TPO45阅读文本及答案解析

TPO45阅读文本2. Wind pollination

Pollen, a powdery substance, which is produced by flowering plantsand contains male reproductive cells, is usually carried from plant to plant byinsects or birds, but some plants rely on the wind to carry their pollen. Windpollination is often seen as being primitive and wasteful in costly pollen andyet it is surprisingly common, especially in higher latitudes. Wind is verygood at moving pollen a long way; pollen can be blown for hundreds ofkilometers, and only birds can get pollen anywhere near as far. The drawback isthat wind is obviously unspecific as to where it takes the pollen. It is liketrying to get a letter to a friend at the other end of the village by climbingonto the roof and throwing an armful of letters into the air and hoping thatone will end up in the friend's garden. For the relatively few dominant treespecies that make up temperate forests, where there are many individuals of thesame species within pollen range, this is quite a safe gamble. If a number ofpeople in the village were throwing letters off roofs, your friend would bebound to get one. By contrast, in the tropics, where each tree species has few,widely scattered individuals, the chance of wind blowing pollen to anotherindividual is sufficiently slim that animals are a safer bet as transporters ofpollen. Even tall trees in the tropics are usually not wind pollinated despitebeing in windy conditions. In a similar way, trees in temperate forests thatare insect pollinated tend to grow as solitary, widely spread individuals.

Since wind-pollinated flowers have no need to attract insects orother animals, they have dispensed with bright petals, nectar, and scent. Theseare at best a waste and at worst an impediment to the transfer of pollen in theair The result is insignificant-looking flowers and catkins (dense cylindricalclusters of small, petalless flowers).

Wind pollination does, of course, require a lot of pollen. ■Birch and hazel trees can produce 5.5 and 4 million grains per catkin,respectively. ■There are various adaptations to help as much of the pollen go asfar as possible. ■ Most deciduous wind-pollinated trees (which shed their leaves everyfall) produce their pollen in the spring while the branches are bare of leavesto reduce the surrounding surfaces that “compete" with the stigmas (thepart of the flower that receives the pollen) for pollen. ■ Evergreenconifers, which do not shed their leaves, have less to gain from springflowering, and, indeed, some flower in the autumn or winter.

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 hittingthe ground. Moreover, dangling catkins like hazel hold the pollen in until thewind is strong enough to bend them, ensuring that pollen is only shed into theair when the wind is blowing hard. Weather is also important. Pollen is shedprimarily when the air is dry to prevent too much sticking to wet surfaces orbeing knocked out of the air by rain. Despite these adaptations, much of thepollen fails to leave the top branches, and only between 0.5 percent and 40percent gets more than 100 meters away from the parent. But once this far,significant quantities can go a kilometer or more. Indeed, pollen can travelmany thousands of kilometers at high altitudes. Since all this pollen isfloating around in the air, it is no wonder that wind-pollinated trees are amajor source of allergies.

Once the pollen has been snatched by the wind, the fate of thepollen is obviously up to the vagaries of the wind, but not everything is leftto chance. Windbome pollen is dry, rounded, smooth, and generally smaller thanthat of insect-pollinated plants. But size is a two-edged sword. Small grainsmay be blown farther but they are also more prone to be whisked past thewaiting stigma because smaller particles tend to stay trapped in thefast-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 closest inmeaning to

O other side of the issue

O objection

O concern

O problem

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

O Pollen production requires a significant investment of energy andresources on the part of the plant.

O The capacity to produce pollen in large quantities is a recentdevelopment in the evolutionary history of plants.

O Plants in the tropics generally produce more pollen than those intemperate zones.

O The highest levels of pollen production are found in plants thatdepend on insects or birds to carry their pollen.

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

O in temperate forests

O at lower latitudes

O in the tropics

O surrounded by trees of many different species

4. Paragraph 1 supports which of the following as the reason animalsare a safer bet than wind as pollinators when the individual trees of a speciesare widely separated?

O Animals tend to carry pollen from a given flower further than thewind does.

O Animals serve as pollinators even where there is little wind todisperse the pollen.

O An animal that visits a flower is likely to deliberately visitother flowers of the same species and pollinate them.

O Birds and insects fly in all directions, not just the directionthe wind is blowing at a given moment.

5. In paragraph 1, the author compares pollen moved by wind withletters thrown off roofs in order to

O explain why there are relatively few species of trees that dependon wind pollination

O compare natural, biological processes with human social practices

O make a point about the probability of wind-blown pollen reaching atree of the same species

O argue against the common assumption that the tallest trees are themost likely to employ wind pollination

6. Paragraph 2 suggests that wind-pollinated plants do not havebright petals, nectar, and scent for which TWO of the following reasons? Toreceive credit, you must select TWO answers.

□ They interfere with pollination by wind.

□ They are easily damaged by wind.

□ They are unnecessary.

□ They reduce the amount of pollen that can be produced.

7. The word "respectively” in the passage is closest in meaningto

O over time

O separately

O in that order

O consistently

8.According to paragraph 3, why do most deciduous wind-pollinatedtrees produce their pollen in the spring?

O To avoid competing with evergreen conifers, which flower in thefall or winter

O So that the leaves of the trees receiving the pollen will notprevent the pollen from reaching the trees' stigmas

O Because they do not have enough energy to produce new leaves andpollen at the same time

O In order to take advantage of the windiest time of year

9. According to paragraph 4，which of the followingis NOT an adaptation that helps ensure that pollen travels as far as possible?

O Pollen-producing flowers and catkins are located at or near thetop of the tree.

O Trees grow at least 100 meters away from each other.

O Dangling catkins release pollen only when the wind is blowinghard.

O Pollen is not released during rain storms or when the air is damp.

10. The word "significant" in the passage is closest inmeaning to

O sufficient

O considerable

O increasing

O small

11. The phrase “no wonder" in the passage is closest in meaningto

O unsurprising

O understandable

O well-known

O unfortunate

12. Which of the sentences below best expresses the essentialinformation in the highlighted sentence in the passage? Incorrect choiceschange the meaning in important ways or leave out essential information.

O Because smaller particles tend to stay trapped in the fast movingair, they are blown much farther than other grains.

O Smaller particles are trapped by the stigma when fast-moving air flowspast it.

O Small particles that are whisked past the waiting stigma gainspeed and are often trapped in the fast-moving air.

O While smallness helps pollen travel farther, it also makes it morelikely to be blown past the stigma.

13. Look at the four squares [■] thatindicate where the following sentence could be added to the passage.

This level of volume is important to ensure that at least some ofthe pollen reaches target tree, but dispersing the pollen is crucial as well.

Where would the sentence best fit? Click on a square [■]to add the sentence to the passage.

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

Drag your answer choices to the spaces where they belong. To removean answer choice, click on it.

To review the passage, click VIEW TEXT

Some plants depend on the wind to carry their pollen.

Answer Choices

Because there are few trees in temperate forests, it is safer totransport pollen by insects or birds.

Wind pollination is a safe reproductive strategy for trees intemperate forests where there are only a few dominant species and, therefore,many individuals of the same species.

Wind pollination requires production of a large amount of pollen,which must be released at the right time and under the right conditions toextend its range.

Most wind-pollinated trees are deciduous because evergreen needlescompete with the stigma for pollen, making wind pollination uncertain.

Wind-pollinated plants usually have small petalless flowers whichoften grow in catkins that produce a very fine-grained pollen.

Wind-pollinated trees must grow in regions that are only moderatelywindy because strong winds will blow the tiny pollen grains past the stigma.