托福阅读：模拟试题及答案解析（1）

　　托福阅读原文

　　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.