VOA标准英语2011--Autumn Leaves Show Their True Colors

Autumn Leaves Show Their True Colors

 

As days grow shorter and nights grow cooler in Boulder¹, Colorado, the green leaves of summer have been putting on party colors.

On cottonwood trees and aspens, they’re golden yellow coins. On Mapleton Avenue, the maple² trees are sunset shades of orange and red.

The same seasonal³ phenomenon occurs in many countries in late autumn, yet people don't always know the reason leaves change color when the weather gets cooler.

Fall splendor⁴

Some do have their theories about the autumnal transformation⁵.

“I’ve heard that the reason why the leaves change throughout the seasons is because God likes to see different colors all over the world," says one girl, "so that he can tell what season it is.”

“According to legend," says a woman, "three hunters and their dog chased the Great Bear into the sky, slaughtered⁶ it and its blood dripped onto the leaves, turning them red.”

Dave Sutherland, a naturalist⁷ with Boulder Mountain Parks, recalls the explanation he heard in childhood. “My mom told me it was because at night, when everybody’s asleep the fairies come out and paint them.”

Magical science

It turns out the scientific explanation for why leaves change color is almost as magical as artistic⁸ fairies.

The story begins in summer, when leaves are green thanks to a pigment⁹ known as chlorophyll. It captures just the right spectrum¹⁰ of color from the sun’s light, to power the process known as photosynthesis¹¹.

As the days grow shorter in autumn, leaves stop making chlorophyll, revealing a dazzling secret: green leaves are actually yellow.

“As the chlorophyll is grabbing that sunlight, it’s pulling water from the soil and carbon dioxide from the air and energy from the sunlight and shoving it together to make sugar,” Sutherland says.

That helps the tree blossom and grow. As the days grow shorter in autumn, leaves stop making chlorophyll, revealing a dazzling secret.

Dazzling secret

“Green leaves are yellow. You just don’t see the yellow," Sutherland says. "The yellow pigments¹² are there all along, hidden beneath the mask of green. They’re just waiting for a chance for the chlorophyll to start breaking down so the yellow color can start shining through.”

That yellow comes from pigments known as carotinoids, which make carrots orange and autumn aspen leaves golden yellow. Some scientists say leaves keep making carotinoids after they’re done with chlorophyll, because the yellow pigments help them capture a little more solar energy.

And when they turn red? That’s another pigment called anthocyanin.

“Everybody knows anthocyanin," Sutherland says. "It makes strawberries red and blackberries black.”

And fall maple leaves red.

Natural sunscreen

But, unlike carotinoids, leaves don’t make this pigment during summer, only in autumn. For decades, scientists believed anthocyanin made a sort of accidental color, caused by sugars trapped in dying autumn leaves.

But Bill Hoch, a plant scientist at Montana State University, is among those who believe leaves make anthocyanin on purpose, as a kind of natural “sunscreen.” As an autumn leaf loses its ability to make energy from photosynthesis, he says, the red anthocyanin pigment protects the leaf from the harmful effects of sunlight - especially the blue spectrum of light.

This helps the leaf’s energy-making machinery¹³ stay intact just long enough to send the last bits of nutrients¹⁴, especially nitrogen and phosphorus, into the tree’s twigs¹⁵ and roots.

“It’s only to recover those nutrients, because those leaves are going to be discarded pretty quickly," Hock says. "So they’re just trying to protect their photosystems long enough to just recover as many nutrients as possible.”

Hoch adds that carotinoids protect the photo systems, which are light-powered energy cells. But instead of being a sunscreen for the final days of a dying leaf, the yellow pigment is an anti-oxidant that protects the leaf throughout its life.

As for why some leaves turn yellow and others turn red, Hoch says plants that grow best in full sun tend to have more robust¹⁶ leaves. So, in the fall, they can get by with the protection of just the yellow carotinoids. Plants that thrive in shade or in nutrient-poor soil tend to have more delicate photo machinery, which need more protection, so their leaves make more anthocyanins and turn dark.

There’s much more that scientists want to learn about how leaves work. In the meantime, people in Boulder - and across the northern hemisphere - can simply enjoy the natural splendor of spectacular autumn colors.

1 boulder   

n.巨砾；卵石，圆石

参考例句：

• We all heaved together and removed the boulder.大家一齐用劲，把大石头搬开了。
• He stepped clear of the boulder.他从大石头后面走了出来。

2 maple   

n.槭树，枫树，槭木

参考例句：

• Maple sugar is made from the sap of maple trees.枫糖是由枫树的树液制成的。
• The maple leaves are tinge with autumn red.枫叶染上了秋天的红色。

3 seasonal   

adj.季节的，季节性的

参考例句：

• The town relies on the seasonal tourist industry for jobs.这个城镇依靠季节性旅游业提供就业机会。
• The hors d'oeuvre is seasonal vegetables.餐前小吃是应时蔬菜。

4 splendor   

n.光彩；壮丽，华丽；显赫，辉煌

参考例句：

• Never in his life had he gazed on such splendor.他生平从没有见过如此辉煌壮丽的场面。
• All the splendor in the world is not worth a good friend.人世间所有的荣华富贵不如一个好朋友。

5 transformation   

n.变化；改造；转变

参考例句：

• Going to college brought about a dramatic transformation in her outlook.上大学使她的观念发生了巨大的变化。
• He was struggling to make the transformation from single man to responsible husband.他正在努力使自己由单身汉变为可靠的丈夫。

6 slaughtered   

v.屠杀，杀戮，屠宰( slaughter的过去式和过去分词 )

参考例句：

• The invading army slaughtered a lot of people. 侵略军杀了许多人。 来自《简明英汉词典》
• Hundreds of innocent civilians were cruelly slaughtered. 数百名无辜平民遭残杀。 来自《简明英汉词典》

7 naturalist   

n.博物学家(尤指直接观察动植物者)

参考例句：

• He was a printer by trade and naturalist by avocation.他从事印刷业，同时是个博物学爱好者。
• The naturalist told us many stories about birds.博物学家给我们讲述了许多有关鸟儿的故事。

8 artistic   

adj.艺术(家)的，美术(家)的；善于艺术创作的

参考例句：

• The picture on this screen is a good artistic work.这屏风上的画是件很好的艺术品。
• These artistic handicrafts are very popular with foreign friends.外国朋友很喜欢这些美术工艺品。

9 pigment   

n.天然色素，干粉颜料

参考例句：

• The Romans used natural pigments on their fabrics and walls.古罗马人在织物和墙壁上使用天然颜料。
• Who thought he might know what the skin pigment phenomenon meant.他自认为可能知道皮肤色素出现这种现象到底是怎么回事。

10 spectrum   

n.谱，光谱，频谱；范围，幅度，系列

参考例句：

• This is a kind of atomic spectrum.这是一种原子光谱。
• We have known much of the constitution of the solar spectrum.关于太阳光谱的构成，我们已了解不少。

11 photosynthesis   

n.光合作用

参考例句：

• In apple trees photosynthesis occurs almost exclusively in the leaves.苹果树的光合作用几乎只发生在叶内。
• Chloroplasts are the structures in which photosynthesis happens.叶绿体就是光合作用发生的地方。

12 pigments   

n.（粉状）颜料( pigment的名词复数 )；天然色素

参考例句：

• The Romans used natural pigments on their fabrics and walls. 古罗马人在织物和墙壁上使用天然颜料。 来自辞典例句
• The original white lead pigments have oxidized and turned black. 最初的白色铅质颜料氧化后变成了黑色。 来自辞典例句

13 machinery   

n.(总称)机械，机器；机构

参考例句：

• Has the machinery been put up ready for the broadcast?广播器材安装完毕了吗？
• Machinery ought to be well maintained all the time.机器应该随时注意维护。

14 nutrients   

n.（食品或化学品）营养物，营养品( nutrient的名词复数 )

参考例句：

• a lack of essential nutrients 基本营养的缺乏
• Nutrients are absorbed into the bloodstream. 营养素被吸收进血液。 来自《简明英汉词典》

15 twigs   

细枝，嫩枝( twig的名词复数 )

参考例句：

• Some birds build nests of twigs. 一些鸟用树枝筑巢。
• Willow twigs are pliable. 柳条很软。

16 robust   

adj.强壮的，强健的，粗野的，需要体力的，浓的

参考例句：

• She is too tall and robust.她个子太高，身体太壮。
• China wants to keep growth robust to reduce poverty and avoid job losses,AP commented.美联社评论道，中国希望保持经济强势增长，以减少贫困和失业状况。