures that producenectar) on stems, leaves, leaf stems, or other structures.These plants usually occur where antsare abundant, most in the tropics but some in temperate areas. Among those of northeasternNorth America are various plums, cherries, roses, hawthorns, poplars, and oaks. Like floralnectar, extrafloral nectar consists mainly of water with a high content of dissolved sugars and, in some plants, small amounts of amino acids. The extrafloral nectaries of some plants areknown to attract ants and other insects, but the evolutionary history of most plants with thesenectaries is unknown. Nevertheless, most ecologists believe that all extrafloral nectaries attractinsects that will defend the plant.
【2】Ants are portably the most frequent and certainly the most persistent defenders ofplants. Since the highly active worker ants require a great deal of energy, plants exploit thisneed by providing extrafloral nectar that supplies ants with abundant energy. To return thisfavor, ants guard the nectaries, driving away or killing intruding insects that might competewith ants for nectar. Many of these intruders are herbivorous and would eat the leaves of theplants.
【3】Biologists once thought that secretion of extrafloral nectar has some purely internalphysiological function, and that ants provide no benefit whatsoever to the plants that secreteit. This view and the opposing “protectionist” hypothesis that ants defend plants had beendisputed for over a hundred years when, in 1910, a skeptical William Morton Wheelercommented on the controversy. He called for proof of the protectionist view: that visitations ofthe ants confer protection on the plants and that in the absence of the insects a much greaternumber would perish or fail to produce flowers or seeds than when the insects are present. That we now have an abundance of the proof that was called for was established whenBarbara Bentley reviewed the relevant evidence in 1977, and since then many moreobservations and experiments have provided still further proof that ants benefit plants.
【4】One example shows how ants attracted to extrafloral nectaries protect morning gloriesagainst attacking insects. The principal insect enemies of the North American morning gloryfeed mainly on its flowers or fruits rather than its leaves. Grasshoppers feeding on flowersindirectly block pollination and the production of seeds by destroying the corolla or thestigma, which receives the pollen grains and on which the pollen germinates. Without theircolorful corolla, flowers do not attract pollinators and are not fertilized. An adult grasshoppercan consume a large corolla, about 2.5 inches long, in an hour. Caterpillars and seed beetlesaffect seed production directly. Caterpillars devour the ovaries, where the seeds are produced, and seed beetle larvae eat seeds as they burrow in developing fruits.
【5】Extrafloral nectaries at the base of each sepal attract several kinds of insects, but 96 percent of them are ants, several different species of them. When buds are still small, lessthan a quarter of an inch long, the sepal nectaries are already present and producing nectar. They continue to do so as the flower develops and while the fruit matures. Observations leavelittle doubt that ants protect morning glory flowers and fruits from the combined enemy forceof grasshoppers, caterpillars, and seed beetles. Bentley compares the seed production of sixplants that grew where there were no ants with that of seventeen plants that were occupied byants. Unprotected plants bore only 45 seeds per plant, but plants occupied by ants bore 211 seeds per plant. Although ants are not big enough to kill or seriously injure grasshoppers, theydrive them away by nipping at their feet. Seed beetles are more vulnerable because they aremuch smaller than grasshoppers. The ants prey on the adult beetles, disturb females astheylay their eggs on developing fruits, and eat many of the eggs they do manage to lay.
试题
1.According to paragraph 1，floral nectar andextrafloral nectar are alike in that
A.they are likely to be produced by the same plants.
B.they basically consist of the same chemicalcomponents.
C.they attract only insects that will defend the plant.
D.they are produced by the same parts of the plant.
2.To say that ants are "persistent" defenders of plants means that
A.they defend plants against a wide variety of threats.
B.they continue to defend plants for as long as the plants are threatened.
C.they are successful defenders of plants.
D.they are easily observable defenders of plants.
3.What can be inferred from paragraph 2 about the ants that are attracted to theextrafloral nectaries?
A.They do not eat the leaves of the plants that produce extrafloral nectar.
B.They live almost entirely on extrafloral nectar.
C.They spend most of their energy guarding extrafloral nectaries.
D.They frequently fight among themselves over extrafloral nectar.
4.According to paragraph 3, what was the position of the opponents of the"protectionist" hypothesis?
A.Extrafloral nectar provides plants with a direct defense against attack by insects.
B.Ants substantially benefit plants that secrete extrafloral nectar.
C.The secretion of extrafloral nectar plays a role in the plant’s internal functioning.
D.Ants visit plants that secrete extrafloral nectar as often as they visit plants that do not.
5.The word "skeptical" in the passage is closest in meaning to
A.curious.
B.doubtful.
C.open-minded.
D.practical.
6.Which of the sentences below best expresses the essential information jn thehighlighted sentence in the passage Incorrect choices change the meaning inimportant ways or leave out essential information.
A.We now have ample proof that ants benefit plants.
B.Barbara Bentley has called for additional proof that ants benefit plants.
C.In 1977 Barbara Bentley conducted research that proved that all prior studies were wrong.
D.Proof that ants benefit plants will require many more observations and experiments.
7.According to paragraph 4, what effect does the destruction of the corolla have onplants.
A.It leaves the seeds exposed and unprotected.
B.It prevents the stigma from developing.
C.It keeps pollen grains from attaching properly.
D.It prevents the flower from attracting pollinators.
8.The word "devour" in the passage is closest in meaning to
A.attack.
B.eat.
C.damage.
D.prefer.
9.What role does paragraph 5 play in the passage?
A.It offers various kinds of evidence for the protectionist view.
B.It presents the study that first proved that ants benefit plants.
C.It explains how insects find sources of nectar.
D.It presents information that partly contradicts the protectionist view.
10.The word "vulnerable" in the passage is closest in meaning to
A.numerous.
B.harmful.
C.open to attack.
D.difficult to locate.
11.According to paragraph 5, what did Bentley’s comparative study show?
A.Many more plants grew in places where ants were present than where they were absent.
B.The ants preferred plants with low seed production to plants with high seed production.
C.The plants occupied by ants produced many more seeds than those that were not occupiedby ants.
D.The plants that grew in places without ants were much smaller and weaker than those thatgrew in places where ants were present.
12.According to paragraph 5, ants defend morning glory plants from seed beetles ineach of the following ways EXCEPT:
A.driving adult beetles off the plants by nipping at their feet.
B.catching and eating adult beetles.
C.eating beetle eggs they find on developing fruits.
D.making it difficult for beetles to lay eggs on developing fruits.
13. Look at the four squares [■] that indicate where the following sentence could beadded to the passage. Where would the sentence best fit? Sometimes they capture theinsects to feed their protein-hungry larvae.
Ants are portably the most frequent and certainly the most persistent defenders of plants. ■【A】 Since the highly active worker ants require a great deal of energy, plants exploit thisneed by providing extrafloral nectar that supplies ants with abundant energy. ■【B】 Toreturn this favor, ants guard the nectaries, driving away or killing intruding insects that mightcompete with ants for nectar. ■【C】 Many of these intruders are herbivorous and would eatthe leaves of the plants. ■【D】
14. Directions: An introductory sentence for a brief summary of the passage of thepassage is provided below. Complete the summary by selecting the THREE answerchoices that express the most important ideas in the passage. Some answer choicesdo not belong in the summary because they express ideas that are not presented inthe passage or are minor ideas in the passage. This questions is worth 2 points.
Many plants have extrafloral nectaries that produce nectar even during periods inwhich the plant is not flowering.
A.Evolutionary history shows that plants that produce extrafloral nectar originated in thetropics.
B.Extrafloral nectar has a higher concentration of sugar than floral nectar and is moreattractive to ants and other insects.
C.The protectionist hypothesis is that extrafloral nextar attracts ants, and that the ants, inorder to preserve this energy-rich food source, attack insects that might harm the plant.
D.Evidence accumulated during the twentieth century proved that ants provide significantbenefits for plants with extrafloral nectaries.
E.Research has shown that American morning glory plants that are protected by ants producesignificantly more seeds than morning glory plants that are not protected by ants.
F.Ants generally ignore small insects, but they will eat the adults of large insect species aswell as their eggs and larvae.
答案
1.B 2.B 3.A 4.D 5.A 6.A 7.C 8.B 9.B 10.C 11.C 12.A 13.C 14.CDE
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