[ 听力基础 ] 一

Posted 2023-03-07 we1less

We've been talking about various mechanisms of seed dispersal, and before we finish, I'd like to talk about some new findings concerning maple seed dispersal. Now, most of you have probably seen a maple tree and its seeds before. There are about 125 species of them in Asia, Europe, North Africa, and North America. And they have what are called winged seeds because... Well, take a look at this picture. You can see that this maple seed has what looks like a wing with a nut at one end of it. Now, when the maple seeds fall off the tree, they pick up downward speed and start to spin around the nut, which is their center of gravity. The seed looks kind of like a helicopter when it spins. In fact, maple seeds and other spinning seeds are often called helicopter seeds. Now, this spinning is actually very important, because it helps the seed by slowing down its descent. This is beneficial to the seed because a decrease in velocity means it can be carried farther from its tree of origin by the wind. This gives the seed a better chance of landing in a suitable place for sprouting, which increases its chances for survival. 

But how exactly does spinning slow the seed down? By creating lift. Lift is a force that's generated when an object moves through the air. Now, don't worry. I am not going to go into great detail here about the physics of lift. But simply speaking, a moving object both creates and redirects the flow of air. When this flow is redirected in a certain way, the object achieves lift. Think of an airplane wing. You might not have noticed this, but airplane wings are positioned at an angle to produce an upward force on the wing. That upward force is lift. 

There are, of course, other ways to produce lift. Flying animals flap their wings, for example. And maple seeds spin through the air. But regardless of the object, physics tells us that larger objects need to move at a higher velocity in order to produce sufficient airflow for lift. This is why, for example, large jets need to build up a lot of speed on the runway before they can take off. Now, maple seeds and other spinning seeds have puzzled scientists because, although they move through the air relatively slowly, they are able to generate unexpectedly high lift forces. The relationship between velocity and lift doesn't seem to apply here. 

What's interesting is that the wings of certain animals, such as bats and insects, also generate high lift forces even at low velocities. Recently, experts in animal flight came up with an explanation for this -- the extra lift is generated by vortices. Vortices are spinning flows of air like tiny tornadoes. Thanks to their special structure and movement, insect and bat wings create these small spinning air flows, usually above the wings. Each individual vortex creates an area of low air pressure that sucks the wing upwards, so to speak, and creates the extra lift. 

Now, a group of researchers wondered if this same phenomenon could be found in spinning seeds. They conducted an experiment in a device called a wind tunnel, which is used to study the effects of movement through air or resistance to moving air. The researchers used smoke in the wind tunnel to make the air flow visible and watched as a maple seed rotated in the air flow. And they found that the smoke pattern showed a vortex that was similar to the vortices made by the wings of insects and bats. So maple seeds generate vortices that provide the extra lift that slows down their descent. 

This discovery was interesting for a number of reasons. First, here's a case of animals and plants evolving incredibly similar adaptations to the problems of flight performance. We tend to think of plants and animals as very different organisms following very different evolutionary paths. Now, here's evidence that these organisms can actually evolve similar characteristics. And then, there's the possible application of these findings. Aerospace engineers are interested in using what we've learned about vortices to design new forms of parachutes to slow down spacecraft landing on other planets. 

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1. What is the lecture mainly about?

A. How spinning seeds have inspired the design of new kinds of aircraft

B. Recent findings about the airflow properties of spinning seeds

C. The various physical forces that cause spinning seeds to fall straight down

D. A comparison of the mechanisms used by plants to disperse seeds

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2. According to the professor, what aspect of maple seeds is important for their survival?

A. The ability to adapt to changing environmental conditions

B. A shape that ensures they land near their tree of origin

C. The way they fall, which allows the wind to disperse them widely

D. A size that makes them easily dispersed by insects and bats

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3. What question about maple seeds did researchers focus on?

A. How they generate lift at low velocities

B. How wind speed affects their flight

C. How they differ from nonspinning seeds

D. Which species' seeds generate the most lift

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4. What did the wind tunnel experiment reveal about maple seeds?

A. They fall faster when there is no wind.

B. They generate vortices that slow their descent.

C. Their flight becomes unstable in high winds.

D. Their rotational speed is increased by vortices.

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5. What does the professor imply about the discovery that some plants and animals generate lift in similar ways?

A. It was predicted by scientists many years ago.

B. It contradicts assumptions often made about animal and plant evolution.

C. It does not provide a satisfactory explanation of seed dispersal.

D. It will probably inspire many more scientists to study plants.

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6. According to the professor, what practical application might the researchers' findings about vortices have in the future?

A. The findings could help scientists predict the spread of maple trees.

B. The findings could help launch aircraft more efficiently.

C. The findings could help farmers improve seed dispersal.

D. The findings could be used to design new parachutes for spacecraft.

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Now, this spinning is actually very important, because it helps the seed by slowing down its descent. This is beneficial to the seed because a decrease in velocity means it can be carried farther from its tree of origin by the wind. This gives the seed a better chance of landing in a suitable place for sprouting, which increases its chances for survival.

        spinning  adj. 旋转的;纺织的

        seed  n. 种子;

        slowing down  减速;放缓进程;放慢;慢化;慢下来

        velocity  n. (沿某一方向的)速度;高速;快速

        farther  adj. 更远的;(空间、方向或时间上)较远的

        sprouting  v. 发芽;抽芽;抽条;生长;出现;

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