ols and colleges B.social recognition
C. socio-economic status D. certain behavioral changes
38. No one can expect to fully enjoy the adulthood privileges until he is __.
A. eleven years old B. sixteen years old
C. twenty-one years old D.between twelve and twenty-one years old
39. Starting from 22,__.
A. one will obtain more basic rights
B. the older one becomes, the more basic rights he will have
C. one won't get more basic rights than when he is 21
D. one will enjoy more rights granted bv society.
40. Acoording to the passage, it is true that
A. in the late 19th century in the United States the dividing line between adolescence and
adulthood no longer existed
B. no one can marry without the permission of his parents until the age of twenty-one
C. one is considered to have reached adulthood when he has a driver' s license
D. one is not free from the restrictions of child labor laws until he can join the army
3
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 com-
posed 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 car-
bon dioxide from the air may enter the leaf as a gas but is dissolved in water in the leaf be-
fore 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 car-
bon 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 l
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