新托福阅读背景知识：植物适应沙漠

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　　Plant adaptation to the desert(背景材料)

　　Cactus adaptations.

　　The secret to the superior endurance of cacti lies in their adaptations. Over millions of years, through natural selection, only the strongest and best adapted species survived.

　　As you know, it is very dry in the desert. Plants that adapt to this are known as xerophytes (from zeros, dry and python, plant). There are plants that avoid the dry season by sprouting from seed just after the spring rain and growing very fast so that by the time the dry season comes, they have already produced a lot of seeds and died. These seeds lie on the soil for the dry season and sprout again in spring and the cycle repeats. Other xerophytes simply drop their leaves and stay dormant for the winter. But there is another special type of xerophytes which stores water in its fleshy tissues. Such plants are called succulents (from success, juicy). The cactus is a typical example of a succulent.

　　If you cut a cactus open, you see a juicy, slimy tissue. This is where the moisture is stored for the dry season. The part between the middle circle (and pith) and just under the very green part of the plant (or palisade parenchyma) just under the skin is allocated for the storage of water and food for the plant. This is a type of spongy parenchyma and can take up to 85% of the plant's volume. This is a major adaptation in the desert. Because the plant remains completely alive during the dry season and there is no need for it to dry up and lose everything, makes it possible for the plant to grow to large sizes. Another advantage is that the plant retains supplies (in the form of starch) for the winter so that it can flower right away in spring without accumulating more supplies (as most plants need to do in spring). The whole purpose of storing supplies for the winter is mostly to energize flowering in spring but it also lets the cactus start growing much sooner.

　　Flowering plants breathe and transpire (evaporate water from their surface) through closeable microscopic pores called stoats on the leaves or stems. To do this, their pores have to be open. In most plants these are open all day and on warm nights. But for cacti this is inconvenient as in daytime it is very hot and thus the plant would lose a lot of water through evaporation. So the cactus must close them in the daytime. But then it cannot breathe or photosynthesize (the process where sugars are made from carbon dioxide and water and releasing oxygen using the sun's energy). Succulents have an adaptation to that. Their stoats are closed during the day and are open at night, when it is not that hot and store carbon dioxide in its tissues as crass lean acid and then turn it back to carbon dioxide in the daytime. This process is called crass lean acid metabolism or CAM and it is a very smart way of respiring in the desert.

　　If we look at the outside of the plant, we notice that there is a tough leathery skin covering the plant, we can also notice the presence of ribs and spines and sometimes fur. These are all very smart adaptations. They serve mainly for surviving heat but are also used as defense.

　　The tough leathery skin is very impermeable to water, thus reducing evaporation from the surface of the plant. This skin often has a layer of plant wax on it which is often lightly colored (Pilosocereus azures is an example of a plant with such wax), white or blue. This reflects light and also reduces evaporation from the inside. 第二页继续...