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Most growing plants contaln much more water than all other materials combined. C. R.Darnes 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 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.
Stnlctural parts of plants, such as woodv stems no longer actively growing, may have much less water than growlng tissues.
The actual amount of water in the plant at any one time, however, is only a very small part of what passes through it during its development. The processes of photosynthesis, by which carbon dioxide and water are combined-in the presence of chlorophvll (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 1O ,000 parts of air) and water vapor is near saturation in the air spaces within the leaf (at 80"F , saturated air would contain about 186 parts of water vapor in 1O, OOO 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 (碳水化合物) .
41 . A growing plant needs water for all of the following except__.
A. forming sugars B. sustaining woody stems
C. keeping green D. producing carbon dioxide
42 . The essential function of photosynthesis in terms of plant needs is__.
A. to form sugars B. to derive energy from light
C. to preserve water D. to combine carbon dioxide with water
43 . The second paragraph uses facts to develop the essential idea that__.
A. a plant efficiently utilizes most 0f 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
44 . According to the passage, which of the following statements is TRUE?
A. The mineral eLements will not be absorbed by the plant unless they are dissolved in its root .
B. The woody stems contain more water than the leaves.
C. Air existing around the leaf is found to be saturated.
D. Only part of the carbon dioxide in the plants is synthesized.
45 . This passage is mainly about__.
A. the functions of carbon dioxide and water
B. the role of water in a growing plant
C. the process of simple sugar formation
D. the synthesis of water with carbon dioxide
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