问题 问答题 简答题

水稻种子萌发时,表现出“干长根,湿长芽”现象的原因何在?

答案

参考答案:

“干长根,湿长芽”现象是由于根和胚芽鞘的生长所要求的含氧量不同所致。

根的生长,既有细胞的伸长和扩大,也包括细胞分裂,而细胞分裂需要有氧呼吸提供能量和重要的中间产物。因而水多、氧不足时,根的生长受到抑制。但是胚芽鞘的生长,主要是细胞的伸长与扩大,在水层中,水分供应充足,故而芽生长较快。此外,“干根湿芽”还与生长素含量有关。在水少供氧充足时,IAA氧化酶活性升高,使IAA含量降低,以至胚芽鞘细胞伸长和扩大受抑制,根生长受影响小。

而在有水层的条件下,氧气少,IAA氧化酶活性降低,IAA含量升高,从而促进胚芽鞘细胞的伸长,并且IAA运输到根部,因根对IAA比较敏感,使根的生长受到抑制。还有人认为,胚芽鞘呼吸酶以细胞色素氧化酶为主,与O2亲和力高,幼根则以抗氰氧化酶为主,与O2亲和力较低,因而在水多时,胚芽鞘生长快于幼根。

阅读理解

Today almost everyone knows computers and the Internet. If I ask you “what is the most important in your life?” maybe you will say “Computers and the Internet.”

The first computer was made in 1946. It was very big but it worked slowly. Today computers are getting smaller and smaller. But they work faster and faster, What can computers do? A writer has said, “People can’t live without computers today.”

The Internet came a little later than computers. It is about twenty-five years later than computers. But now it can be found almost everywhere. We can use it to read books, write letters, do shopping, play games or make friends.

Many students like the Internet very much. They often go into the Internet as soon as they are free. They make friends on the Internet and maybe they have never seen these friends. They don’t know their real names, ages, and even sex(性别). They are so interested in making the “unreal friends “that they can’t put their hearts into study. Many of them can’t catch up with others in many subjects because of that.

We can use computers and the Internet to learn more about the world. But at the same time, we should remember that not all the things can be done by computers and the Internet.

小题1:The Internet can not be used for______.

A. studying

B. shopping

C. thinking

D. playing小题2:When the computer was invented, it was______.

A. large and worked quickly

B. small and worked slowly

C. large but worked slowly

D. small but worked quickly小题3:The Internet was born in about______.

A.1960

B.1970

C.1980

D.1985小题4:Which of the following is true ?

A. Few students like going into the Internet.

B. Students use the Internet to make “unreal friends.”

C. These “unreal friends” often meet each other.

D. Students know the friends on the Internet very well.小题5:What does the writer think of the Internet?

A. It is wonderful.

B. It can make students study harder.

C. It is not good for students.

D. It is helpful, but we can’t do everything on it.

填空题