请阅读 Passage 2 ,完成第 26 ~ 30小题。

                                                                                  Passage 2

 Most growing plants contain much more water than all other materials combined. C. R. Barnes hassuggested 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 rooL. They are carried Lo all parts of the growing plant and are built into essential plant materials while in a dissolved state. The carbon dioxide( C02 ) 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 t0 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 carhon dioxide and water are combined-in the presence of chlorophyll and with energy derived from light-to form sugars,require thaL 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 minuLe openings,through which the carbon dioxide enters. The same structure that permits the one gas to enter the leaf,however,permits another gas-water vapm-to be lost from it. Since carbon dioxide is present in the air only in trace quantities(3 to4 parts in 10000 parts of air) and water vapor is near saturation in the air spaces within the leaf(at 80degree  F,saturated air would contain about 186 parts of water vapor in 10000 parts of air) ,the total amount of water vapor lost is many times the carbon dioxide intake. Actually,because of wind and otherfactors,the loss of water in proportion to carbon dioxide intake may be even greater than the relativeconcentrations of the two gases. Also,not all of the carbon dioxide that enters the leaf is synthesizedinto carbohydrates.