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  3. Lichens To be certain, a lichen is not the most conspicuous of plants. Lichens grow in unassuming fashion on rocks, logs and

Lichens To be certain, a lichen is not the most conspicuous of plants. Lichens grow in unassuming fashion on rocks, logs and

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问题                                Lichens
    To be certain, a lichen is not the most conspicuous of plants. Lichens grow in unassuming fashion on rocks, logs and other exposed surfaces in a wide range of habitats around the world. To the untrained eye they look like little more than crusty patches that, at first glance, might easily be mistaken for a discoloration of the surface. Even if the average person should happen to notice the lichen’s presence and correctly identify it as some form of life, he is unlikely to go much further in contemplating it. Though almost totally ignored by the layperson, for the botanist, lichens are one of the most fascinating of all plants, and one of the most intensely studied.
    They are the subject of so much scientific scrutiny primarily because a lichen is not just one plant. It is, in fact, a composite organism made up of fungus and algae living together in a close association that is, presumably, beneficial to both. When these two very different plants combine, the result is a unique and very long-lived composite organism that appears, at least on a macroscopic scale, to be a unitary plant. It is an organism that bears no resemblance to either of its constituents when they are observed individually. The separate fungal and alga) elements can be recognized only when the body of the plant, called a thallus because there are no stems or roots, is sectioned and examined under a microscope. When viewed this way, the fungus component dominates the picture, as it accounts for nine tenths of the total body mass of the lichen. But, entrapped within it, clearly visible as dark spots, are the algae cells. Essentially, nothing is known of how an amorphous mass of fungi and algae come together to form a highly differentiated, structurally stable body.
    Despite all the scientific scrutiny lichens have received, it is still not entirely certain what each member gains from the association.  Some researchers have speculated that the fungi join in the relationship because they are able to consume the algae cells as they die and therefore are guaranteed a food supply.  It is well-known that the chlorophyll-containing algae cells produce food by means of photosynthesis. There may be some mechanism, still unknown to us, through which this energy source is utilized by the fungus. Fungus possesses no chlorophyll of its own. How or even whether the algae benefit from this association is still less certain, though we can easily imagine that they gain mechanical protection from the elements by being tightly enveloped in the structural fibers of the fungus body. They should also benefit from retention of water between the fibers.
    The hardiness of lichens has made them what botanists term "pioneer plants". This refers to their ability to colonize habitats where other plants do not exist. They are common on barren rocky surfaces, where the lack of soil precludes the establishment of most other kinds of plant life. They can even be found in places as hostile and extreme as the interior of the Antarctic continent. Although they are most often associated with far northern or southern environments, they have been found living in sun baked desert soils that are otherwise devoid of life. The most highly specialized lichens are the endolithic species of the Antarctic, which as the name indicates, live inside rocks, forming more or less continuous tissue structures between the rock crystals.
    As remarkable in their robustness as lichens are, there is one kind of an environment which they are generally unable to tolerate. Habitats that are heavily affected by pollution axe noticeably devoid of lichens. These organisms are especially susceptible to sulfur dioxide poisoning and they absorb and accumulate other toxins as well; both air and waterborne. This heightened sensitivity arises from the fact that lichens have no means of ridding their tissues of these substances. It is thought that the pollutants accumulate and destroy the chlorophyll in the algae cells, thus disrupting the relationship with the fungus. This particular characteristic makes lichens an especially good indicator of environmental health. Surveys currently indicate that lichens are completely absent from urban centers with populations of 100,000 or more.
The word susceptible in paragraph 5 is closest in meaning to
选项 A、resistant.
B、vulnerable.
C、impervious.
D、impermeable.
答案B
解析
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