题目内容
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 stem 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 tress, 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 form. the evaporation of water at the top of the plant. As water is lost form. 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 cohesive strength permits columns of water to be pulled to great heights without being broken.
How many theories does the author mention？
查看答案
搜索结果不匹配？点我反馈
更多问题
According to the last paragraph, consumers who believe that organic foods are better than
Part B
Directions: You will hear four dialogues or monologues. Before listening to each one, you will have 5 seconds to read each of the questions which accompany it. While listening, answer each question by choosing A, B, C or D. After listening, you will have 10 seconds to check your answer to each question. You will hear each piece ONLY ONCE.
听力原文： No organ of the body is less appreciated than the skin, one of our biggest organs -- and the most telltale -- it mirrors the state of our health, our age, and our race. Occupation and way of life may also become indelibly imprinted for all to see. Here some facts about our outer covering organ to show that there is more to the skin than meets the eye.
Your skin is your body's thermostat. It adjusts heat gain and loss so that the internal temperature remains within the normal range of 97 to 99 degrees.
It takes about two square yards of skin to cover the average adult from head to toe. Undoubtedly the most intricate wrapping in existence, the skin contains 3 million cells, three feet of blood vessels, and twelve feet of nerves . in each area the size of a postage stamp.
The widest difference between the darkest human skin and that of an albino is brought about by not more than 1/25 of an ounce of melanin. As far as is known, the amount of this dark brown or black pigment accounts for all our variations in racial colors, including yellow, brown, and black.
What is the skin？
A．His stories were inspired by his travels,B．His travels prevented him from writing.C．
Section B
Directions: In this section, you will hear 3 short passages. At the end of each passage, you will hear some questions. Both the passage and the questions will be spoken only once. After you hear a question, you must choose the best answer from the four choices marked A, B, C and D.
听力原文： The first English dictionary was published in 1604. The dictionary was actually nothing more than a list of about 3,000 difficult words, each followed by a one word definition. The author, Robert Cadre, made no attempt to include everyday words in his dictionary. During the 1600s more dictionaries were published. Each followed Cadre's lead and presented a few thou sand hard words. Around 1700 one dictionary maker, John Kersey, did define easy words as well as hard ones. But until the 1750s all dictionaries were rather crude and not very valuable.
A man named Dr. Samuel Johnson changed all this. In 1755, Dr. Johnson produced the first modern dictionary. He included in his dictionary all. important words, and he gave good meanings. By the end of the 1700s most dictionary makers had followed Johnson's lead. Dictionaries were getting better and better.
The 1800s saw the greatest improvement in the quality of dictionaries. In England scholars planned and prepared the Oxford English Dictionary. One of the most interesting features of the Oxford Dictionary is its word histories. It traces the history of each word from its earliest recorded use up to the time of the printing of the dictionary.
(27)
微信支付 
支付宝支付