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  3. The forest from which Man takes his timber is the tallest and most impressive plant community on Earth. In terms of Man’s brief

The forest from which Man takes his timber is the tallest and most impressive plant community on Earth. In terms of Man’s brief

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问题     The forest from which Man takes his timber is the tallest and most impressive plant community on Earth. In terms of Man’s brief life it appears permanent and unchanging, save for the seasonal growth and fall of the leaves, but to forester it represents the climax of a long succession of events.
    No wooded landscape we see today has been forest for all time. Plants have minimum requirements of temperature and moisture and, in ages past, virtually every part of Earth’s surface has at some time been either too dry or too cold for plants to survive. However, as soon as climatic conditions change in favour of plant life, a fascinating sequence of changes occurs, called a primary succession.
    First to colonize the barren land are the lowly lichens, surviving on bare rock. Slowly, the acids preduced by these organisms crack the rock surface, plant debris accumulates, and mosses establish a shallow root-hold. Ferns may follow and, with short grasses and shrubs, gradually form a covering of plant life. Roots probe even deeper into the developing soil and eventually large shrubs give way to the first trees. These grow rapidly, cutting off sunlight from the smaller plants, and soon establish complete domination-closing their ranks and forming a climax community which may endure for thousands of years.
    Yet even this community is not everlasting. Fire may destroy it outright and settlers may cut it down to gain land for pasture or cultivation. If the land is then abandoned, a secondary succession will take over, developing nmch faster on the more hospitable soil. Shrubs and trees are among the early invaders, their seeds carried by the wind, by birds and lodged in the coats of mammals.
    For as long as it stands and thrives, the forest is a vast machine, storing energy and many elements essential for life.
In a "primary succession", what makes it possible for mosses to take root?
选项 A、The type of rock.
B、The amount of sunlight.
C、The amount of moisture.
D、The effect of lichens.
答案D
解析 第三段的一、二句话中，作者提到了首先能在岩石上生存的植物是lichen(地衣)，然后在地衣生长之后，由于地衣的影响才能使moss(苔藓))的生长成为可能。
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