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  3. Lichens. probably the hardiest of all plants, live where virtually nothing else can---not just on rugged mountain peaks but

Lichens. probably the hardiest of all plants, live where virtually nothing else can---not just on rugged mountain peaks but

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问题          Lichens. probably the hardiest of all plants, live where virtually nothing else can---not
     just on rugged mountain peaks but also on sunbaked desert rocks. They are usually the
     first life to appear on a mountainside that has been scraped bare by an avalanche.
     Unlike other members of the plant kingdom, lichens are actually a partnership between
5    two plants. The framework of a lichen is usually a network of minute hairlike fungus that
     anchors the plant, The other component is an alga (similar to the green film of plant life
     that grows on stagnant pools) that is distributed throughout the fungus. Being green plants,
     algae are capable of photosynthesis--that is, using energy from the Sun to manufacture
     their own food. The fungi arc believed to supply water, minerals, and physical support to
10   the partnership.
         Lichens are famous for their ability to survive ~ water shortage. When water is scarce
     (as is often the case on a mountain), lichens may become dormant and remain in that
     condition for prolonged periods of time. Some lichens can even grow where there is no
     rain at all, surviving on only occasional dew--the moisture that condenses on the surface
15   of the plants at night, And unlike most other plants, lichens are little affected by the strong
     ultraviolet rays in the mountains.
         Lichens use little energy, for they grow slowly. Some grow so slowly and are so old
     that they are called "time stains." You may find lichens that are centuries old; certain of
     these lichen colonies have been established for an estimated 2,000 years.
20       For decades, scientists wondered how the offspring of an alga and a fungus got together
     to form a new lichen, it seemed unlikely that they would just happen lo encounter one
     another. It was finally discovered that in many cases the two partners have never been
     separated. Stalklike "buds" that form on certain lichens are broken off by the wind or by
     animals; these toll or are blown to a new location
选项 A、Where can the oldest lichens be found?
B、How long does it take for lichens to establish themselves?
C、How large can lichens he?
D、Where do lichens usually occur?
答案D
解析
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