托福阅读真题Official 45 Passage 2（一）

托福阅读真题Official 45 Passage 2（一）

Wind Pollination

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 is surprisingly 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 letter to 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 species that 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 friend would be bound to get one.By contrast,in the tropics,where each tree species has few,widely scattered individuals,the chance of wind blowing pollen to another individual is sufficiently slim that animals are a safer bet as transporters of pollen.Even tall trees in the tropics are usually not wind pollinated despite being in windy conditions.In a similar way,trees in temperate forests that are insect pollinated tend to grow as solitary,widely spread individuals.

Since wind-pollinated flowers have no need to attract insects or other animals,they have dispensed with bright 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).

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 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,which do not shed their leaves,have less to gain from spring flowering,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 hitting the ground.Moreover,dangling catkins like hazel hold the pollen in until the wind is strong enough to bend 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.

Once the pollen has been snatched by the wind,the fate of the pollen is obviously up to the vagaries of the wind,but not everything is left to chance.Windborne pollen is dry,rounded,smooth,and generally smaller than that of insect-pollinated plants.But size is a two-edged sword.Small grains may be blown farther but they are also more prone to be whisked past the waiting stigma 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.

Question 1 of 14

The word“drawback”in the passage is closest in meaning to

A.other side of the issue

B.objection

C.concern

D.problem

正确答案：D

题目详解

题型分类：词汇题

选项分析：

原句为drawback是风明显地不确定会把花粉带到哪里，而前一句说到了风传粉的好处，可以推测这一句是在说风传粉的坏处。即D选项problem意思为“问题，难题”。drawback原意为“缺点；不利条件”。

A选项other side of the issue事情的另外一方面。

B选项objection反对的理由，反对；异议。

C选项concern关心，关注。

Question 2 of 14

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.

正确答案：A

题目详解

题型分类：推论题

原文定位：本段都在说花粉制造，根据选项定位

选项分析：

A选项a significant investment of energy and resources对应第二句costly pollen。A选项正确。

B选项a recent development in the evolutionary history of plants无中生有。

C选项Plants in the tropics出自倒数第三句，但原文只说了热带地区的植物一般是动物传粉，而没有说more pollen。

D选项insects or birds出自最后一句，但是没有说The highest levels of pollen production。