叶片和外界进行气体交换的“窗户”是（ ）
A．气孔
B．导管
C．保卫细胞
D．筛管
叶片的结构由表皮、叶肉和叶脉组成．
表皮有表皮细胞和保卫细胞组成，表皮细胞排列紧密，起保护作用．
保卫细胞成对出现，呈半月形，中间是气孔，气孔的张开和闭合受保卫细胞控制：保卫细胞吸水膨胀时，气孔张开；保卫细胞失水时，气孔闭合．气孔不但是气体交换的“窗口”，也是水分散失的“门户”．
叶脉中有导管运输水分和无机盐，筛管运输有机物，并且具有保护和支持作用．
叶肉主要有海绵组织和栅栏组织构成，里面含有叶绿体，是进行光合作用的主要场所．
故选：A
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| Adults are often surprised by how well they remember something they learned as children but have never practised in the meantime. A man who has not had an opportunity to go swimming for years can 1 swim as well as ever when he gets back in the water. He can get on a bicycle after several decades and still 2 away. A mother who has not 3 the words for years can teach her daughter the poem that begins "Twinkle, twinkle, little star" or recite the story of Cinderella or Snow White. One explanation is the law of overlearning, which can be stated as follows: 4 we have learned something, additional learning increases the 5 of time we will remember it. In childhood, we usually continue to practise such skills as swimming, bicycle riding long after we have learned them. We continue to listen to and 6 ourselves of poems such as "Twinkle, twinkle, little star" and childhood tales such as Cinderella or Snow White. We not only learn but 7 . The law of overlearning explains why cramming (突击学习) for an examination, 8 it may result in a passing grade, is not a 9 way to learn a school course. By cramming, a student may learn the subject well enough to get by on the examination, but he is likely soon to forget almost everything he learned. A little overlearning; 10 , is usually a good investment toward the future. | ||||
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