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
2023-08-10
11
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] The nutrients for the trees are produced in ______.
选项
A、the tree fluid
B、the leave cells
C、the tree trunks
D、the chlorophyll
答案
B
解析
题干中的nutrients相当于第3段第2句中的by which the whole tree is fed,该句开头的in their tiny cells表明这些营养物形成的地方,而their指的就是上句提到的leaves,由此可推断选项B为本题答案。
转载请注明原文地址:https://tihaiku.com/zcyy/2912696.html
相关试题推荐
ContrarytocommonbeliefsocialmediawebsitessuchasFacebookdonot【C1】__
ContrarytocommonbeliefsocialmediawebsitessuchasFacebookdonot【C1】__
ContrarytocommonbeliefsocialmediawebsitessuchasFacebookdonot【C1】__
ContrarytocommonbeliefsocialmediawebsitessuchasFacebookdonot【C1】__
ContrarytocommonbeliefsocialmediawebsitessuchasFacebookdonot【C1】__
I’vealwaysbeenanoptimistandIsupposethatisrootedinmybeliefthat
I’vealwaysbeenanoptimistandIsupposethatisrootedinmybeliefthat
[originaltext]Sometimeswhenyoutakeacommondrug,youmayhaveasideeff
[originaltext]Sometimeswhenyoutakeacommondrug,youmayhaveasideeff
[originaltext]Sometimeswhenyoutakeacommondrug,youmayhaveasideeff
随机试题
Forthispart,youareallowed30minutestowriteashortessay.Youshouldsta
SharksSharksareamazingfishthathav
HowcanEnglishlearnersmakeuseofESLJournal?[br][originaltext]M:Thisis
InaBertelsmannFoundationstudyonsocialjusticereleasedthisfall,the
下列有关肌梭感受器,错误的是()A.传入神经为Ia类和Ⅱ类纤维 B.是感受
完全性右束支传导阻滞可出现哪种分裂A.S反常分裂 B.S生理分裂 C.S分裂
“学生是仓库,教师就是储蓄者”。这体现了()。A.“假学习” B.教师垄断
D本题考查的是元素轮换规律,每一组图共3类图形,且每个图形都分为内外两部分,注意各自的变化,其中内部小图形都是不一样的。故答案为D。
控制性详细规划的成果要求中,控制性详细规划的深度应以是否具有( )为准绳。A.
(2020年真题)工程设备验收时,核对验证内容不包括( )。A.核对设备型号规格
最新回复
(
0
)