当前位置:中华考试网 >> 托福考试 >> 托福机经 >> 阅读机经 >> 2018年12月8日托福阅读机经小范围精准预测

2018年12月8日托福阅读机经小范围精准预测

中华考试网   2018-11-29   【

2018年12月8日托福阅读机经小范围精准预测

  Wind pollination

  1 Pollen, a powdery substance, which is produced by flowering plants and contains male reproductive cells,is usually carried from plant to plant by insects or birds, but some plants rely on the wind to carry their

  pollen. Wind pollination is often seen as being primitive and wasteful in costly pollen and yet it issurprisingly common, especially in higher latitudes. Wind is very good at moving pollen a long way;pollen can be blown for hundreds of kilometers, and only birds can get pollen anywhere near as far. The

  drawback is that wind is obviously unspecific as to where it takes the pollen. It is like trying to get a letterto a friend at the other end of the village by climbing onto the roof and throwing an armful of letters into the air and hoping that one will end up in the friend’s garden. For the relatively few dominant tree speciesthat make up temperate forests, where there are many individuals of the same species within pollen range,this is quite a safe gamble. If a number of people in the village were throwing letters off roofs, your friendwould be bound to get one. By contrast, in the tropics, where each tree specieshas few, widely scatteredindividuals, the chance of wind blowing pollen to another individual is sufficiently slim that animals area safer bet as transporters of pollen. Even tall trees in the tropics are usually not wind pollinated despitebeing in windy conditions. In a similar way, trees in temperate forests that are insect pollinated tend togrow as solitary, widely spread individuals.

  2 Since wind-pollinated flowers have no need to attract insects or other animals, they have dispensed withbright petals, nectar, and scent. These are at best a waste and at worst an impediment to the transfer of pollen in the air. The result is insignificant-looking flowers and catkins (dense cylindrical clusters of small,petalless flowers).

  3 Wind pollination does, of course, require a lot of pollen. ■ Birch and hazel trees can produce5.5 and 4 million grains per catkin, respectively. ■There are various adaptations to help as much of the pollen go as far as possible. ■Most deciduous wind-pollinated trees (which shed their leaves every fall) produce their pollen in the spring while the branches are bare of leaves to reduce the surrounding surfaces that “compete”with the stigmas (the part of the flower that receives the pollen) for pollen. ■ Evergreen conifers, whichdo not shed their leaves, have less to gain from spring flowering, and, indeed, some flower in the autumnor winter.

  4 Pollen produced higher in the top branches is likely to go farther, it is windier (and gustier) and the pollen can be blown farther before hitting the ground. Moreover, dangling catkins like hazel hold the pollen in until the wind is strong enough to ben them, ensuring that pollen is only shed into the air when the wind is blowing hard. Weather is also important. Pollen is shed primarily when the air is dry to prevent too much sticking to wet surfaces or being knocked out of the air by rain. Despite these adaptations, much of

  the pollen fails to leave the top branches, and only between 0.5 percent and 40 percent gets more than 100 meters away from the parent. But once this far, significant quantities can go a kilometer or more. Indeed,pollen can travel many thousands of kilometers at high altitudes. Since all this pollen is floating around in the air, it is no wonder that wind-pollinated trees are a major source of allergies.

  5 Once the pollen has been snatched by the wind, but not everything is left to chance. Windborne pollen isdry, rounded, smooth, and generally smaller than that of insect-pollinated plants. But size is a two-edgedsword. Small grains may be blown farther but they are also more prone to be whisked past the waitingstigma because smaller particles tend to stay trapped in the fast-moving air that flows around the stigma.But stigmas create turbulence, which slows the air speed around them and may help pollen stick to them.

  1. The word “drawback” in the passage is closet in meaning to

  A. other side of the issue

  B. objection

  C. concern

  D. problem

  2. Which of the following can be inferred from paragraph 1 about pollen production?

  A. Pollen production requires a significant investment of energy and resources on the part of the plant.

  B. The capacity to produce pollen in large quantities is a recent development in the evolutionary history of

  plants.

  C. Plants in the tropics generally produce more pollen than those in temperate zones.

  D. The highest levels of pollen production are found in plants that depend on insects or birds to carry their

  pollen.

  3. According to paragraph 1, wind-pollinated trees are most likely to be found

  A. in temperate forests

  B. at lower latitudes

  C. in the tropics

  D. surrounded by trees of many different species

  4. Paragraph 1 supports which of the following as the reason animals are a safer bet than wind as pollinators

  when the individual trees of a species are widely separated?

  A. Animals tend to carry pollen from a given flower further than the wind does.

  B. Animals serve as pollinators even where there is little wind