题干

科学家温特做了如下实验:把切下的燕麦尖端放在琼脂块上,几小时后,移去胚芽鞘尖端,将琼脂块切成小块.再将经处理过的琼脂块放在切去尖端的燕麦胚芽鞘一侧,结果胚芽鞘会朝对侧弯曲生长.但是,如果放上的是没有接触过胚芽鞘尖端的琼脂块,胚芽鞘则既不生长也不弯曲.该实验证明了(    )

A:生长素只能从形态学上端运输到形态学下端

B:造成胚芽鞘弯曲的刺激是某种化学物质

C:生长素的化学本质是吲哚乙酸

D:胚芽鞘会弯向光源生长

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答案(点此获取答案解析)

B

同类题3

完形填空
阅读下面短文,从各题A、B、C、D四个选项中,选出可以填入空白处的最佳答案。
    “How does the yard look?” My father asked, with his eyes shining. I could tell he was eagerly waiting for my 1. “Wonderful!” I replied after I gave it a thorough 2 . Then I recited all the changes I had 3 in his yard and he smiled contently.
    When I was young, my mother 4 in a car accident, leaving my father alone to 5 their three young daughters. At the beginning, life was not always6and my younger sisters usually complained. However, Dad always 7 us to believe that life was good. Meanwhile, he tried his best to 8that belief.
    In 1972, my dad developed a piece of waste land that had been9 on Okaloosa Island. Every year at the first sign of 10 , he would begin spending countless hours working there to make it be bursting with colors. Through his continuous hard work, the land 11turned into a beautiful garden with different types of flowers and other unique plants. While our friends were enjoying their vacation through traveling to different places, we were 12 working with my dad in his yard, watering the flowers, weeding or cutting off the branches. Sometimes, our friends were envious of our working vacation, and we would 13 them. Dad made his yard very 14 so that it became our vacation paradise during our childhood.
    For so many years, when we were 15 , we liked to visit Dad's yard, because it could 16us of Dad's belief. It was Dad and his yard that gave us 17 that enabled us to survive and 18 major challenges in life. One day, it shocked us that Dad said he would not 19 the yard. Then he explained, “Twenty years ago I decided to start my work in that yard to tell you life is good. Now my work is done.” We were sitting in 20, recalling what we had experienced. Life was good. Dad was right.