长喜英语.2012大学英语6级考试:全真试题&标准模拟2011年12月-2006年6月(12套真题+2套模拟+6套听力+30篇作文+3000分频词)(第二版)(附MP3光盘1张)

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The Forest for the Trees

The ample sunlight and extremely wet climate of many tropical areas encourage the growthof towering trees with wide canopies. This thick top layer of the rainforest dictates the lives of allother plants in the forest. New tree seedlings rarely survive to make it to the top unless some oldertrees die, creating a "hole" in the canopy. When this happens, all of the seedlings on the groundlevel compete intensely to reach the sunlight.
Many plant species reach the top of the forest by climbing the tall trees. It is much easier toascend this way, because the plant doesn't have to form its own supporting structure.
Some plant species, called epiphytes, grow directly on the surface of the giant trees. These plants,which include a variety of orchids and ferns, make up much of the under story, the layer of the rainforestright below the canopy. Epiphytes are close enough to the top to receive adequate light, and the runofffrom the canopy layer provides all the water and nutrients (养分 ) they need, which is important sincethey don't have access to the nutrients in the ground.
Stranglers and Buttresses
Some epiphytes eventually develop into stranglers. They grow long, thick roots that extend downthe tree trunk into the ground. As they continue to grow, the roots form a sort of web structure all aroundthe tree. At the same time, the strangler plant's branches extend upward, spreading out into the canopy.Eventually, the strangler may block so much light from above, and absorb such a high percentage of nu-trients from the ~round below, that the host tree dies.

装　　帧：袋装

开　　本：16开

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