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  3. Most growing plants contain much more water than all other materials combined. C. R. Barnes has suggested that it is as proper t

Most growing plants contain much more water than all other materials combined. C. R. Barnes has suggested that it is as proper t

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问题    Most growing plants contain much more water than all other materials combined. C. R. Barnes has suggested that it is as proper to term the plant a water structure as to call a house composed mainly of brick a brick building. Certain it is that all essential processes of plant growth and development occur in water. The mineral elements from the soil that are usable by the plant must be dissolved in the soil solution before they can be taken into the root. They are carried to all parts of the growing plant and are built into essential plant materials while in a dissolved state. The carbon dioxide (CO2) from the air may enter the leaf as a gas but is dissolved in water in the leaf before it is combined with a part of the water to form simple sugars—the base material from which the plant body is mainly built. Actively growing plant parts are generally 75 to 90 percent water. Structural parts of plants, such as woody stems no longer actively growing, may have much less water than growing tissues.
   The actual amount of water in the plant at one time, however, is only a very small part of what passes through it during its development. The processes photosynthesis, by which carbon dioxide and water are combined—in the presence of chlorophyll (叶绿素) and with energy derived from light—to form sugars, require that carbon dioxide from the air enter the plant. This occurs mainly in the leaves. The leaf surface is not solid but contains great numbers of minute openings, through which the carbon dioxide enters. The same structure that permits the one gas to enter the leaf, however, permits another gas—water vapor—to be lost from it. Since carbon dioxide is present in the air only in trace quantities(3 to 4 parts in 10, 000 parts of air) and water vapor is near saturation in the air spaces within the leaf (at 80 ℉ , saturated air would contain about 186 parts of water vapor in 10, 000 parts of air) , the total amount of water vapor lost is many times the carbon dioxide intake. Actually, because of wind and other factors, the loss of water in proportion to carbon dioxide intake may be even greater than the relative concentrations of the two gases. Also, not all of the carbon dioxide that enters the leaf is synthesized into carbohydrates.
The second paragraph uses facts to develop the essential idea that ______.
选项 A、a plant efficiently utilizes most of the water it absorbs
B、carbon dioxide is the essential substance needed for plant development
C、a plant needs more water than is found in its composition
D、the stronger the wind, the more the water vapor loss
答案C
解析 段落大意题。本段第一句为段落主题句：植物的含水量只是其需水量很小的一部分。然后用事实说明为什么会出现这种情况(植物叶子允许二氧化碳进人，却让水蒸气散失，再加上风和其他因素的作用，大部分的水都损失掉了)。故选C。
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