Mountaineers have noted that as they climb, for example, up to the 12,633­foot Humphreys Peak in Arizona, plant life changes greatly. In the Sonoran Desert, one climbs into a pine forest at 7,000 feet and a treeless tundra(冻土带) on the top of the mountain. It may seem that plants at a given altitude are associated in what can be called “communities” －groupings of species. The idea is that over time, plants that require particular climate and soil conditions come to live in the same places, and are frequently to be found together. Scientists study the history of plant life and build up a picture of how groups of plants have responded to climate changes and how ecosystems develop. But are these associations, which are real in the present, permanent?

    A great natural experiment took place on this planet between 25,000 and 10,000 years ago, when small changes in the earth's orbit caused great sheets of ice to spread from poles. These glaciers(冰川) covered much of North America and Europe to depths of up to two miles, and then, as the climate warmed, they retreated. During this retreat, they left behind newly uncovered land for living things to occupy, and as those living things moved in they laid down a record we can read now. As the ice retreated and plants started to grow near a lake, they release pollen(花粉). Some would fall into the lake, sink to the bottom and mix with the sand. By drilling into the lake bottom it is possible to read the record of the plant life around the lake. The fossil record seems clear; there is little or no evidence that entire groups of plants moved north together. Things that lived together in the past don't live together now, and things that live together now didn't live together in the past. Each individual living things moved at its own pace. The fossil record seems to be telling us that we should be thinking about preserving species by giving them room to move about－to respond to environmental changes.

    Chinese students always complain that they're under great pressure and don't get enough sleep. But they may be surprised to learn that they're much luckier than their peers (同龄人) in South Korea and Japan, according to a recently published report, conducted by Shanghai Academy of Social Sciences on the physical and mental health of senior high school students in the three Asian countries.

    According to the study among nearly 6,000 high school students in the three countries, Chinese students are the tallest but poorest in physical fitness. Chinese students are on average 168.17cm, compared with 167.61cm for South Korean students and 164.70cm for Japanese students. Chinese students asked for sick leave more than students in other countries. About 72 percent of Korean students never asked for sick and injury leave during high school, compared to 53 percent for Japanese students and 45 percent for Chinese students. Moreover, Chinese students drink alcohol and smoke earlier than Korean and Japanese students. Around 70 percent of Chinese students have drunk alcohol, compared to 50 percent for Korean and Japanese students.

    According to the survey, Japanese and South Korean students suffer more pressure than their Chinese peers. Statistics (统计) show 16 percent of students in China said they experienced mental pressure often over the past year, while the numbers in Japan and South Korea were 33 percent and 47 percent respectively. Their worries come from “study”, “future after graduation”, “friendships” and “relationship with their parents”. To go with stress, 57.9 percent of Korean students sleep the least, less than six hours a day, compared with 46.6 percent of Japanese students and 30 percent of Chinese students.

    More Chinese students were satisfied with their physical appearance, double that of students in the survey from Japan and South Korea. The report said that over half of students in South Korea had tried to lose weight in the past year, while in China the figure was only 22 percent.

    In measures of self-worth, such as “I think I am valuable” and “I think I am successful”, “I can complete many things”, “I won't give up to failure,” Chinese students showed greater self-confidence than students in Japan and South Korea.

    Dreams can be familiar and strange, fantastical or boring, but some dreams might be connected to the mental processes that help us learn. In a recent study, scientists found a connection between nap time dreams and better memory in people who were learning a new skill.

    In the study, 99 college students between the ages of 18 and 30 each spent an hour on a computer, trying to get through a virtual maze (逃宫). The maze was difficult, and the study participants had to start in a different place each time they tried, making it even more difficult. They were also told to find a particular picture of a tree and remember where it was.

    For the first 90 minutes of a five - hour break, half of the participants stayed awake and half were told to take a short nap. Participants who stayed awake were asked to describe their thoughts. Participants who took a nap were asked about their dreams after sleep and they were awakened within a minute of sleep to describe their dreams.

    Stickgold, a neuroscientist, wanted to know what people were dreaming about when their eyes weren't moving during sleep.Four of the 50 people who slept said their dreams were connected to the maze. Some dreamed about the music that had been playing when they were working; others said they dreamed about seeing people in the maze. When these four people tried the computer maze again, they were able to find the tree faster than before their naps.

    Stickgold suggests the dream itself doesn't' help a person learn, it's the other way around. He suspects that the dream was caused by the brain processes associated with learning.

    All four of the people who dreamed about the task had done poorly the first time,which makes Stickgold wonder if the dreams show up when a person finds a new task particularly difficult. People who had other dreams, or people who didn't' take a nap, didn't show the same improvement.

    An earthquake is one of the most common natural disasters. It may cause great damage. So it is wise to learn some simple safety tips to protect yourself or your family members.

    Fragile items, like those made of glass should usually be placed on a lower surface, near the ground instead of placing them on cupboards higher up. Never place them near your bed, sofas and other furniture where you would be sitting or lying down. When there is a strong movement, these pieces will fall on the floor directly and not on you.

    There is a strong chance of short circuit (短路) and fire breakouts during an earthquake. Make sure you turn off electrical connections and gas immediately when an earthquake happens.

    During an earthquake, lie beneath an object that is not easily damaged. Do not go near objects that could directly fall on you. Never use the elevator to go down. Stay inside until the shaking stops and it is safe to go outside. Research has shown that most injuries happen when people inside buildings try to go out. Use the staircases at all times.

    If you are outdoors, do not take shelter under a tree, streetlights, electric poles or tall buildings. If you are driving, stop your car and stay in a safe place. Do not park your car under a tree or any tall object.

    If trapped in debris (瓦砾堆), cover your mouth with a handkerchief or clothing. Tap on a pipe or wall so rescuers can find you. Use a whistle (哨子) if you have one. Never shout for help. Shouting can cause you to breathe in dangerous amounts of dust. Do not light a match because you may burn yourself. Do not move about or kick up dust.

Plan your answers beforehand

    The holiday season is a favorite for many. There are always parties with family, friends, relatives or work colleagues. {#blank#}1{#/blank#} But at the same time, it can also make you feel unhappy, especially because there is always the chance that you will be asked to answer awkward questions during these gatherings.

    I think you had better have a general idea of who you might meet or the questions some people may ask you. You may prepare answers to some awkward questions. {#blank#}2{#/blank#} Think of ways you might be comfortable answering questions, and have some basic answers in your head.

    My family and I recently attended a holiday event. {#blank#}3{#/blank#}However, because I knew several people who would most likely be attending the event, I was able to take some time to think about how I wanted to answer the questions they could ask me. This doesn't mean that I spent the days before the party memorizing answers and recited the answers to the questions like a robot. {#blank#}4{#/blank#}

    {#blank#}5{#/blank#}So I found it most effective to prepare by having several stories and answers, or even questions for your companion, in order to make sure the conversation runs smoothly.

A. Ask some questions of whoever you are speaking with.

B. Communicating with others around you can be fun.

C. And you may even practice a bit beforehand if necessary.

D. You have no way of knowing exactly what questions you may be asked.

E. Some awkward questions might be asked at the holiday parties.

F. I was asked a few awkward questions about my work situation.

G. Rather, I simply decided what I was comfortable telling them.

A healthy lifestyle includes many choices. Among them, choosing a balanced diet or healthy eating plan is of the greatest importance. But how do you choose a healthy eating plan? Let's begin by explaining what a healthy eating plan is.

A healthy eating plan that helps you manage your weight includes different kinds of foods you may not have considered. If "healthy eating" makes you think about the foods you can't have, try refocusing on all the new foods you can eat.

Fresh, Frozen, or Canned Fruits -- don't think about just apples or bananas. All fresh, frozen, or canned fruits are great choices. Be sure to try some fruits from other countries, too. How about a mango, a juicy pineapple or kiwi fruit? What you should pay attention to about canned fruits is that they may contain added sugars. You'd better choose canned fruits packed in water or in their own juice.

Fresh, Frozen, or Canned Vegetables – try something new. Don't eat the same kinds of vegetables all the time. You can try to eat different kinds of vegetables, which is important for a good diet. When trying canned vegetables, look for vegetables without added salt or butter. You can go to the produce department(果蔬部) and try a new vegetable each week.

Calcium(钙)-rich foods -- you may firstly think of a glass of low-fat or fat-free milk when someone says "Eat more dairy products " But what about low-fat and fat-free yogurts without added sugars? There is a wide variety of flavors and they can be a great dessert instead of those with a sweet tooth.

What's more, you don't need to give up your favorite comfort food. You can enjoy your favorite foods even if they are high in calories, fat or added sugars. The key is eating them only once in a while, and balancing them out with healthier foods and more physical activity.

The point is that you can figure out how to include almost any food in your healthy eating plan in a way that still helps you lose weight or maintain a healthy weight.