首页
登录
职称英语
It is a common belief that sharp frosts color the autumn leaves. As a matter
It is a common belief that sharp frosts color the autumn leaves. As a matter
游客
2024-03-12
13
管理
问题
It is a common belief that sharp frosts color the autumn leaves. As a matter of fact, such frosts are far more likely to turn leaves black or dull brown than to give them the gorgeous tints we admire.
The coloring of the leaves in the fall is a chemical process that is favored by gradual cooling rather than sudden cold. It is not entirely confined to the autumn. Bright red and yellow leaves are often found on the swamp maple and other trees, even in summer.
Through the season of growth the leaves serve as food factories for the trees. In their tiny cells the carbon of the air is combined with materials brought up by the tree fluid from the roots to form the starch, sugar, and other substances by which the whole tree is fed. The food-making process is performed by sunshine with the aid of a substance called chlorophyll. Chlorophyll is a mixture of several pigments, or coloring matters. One of these is green, and gives the leaves their ordinary color. Another is yellow and is the same substance that, on account of its abundance in growing grass, makes butter particularly yellow in the spring.
When the cool weather sets in and the growth of vegetation slows down, the trees need less food and gradually suspend work in the leaf factories. Both the food and the chlorophyll in the leaves are drawn into the body of the tree and stored up for use in the spring. This transfer involves many chemical changes. One of them is the breaking up of the chlorophyll into the substances of which it is composed. The green pigment passes out of the leaves before the yellow. Thus yellow becomes one of the prevailing hues of the autumn foliage.
The reds, which also prevail in the autumn, do not come from the chlorophyll, but from pigments contained in the sap. Their appearance indicates an excess of sugar in the leaves, after the withdrawal of other materials. It is supposed, also, that the reddening of the leaves protects the food materials from the harmful effects of strong light during their passage into the tree. The same red coloring is seen in the buds of many plants in the spring, where it probably also serves a protective purpose. [br] Yellow prevails in the autumn leaves mainly because ______.
选项
A、the food-making process ceases when it becomes cool
B、chlorophyll breaks up into various pigments in cool weather
C、the yellow pigment remains in the leaves longer than the green
D、the transfer of chlorophyll involves complicated chemical changes
答案
C
解析
第4段末句开头的thus表明该句与前一句构成因果关系,选项C是对第4段倒数第2句的近义改写,为本题答案。
转载请注明原文地址:https://tihaiku.com/zcyy/3528957.html
相关试题推荐
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
Itisacommonbeliefthatsharpfrostscolortheautumnleaves.Asamatter
随机试题
会谈的地点一般应安排在()。A.主人的办公室 B.主人的会谈室 C.客人住
A.必绝对收敛 B.必条件收敛 C.必发散 D.可能收敛,也可能发散
35岁,曾生育2女孩,近半年来月经不调,8~2/26天,基础体温双相,月经第6天
体内哪种物质是不直接传递信息的()A.神经递质 B.调制物 C.内分泌激素
以下关于共同共有表述正确的是()。A、共同共有指两个或两个以上的人(自
患者,男性,50岁,4天前因胃胀,服用朱砂莲等中药治疗,于3天前突感恶心、呕吐,
招股说明书中,保荐机构的声明可由法定代表人授权代表签署。()
下列选择中,不属于资产托管机构办理集合资产管理计划资产托管业务应当履行的职责的是
过程分析是以产品制造过程为研究改进对象的一种分析技术,采用(),包括整体生产程序
职业性中毒中,生产性毒物进入体内的主要途径是A.皮肤 B.呼吸道 C.消化道
最新回复
(
0
)