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新托福阅读背景知识：植物适应沙漠

2018-08-10 11:47:49来源：网络

　　The ribs are special structures that are also used for enduring extreme heat. The ribs (and spines) trap wind so that the plant is enveloped in a layer of extremely still air, and this is a very important factor in reducing evaporation. On very windy days even the ribs don't help and cacti sometimes wilt because of high water loss.

　　The spines have different functions. They not only help shade the plant from the sun but are also known to help the cactus absorb water. They do it like this. On cool nights, dew settles on the spines of the plant. The spines are actually known to draw droplets of water towards the areole (the point out of which the spines grow) and here the droplets are absorbed. You can try this at home. Spray the plants with a very fine mist of water and watch what happens to the droplets that settle on the spines. They literally get attracted to the areole along the spine. The spine's structure allows them to do this. Even spines pointing downwards seem to suck the droplets up themselves.

　　Adaptation features are visible in this Pilosocereus glauchochorous. Notice the spines, ribs, fur and wax (the blue coloration). The top of a typically adapted plant.

　　Some plants have fur; sometimes all over the plant, sometimes only near the top. This fur shades the plant even further and is also known to attract water towards the areole. Some plants only have fur near the top. This is very beneficial because the top of the plant is very sensitive to sunlight, new tissues get formed there. Young areoles, with their spines not even wooded yet can get dried up completely in the sun. When an areole is born near the top of the plant, it starts developing spines. At this time the fur appears as well. This fur accompanies the areole as it moves down the plant, shading the growing point inside. By the time the areole is about 15cm away from the top, the fur wears out completely and the now inactive areole gets exposed to the sun.

　　As for the roots of cacti, they are also fully adapted to living in the desert. Some species (especially plants from very dry deserts) have very shallow root systems that spread very far from the plant. This way the plant can take advantage of tiny amounts of moisture from dew or light rain as the roots spread far away and are very shallow (less than 10cm deep while spreading up to 5 meters from the parent plant). On the other hand, some cacti send their roots deep down (like many Echinocacti) to reach the ground water.

　　Rainforest cacti often have aerial roots that can collect water all the time when it rains (and it rains very often in South American forests).

　　The shape of cacti itself is an adaptation. You may have noticed that cacti have a barrel like or candle like shape. This allows for maximum internal volume with a minimum surface area, which is also very smart adaption as a cactus can store a lot of water and have a small external surface area to reduce water loss.

　　以上就是为大家整理的“新托福阅读背景知识：植物适应沙漠”，更多精彩内容请关注新东方在线托福频道!


本文关键字: 托福阅读背景知识 托福阅读 托福

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