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

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

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问题     Most growing plants contain much more water than all other materials combined. C.R. Dames has suggested that it is as proper to term the plant a water structure as to call a house composed mainly of bricks as a brick building. It is certain that all the 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 (CO(下标)2) from the air may enter the leaf as a gas but is dissolved 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 any one time, is only a very small part of what passes through it during its development. The processes of photosynthesis (光合作用), 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 leaf. 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 80F, 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. Also, not all of the carbon dioxide that enters the leaf is synthesized into carbohydrates.
The essential function of photosynthesis in terms of plant needs is ______.
选项 A、to derive energy from light
B、to preserve water
C、to combine carbon dioxide with water
D、to form sugars
答案D
解析 句意理解题。根据第2段第2句：在光合作用过程中，二氧化碳与水在叶绿素和光能的作用下相互结合以形成糖分。这就要求空气中的二氧化碳进入植物。可见，就植物生长需要而言，光合作用的基本功能是为了产生糖分，而不是为了使二氧化碳与水结合，也不是为了摄取光能，故选D。
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