单选题
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
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 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 chlorophyll 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°F, 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.
单选题
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
【正确答案】
D
【答案解析】
单选题
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
【正确答案】
A
【答案解析】
单选题
The second paragraph uses facts to develop the essential idea that
______.
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
【正确答案】
C
【答案解析】
单选题
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.