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    Have you ever wondered why the roots of the plants always know which way to grow—into the soil but not above it? Some British scientists have recently solved this mystery.    

      It turns out that roots have special hairs that tie them into the soil and help them grow their way past obstacles, a team at the John Inners Center in Norwich reports in the February 29 issue of Journal Science.

    "The key is in the fuzzy(有绒毛的) coat of hairs on the roots of plants," says professor Liam Dolan. "We have found a growth control system that enables these hairs to find their way and to become longer when their path is clear."

    Root hairs explore the soil in much the same way a person would feel their way in the dark. If they come across an obstacle, they make their way around until they can continue growing in an opening. I the meantime, the plant is held in place as the hairs grip the soil.

    The hairs are guided by a clever chemical trick. A protein(蛋白质) at the tip of the root hairs called RHD2 helps them to take calcium(钙) from the soil. Calcium makes the hairs grow, and produce more RHD2, and take more calcium.

    But when an obstacle blocks the hair's path, or the hair reaches the surface of the soil, the cycle is broken and growth starts in another direction.

    This system gives plants the flexibility to explore a complex environment and to live in even the most unpromising soils, says Dolan.

    In poor soils such as in parts of Australia and Africa, native plants have adapted by producing enormous numbers of root hairs. A better understanding of this adaptation will allow scientists to develop hairy rooted crops that can grow in unfriendly environments.

    According to Dolan, "Research in the John Inners Center is taking a breeding approach(育种方法) to increase hair length in wheat but it will be some time before new cultivars(栽培变种) are developed."

    Old age may not sound exciting.But recent findings offer good news for older people and for people worried about getting older.Researchers found that people become happier and experience less worry after they reach the age of fifty.In fact，they say by the age of eighty­five，people are happier with their life than they were when they were eighteen years old.

    The findings came from a survey of more than three hundred forty thousand adults in the United States.The Gallup Organization questioned them by telephone in 2008.At that time，the people were between the ages of eighteen and eighty­five.The researchers asked questions about emotions like happiness，sadness and worry.They also asked about mental or emotional stress.

    Arthur Stone in the Department of Psychiatry and Behavioral Science at Stony Brook University in New York led the study.His team found that levels of stress were highest among adults between the ages of twenty­two and twenty­five.The findings showed that stress levels dropped sharply after people reached their fifties.

    The study also showed that men and women have similar feeling patterns as they grow older.However，women at all ages reported more sadness，stress and worry than men.

    Researchers say they do not know why happiness increases as people get older.One theory is that，as people grow older，they grow more thankful for what they have and have better control of their feelings.They also spend less time thinking about bad experiences.

    Professor Stone says the emotional patterns could be linked to changes in how people see the world，or maybe even changes in brain chemistry.

    The researchers also considered possible influences like having young children，being unemployed or being single.But they found that influences like these did not affect the levels of happiness and well­being related to age.

    If a diver surfaces too quickly, he may suffer the bends. Nitrogen (氮) dissolved (溶解) in his blood is suddenly liberated by the reduction of pressure. The consequence, if the bubbles (气泡) accumulate in a joint, is sharp pain and a bent body—thus the name. If the bubbles form in his lungs or his brain, the consequence can be death.

    Other air-breathing animals also suffer this decompression (减压) sickness if they surface too fast: whales, for example. And so, long ago, did ichthyosaurs. That these ancient sea animals got the bends can be seen from their bones. If bubbles of nitrogen form inside the bone they can cut off its blood supply. This kills the cells in the bone, and consequently weakens it, sometimes to the point of collapse. Fossil (化石) bones that have caved in on themselves are thus a sign that the animal once had the bends.

    Bruce Rothschild of the University of Kansas knew all this when he began a study of ichthyosaur bones to find out how widespread the problem was in the past. What he particularly wanted to investigate was how ichthyosaurs adapted to the problem of decompression over the 150 million years. To this end, he and his colleagues traveled the world's natural-history museums, looking at hundreds of ichthyosaurs from the Triassic period and from the later Jurassic and Cretaceous periods.

    When he started, he assumed that signs of the bends would be rarer in younger fossils, reflecting their gradual evolution of measures to deal with decompression. Instead, he was astonished to discover the opposite. More than 15% of Jurassic and Cretaceous ichthyosaurs had suffered the bends before they died, but not a single Triassic specimen (标本) showed evidence of that sort of injury.

    If ichthyosaurs did evolve an anti-decompression means, they clearly did so quickly—and, most strangely, they lost it afterwards. But that is not what Dr Rothschild thinks happened. He suspects it was evolution in other animals that caused the change.

    Whales that suffer the bends often do so because they have surfaced to escape a predator (捕食动物) such as a large shark. One of the features of Jurassic oceans was an abundance of large sharks and crocodiles, both of which were fond of ichthyosaur lunches. Triassic oceans, by contrast, were mercifully shark- and crocodile-free. In the Triassic, then, ichthyosaurs were top of the food chain. In the Jurassic and Cretaceous, they were prey (猎物) as well as predator—and often had to make a speedy exit as a result.

    There have always been a lot of commonly believed but false ideas about being fat and doing exercise. Some people believe that they can't help putting on weight as they get older, while others hold that if they stop exercising, their muscles will turn into fat. Here are some more myths:

I'll never lose weight — I come from a fat family

    Wrong! While we can't change the body type we are born with, we can't blame our genes for making us fat. There's plenty of evidence that fatness runs in families, and the main reason is that they share the same habits of eating too much and exercise too little.

    I am fat because I burn calories slowly

    Wrong! Fatness is not caused by a slow metabolism（新陈代谢）. If fact, although fat people consume more energy that slim people, they also fail to realize how much they eat! Keeping a diary can help you work out your daily food intake more accurately.

    Exercise is boring

    Wrong! Anything will become boring if you do it repetitively. The key is to develop a balanced and varied program that's fun as well as progressive. If you enjoy a Sunday walk, take a different route. If you do Yoga, try a tai chi class. If you like swimming, set yourself a distance or time challenge.

    No pain, no gain

    Wrong! Exercise is not meant to hurt. Indeed, pain is your body telling you something's wrong, and continuing to exercise could lead to serious injury. You may experience mild discomfort as you begin to exercise regularly, but this your body adapting to the positive changes in your lifestyle and the aches should disappear relatively quickly. If you don't, rest and seek medical advice.

    Sweet or salty? What kind of tastes do you like? If like me, you have a sweet tooth and you probably can't resist eating cakes, biscuits or chocolate and will sweeten your tea or coffee with spoonfuls of sugar-delicious! But the taste makes it very easy to ignore the warnings that too much of the white stuff（东西）is bad for our health.

    Consuming sugar is an addiction-the more we eat, the more we want. Today's processed food, like ready meals, is related to the stuff and many fizzy（起泡的）drinks contain seven teaspoons of sugar in just one can. In the UK, statistics show that sugar consumption is at its highest level in history and the government is trying to get the food industry to cut the amount of sugar in popular products like chocolate bars by 20% by 2020.

    Of course, sugary food tastes nice, it can help lift our mood, and a part in it can refresh us. But there are dangers too :a high-sugar diet is linked to putting on weight, and being overweight can increase the risk of getting type 2 diabetes（糖尿病）. With these warning signs, I have considered changing my diet by replacing sugary snacks with fruit and salty biscuits-but that's boring!

I'm not alone. BBC journalist Radhika Shanghani, has gone one step further. Encouraged by some well-knowns and nutritionists promoting a 'zero tolerance' approach to sugar, she gave it up altogether, thinking it would make her healthier. Initially she says, “My first fortnight involves mood swings. I have disturbing headaches and feel permanently hung-over.” These symptoms disappeared but she still found food shopping hard as she was stressing about buying the right things.

    Her experiment wasn't a success. She eventually sought advice from Susan Jebb, professor of diet and population health at Oxford University who said: “Lots of people enjoy sugar and gain pleasure from it, so one has to find a balance between enjoyment and eating the right amount.”

    We now think that chocolate is sweet, but once it was bitter. We think of it as a candy, but once it was a medicine. Today, chocolate can be a hot drink, a frozen dessert, or just a snack. Sometimes it's an ingredient（构成部分）in the main course of a meal. Mexicans make a hot chocolate sauce called Mole and pour it over chicken. The Mexicans also eat chocolate with spices like chili peppers.

    Chocolate is a product of the tropical cacao tree. The beans taste so bitter that even the monkeys say "Ugh!" and run away. Workers must first dry and then roast the beans. This removes the bitter taste.

    The word "chocolate" comes from a Mayan word. The Mayas were an ancient people who once lived in Mexico. They valued the cacao tree. Some used the beans for money, while others crushed them to make a drink.

    When the Spaniards came to Mexico in the sixteenth century, they started drinking cacao, too. Because the drink was strong and bitter, they thought it was a medicine. No one had the idea of adding sugar. The Spaniards took some beans back to Europe and opened cafes. Wealthy people drank cacao and said it was good for the digestion.

    In the 1800s, the owner of a chocolate factory in England discovered that sugar removed the bitter taste of cacao. It quickly became a cheap and popular drink. Soon afterwards, a factory made the first solid block of sweetened chocolate. Later on, another factory mixed milk and chocolate together. People liked the taste of milk chocolate even better.

    Besides the chocolate candy bar, one of the most popular American snacks is the chocolate chip cookie. Favorite desserts are the chocolate cream pie and, of course, an ice cream sundae with hot fudge sauce.