托福考试

Pitcher plants are carnivorous plants whose prey-trapping mechanism features a deep cavity filled with liquid known as a pitfall trap. It has been widely assumed that the various sorts of pitfall trap evolved from rolled leaves, with selection pressure favouring more deeply cupped leaves over evolutionary time. However, some pitcher plant genera (such as Nepenthes) are placed within clades consisting mostly of flypaper traps: this indicates that this view may be too simplistic, and some pitchers may have evolved from flypaper traps by loss of mucilage....

After spawning the salmon die. All North Pacific salmon die after spawning. The females may guard their nests for a week or two after spawning if they still are alive. They get weaker each day and eventually are washed downstream and end up on the river banks....

Male salmon arrive first and start to stake out territories. They can be seen biting each other and displaying their kype. The best areas are in gravel beds where the gravel is not too small nor too large and there is the right amount of water flow to oxygenate the eggs without washing them out of the gravel. Sockeye salmon pair. ...

Adult salmon return to their natal stream for reproduction. This migration occurs in a slightly different time frame for each species and for each stream. In general it occurs during summer or fall in the five common North Pacific salmon. It may be triggered by day length, water temperature or other environmental changes. Some salmon swim thousands of miles to get to the mouth of the stream where they were born. ...

Just why marine animals migrate such vast distances to return to their own birthplace, sometimes bypassing other suitable locations along the way, is not known. Scientists speculate that natal homing evolved because individuals that returned to their home areas to reproduce left more offspring than those that tried to reproduce elsewhere....

把美国西南部周围年轮的数据收集起来，同100年来的气象记录进行比较，就会看出年轮如何反映出气候。因此，对于没有气象记录的时期，我们从一环年轮形成的情况可以推断出当时的气候。弗里茨就这样把美国西部和太平洋北部的气象图编制到大约公元1600年。...

关于动物间的合作有很多，如某些小虾和海葵之间；珊瑚鳟和隆头鱼之类担任“清洁工作”的鱼之间的关系。太平洋中有一种大珊瑚——石芝，呈美丽的翠绿色，非常漂亮，这是因为其组织中共生着一种微小的海藻的缘故等等，今天我们介绍的是蜜獾与响蜜鴷的共同觅食。...