200-word popular science exposition

In the west of Asia, at the junction of Palestine and Jordan, there is a "Dead Sea". From a distance, the waves of the Dead Sea are endless. However, who would have thought that there were no fish, shrimp, aquatic plants or even grass on the beach? This may be why the Dead Sea got its name.

Surprisingly, however, people can swim freely in this sea without fish and grass; Even people who can't swim always float on the water, so don't worry about drowning.

Legend has it that about 2000 years ago, the Roman commander Imperial Capital marched into Jerusalem, attacked the coast of the Dead Sea and ordered the execution of captured slaves. Slaves were thrown into the Dead Sea, not drowned in the water, but sent back to the shore by the waves. The emperor flew into a rage and ordered the prisoners to be thrown into the sea, but the slaves were still safe and sound. The imperial city was frightened. He thought that slaves were blessed by the gods and could not die repeatedly, so he had to order them all to be released.

So, why is the buoyancy of the Dead Sea so great? According to statistics, dead seawater contains many minerals:135.46 million tons of sodium chloride (salt); There are 6.37 billion tons of calcium chloride; Potassium chloride has 2 billion tons; In addition, there are bromine and strontium. All kinds of salts together account for 23% ~ 25% of all the seawater in the Dead Sea. In this way, the density of seawater is greater than that of human body, so it is no wonder that when people get into the sea, they will naturally float and not sink.

How was the Dead Sea formed? Please listen to an old legend first. In ancient times, it used to be a continent. Men in the village have a bad habit. The prophet Root advised them to turn over a new leaf, but they refused to repent. God decided to punish them, so he secretly told Root to leave the village with his family on a certain day, and warned him not to look back after leaving the village, no matter how serious the accident happened behind him. Root left the village at the appointed time and left. Out of curiosity, the wife secretly turned around and took a look. Alas, in an instant, the good village collapsed. What appeared in front of her was a piece of Wang Yang sea, namely the Dead Sea. Because she disobeyed God's warning, she immediately became a stone man. Despite centuries of wind and rain, she stood on the hillside near the Dead Sea and watched it day and night. God punished those stubborn people: they had no fresh water to drink.

This is, of course, a myth and a guess that people can't understand the formation process of the Dead Sea. In fact, the Dead Sea is a saltwater lake, and its formation is the result of natural changes. The Dead Sea is located in the middle of the North-South Rift Valley between Jordan and Palestine. It is 75 kilometers long from north to south, 5 to 16 kilometers wide from east to west, with an average depth of 146 meters and the deepest point of about 400 meters. River water contains a lot of minerals. The river flows into the Dead Sea, constantly evaporating, and minerals are precipitated. As time goes by, it accumulates more and more, forming the saltiest saltwater lake in the world today-the Dead Sea.

Decades ago, the Dead Sea was desolate. In order to develop and utilize its resources, some factories have appeared beside it, and some modern swimming pools, hotels and amusement places have also been built. The sunshine over the Dead Sea is high, the air on the sea surface is fresh, and the oxygen content is high. The therapeutic effect of seawater is not inferior to that of hot springs, attracting many tourists. As a result, it has become a lot of life.

However, because the evaporation of the Dead Sea is greater than the input water of the Jordan River, the water level is decreasing day by day. According to experts' statistics, the water level of the Dead Sea has dropped by 40 to 50 centimeters every year in the last decade. If this continues, the shallow waters in the south will disappear in the near future. Hundreds of years later, the far north may dry up. At that time, the Dead Sea will really die. Zhou Nan June litchi dance

When I was a child, I only knew that the shell and meat of dried litchi were brown. When I was in primary school, my teacher taught Bai Juyi's Introduction to Litchi, and read that "the shell is like a red ribbon, the membrane is like a purple ribbon, the meat is like Bai Rubing snow, and the pulp is like bacon cheese. I really can't understand where litchi is red! Litchi meat, Bai Rubing snow, isn't it even more strange? Ask the teacher a question. The teacher has never seen fresh litchi, and the explanation is unclear, so he has to give up. If it is now, even if the teacher has never seen fresh lychees, he can find out scientific materials and explain them to the pupils who are a little obsessed.

Bai Juyi described the shape of litchi with concrete brushstrokes, but there are some shortcomings. Tapirs are made of silk, which is smooth, but litchi shells are rough. In the terminology of fruit science, there are tiny decisive lobes on the surface of litchi shell, which are called lobes like tortoise shells. There are protrusions in the center of leaves, and some are as sharp as thorns, which are called thin slices. The leaf size is dense and the peak value is flat.

Ripe lychees are mostly dark red or purple. Born in the head of a tree, of course, the structure of its shell surface can not be seen clearly from a distance. Only red comes into view, so it is realistic to compare it to "crimson capsule", "red star" and "coral bead". As for the whole tree, even a forest, it will become a beautiful scenery of "flying flames want to wear the sky" and "red clouds are heavy". Ripening of litchi. "Zhou Nan June Litchi Dan" refers to June of the lunar calendar, which coincides with July of the solar calendar. Litchi is also reddish, such as "March Red" and "Guilv" produced in Guangdong, with a reddish yellow color. Litchi is heart-shaped, oval or round, usually with a large pedicle and a slightly smaller top. It is called the fruit shoulder. Some are high and low. The top is called the top of the fruit, which is round or pointed. There is a groove from the top of the fruit to both sides of the pedicle, which is called suture. Appearance and concealment vary from species to species. There are some strange varieties in the old records, such as "dragon teeth" with slender fingers and "pearls" with small circles, which are now extinct because of lack of economic value.

Litchi is generally three or four centimeters in diameter and weighs 10 to more than 20 grams. In the 1960s, Guangdong investigated and found that there were goose-egg-shaped lychees and lilac-shaped lychees, weighing 40 to 50 grams. There is also "Nanmu Leaf" produced in Hejiang, Sichuan, which is weighed about19g in Atlas of Improved Fruit Trees in Sichuan, and 60g in china fruit Cultivation Science. The so-called "membrane is like purple gauze". The pattern of the inner wall of the shell is mistaken for the pattern of the film. In the Ming Dynasty, Xu Bo wrote a poem "Ode to Litchi Membrane", which describes throwing the shell and membrane on the ground when eating litchi, as if "the lotus petals fall before the wind and the peach blossoms are beautiful after the rain", which is an exaggeration.

Litchi pulp is mostly white and translucent, and it is completely correct to say that it is "like jade and snow in Bai Rubing". Some are yellowish. From the botanical point of view, it is not the pulp, but the membrane outside the seed, which should be called aril. The real body is the film thrown away with the shell mentioned above. The cell wall of litchi meat is very thin, so there is no residue left at the entrance. Taste sweet and slightly sour, suitable for raw food. Some are pure.

Litchi is not resistant to storage, as Bai Juyi said: "One day, the color changes, two days, and three days." After four or five days, the color, smell and fragrance are all gone. " Now it has been confirmed by research that the temperature can be kept at 1℃ to 5℃ and can be stored for about 30 days. We should further try to extend the storage period to facilitate long-distance transportation, because litchi is not resistant to storage and the ancient Gongyan people wanted to eat it. It brings people a lot of pain. Emperor Tang Ming did such a thing in order to spoil Yang Guifei. In the Tang Dynasty, Du Mu said: "Chang 'an looks back and leads piles, and the top of the mountain opens a thousand times. When the world of mortals rode the princess, no one knew it was litchi. "This is a mockery of this matter.

Litchi is a seed, oblong, with a smooth brown surface, and a few varieties are green. Excellent litchi, with undeveloped seeds and small shape, looks like lilac, also called scorched core. Now there is a seedless litchi in Hainan Island, and the kernel is even degraded.

The flowering period of litchi is from early February to early April, which varies with varieties. Guangdong has double-season litchi, which blooms twice a year. There are also lychees in four seasons, which bloom four times a year. The flowers are small, green, white or light yellow, not dazzling. Only a few varieties have complete flowers. Male and female flowers often bloom at the same time, so it is best to choose suitable varieties and mix them together to increase the chance of pollination. A litchi inflorescence with flowers.

Litchi originated in China, and it is a special product in China. There are wild litchi forests in Hainan Island and Lianjiang, which can prove that China is the country of origin. According to records, Wei Tuo, king of South Vietnam, paid tribute to Emperor Gaozu, which shows that there were lychees in Guangdong at that time. It has been cultivated for more than 2000 years. There are many records of litchi in Sichuan in the Tang Dynasty. From Cai Xiang's Litchi Tree (650),

There are thirteen books about litchi in ancient times, including Cai Xiang's, and there are eight left. There is only one record in Guangdong. There are records of Chen Dingpu in the early Qing Dynasty in Sichuan, Guangdong and Fujian provinces. Cai Pu is not only the earliest one in the annals of china fruit, but also the earliest one in the world. Including the origin, ecology, function, processing, marketing and related history of litchi, recording 32 varieties of litchi. Among them, "Chen Zi" is still widely planted.

Litchi is a subtropical fruit tree with mild nature, and Chengdu and Fuzhou are the northern limits of its growth. Emperor Wu of the Han Dynasty once built Fuli Palace and transplanted litchi to Chang 'an, but the maintainers were furious and refused to plant it alive or even executed it. During the Song Dynasty, Fujian "put small plants in pottery, sailed to pavilions and transplanted them to Xuanhe Hall". Zong Wei wrote a poem boasting that "intensive migration created Fujian." The new litchi Dan was banned. "In fact, it was only mature once. In the Ming Dynasty, there was a poem "New Litchi Piece", which said that several strains of the Gu family in Changshu survived. "The immortal was originally a seamount and has sprouted in Jiangxiang since then. "But how many years he lived is unknown. Now that science is developed, it may not be completely impossible for litchi to move northward in the future.

China has a vast territory, and different regions have different specialties. It is feasible to develop local characteristics according to local conditions. Litchi production should be vigorously developed in areas rich in litchi. Su Shi has a poem: "There are four seasons of spring in Luofu Mountain, and oranges and bayberries are new. Three hundred lychees a day, it is better to grow up to be a Lingnan person. " But if you eat 300 lychees a day, how many people can you have? Socialist modern litchi production should gradually meet the needs of the broad masses of people.