It’s a sound you will probably never hear, a sickened tree sending out a distress signal. But a group of scientists has heard the cries, and they think some insects also hear the trees and are drawn to them like vultures to a dying animal. Researchers with the US Department of Agriculture’s Forest Service fastened sensors to the bark of drought-stricken trees clearly heard distress calls. According to one of the scientists, most parched trees transmit their plight in the 50-hertz to 50-kilohertz range. (The unaided human ear can detect no more than 20 kilohertz). Red oak, maple, white pine, and birch all make slightly different sounds in the form of vibrations at the surface of the wood.
The scientists think that the vibrations are created when the water columns inside tubes that run along the length of the tree break, a result of too little water following through them. These fractured columns send out distinctive vibration patterns. Because some insects communicate at ultrasonic frequencies, they may pick up the trees’ vibration and attack the weakened trees. Researchers are now running tests with potted trees that have been deprived of water to see if the sound is what attracts the insects. “Water-stressed trees also smell differently from other trees, and they experience thermal changes, so insects could be responding to something other than sound,’’ One scientist said.
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Question 31: Which of the following is the main topic of the passage?
Question 32: The word “them” in line 3 refers to _________ .
Question 33: The word “parched” in paragraph 1 is closest in meaning to _________ .
Question 34: It can be inferred from the passage that the sounds produced by the trees ________.
Question 35: The word “plight” in paragraph 1 is closest in meaning to ________ .
Question 36: All the following are mentioned as possible factors in drawing insects to weakened trees EXCEPT ________ .
Question 37: Which of the following could be considered a cause of the distress signals of trees?
Question 38: The word “fractured” in paragraph 2 is closest in meaning to _________ .
Question 39: It can be inferred from the passage that research concerning the distress signals of trees _________ .
Question 40: The phrase “pick up” in paragraph 2 could best be replaced by which of the following?