新东方背诵50篇——托福听力专用(13)

像所有的熊一样，黑熊胆小，笨拙，很少具有危险性。但如果受到攻击，大部分黑熊会以很快的速度爬上树和奔跑。当发怒或受惊吓时，黑熊会成为可怕的对手。

黑熊以树叶、草、树根、水果、浆果、昆虫、鱼，甚至更大的动物为食。熊类，包括黑熊的最有趣的一个特点是他们的冬眠。与松鼠、旱獭和其它别的林地动物不 同，熊并不真正地冬眠。虽然熊在冬天的几个月中不吃东西，靠体内脂肪维持生命，但它们的体温保持正常，并有规律地一分钟呼吸4或5次。

除交配季节外，大多数黑熊独自生活。他们喜欢住在洞里、空心的大木头里或茂密的树丛里。经过6到9个月的怀孕期后一胎1～4个小熊在1月或2月出生。它们同母熊住在一起，直到它们完全长大，即1岁半左右。黑熊在野外可以活到长达30年，在专门的保护区中甚至能活得更长。

Cohesion-tension Theory 内聚压力理论

2009-09-30 来源: 【大 中 小】 我要投稿 下载音频

Atmospheric pressure can support a column of water up to 10 meters high. But plants can move water much higher; the sequoia tree can pump water to its very top more than 100 meters above the ground. Until the end of the nineteenth century, the movement of water in trees and other tall plants was a mystery. Some botanists hypothesized that the living cells of plants acted as pumps. But many experiments demonstrated that the stems of plants in which all the cells are killed can still move water to appreciable heights. Other explanations for the movement of water in plants have been based on root pressure, a push on the water from the roots at the bottom of the plant. But root pressure is not nearly great enough to push water to the tops of tall trees. Furthermore, the conifers, which are among the tallest trees, have unusually low root pressures.

If water is not pumped to the top of a tall tree, and if it is not pushed to the top of a tall tree, then we may ask: how does it get there According to the currently accepted cohesion-tension theory, water is pulled there. The pull on a rising column of water in a plant results from the evaporation of water at the top of the plant. As water is lost from the surface of the leaves, a negative pressure, or tension, is created. The evaporated water is replaced by water moving from inside the plant in unbroken columns that extend from the top of a plant to its roots. The same forces that create surface tension in any sample of water are responsible for the maintenance of these unbroken columns of water. When water is confined in tubes of very small bore, the forces of cohesion (the attraction between water molecules) are so great that the strength of a column of water compares with the strength of a steel wire of the same diameter. This