1.Glial cell
A supportive cell in the central nervous system -- the brain and spinal cord. Glial cells do not conduct electrical impulses (as opposed to neurons, which do). The glial cells surround neurons and provide support for them and insulation between them. Glial cells are capable of extensive signaling in response to a diversity of stimuli. Bidirectional communication exists between glial cells and neurons, and between glial cells and vascular cells.
Glial cells are the most abundant cell types in the central nervous system. There are three typesof glial cells: astrocytes, oligodendrocytes, and microglia. Astrocytes are concerned with neurotransmission and neuronal metabolism. Oligodendrocytes are involved in the production of myelin, the insulating materialaround neurons.. And microglia are part of the immune system.
2.The cichlid diversity of Lake Malawi/Nyasa/Niassa
Lake Malawi is a large, beautiful African Rift Valley lake, bordered by Malawi, Tanzania and Mozambique. Its biodiversity(생물 다양성) is simply extraordinary by any standard – it has the greatest wealth of fish species of all the lakes in the world. The vast majority of this ichthyofauna (물고기 군락)belongs to just one family, the Cichlidae, and most of the cichlid species are endemic(지방 특유의) to the lake. These fishes are of major economic importance as an important source of protein for the riparian people(강기슭에 사는 사람들). They are also of great value as research subjects in a wide array of biological disciplines and are currently at the forefront (가장 중요한 위치)of evolutionary research. Many of these cichlids, especially the mbuna, enjoy considerable popularity in the specialized aquarium hobby.
Less than half of the 800 or more estimated cichlid species from Lake Malawi are described, and many of those only in superficial terms. All kinds of taxonomic problems(분류학상의 문제) abound at both species and genus level. This makes studying these fishes quite a challenge.
3.Mt.St.Helens
Mount St. Helens is an active stratovolcano(층이 있는 화산) in Skamania County, Washington, in the Pacific Northwest region of the United States. It is 96 miles (154km) south of Seattle and 53 miles (85km) northeast of Portland, Oregon. Mount St. Helens takes its English name from the British diplomat Lord St Helens, a friend of explorer George Vancouver who made a survey of the area in the late 18th century. The mountain is in the Cascade Range and is part the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire that includes over 160 active volcanoes. This volcano is well known for its ash explosions and pyroclastic flows.(화산암 조각으로 이루어진 용암)
Mount St. Helens is most famous for its catastrophic eruption (최악의 화산분출) on May 18, 1980, which was the deadliest and most economically destructive volcanic event in the history of the United States. Fifty-seven people were killed; 250 homes, 47 bridges, 15 miles (24km) of railways, and 185 miles (300km) of highway were destroyed. The eruption caused a massive debris avalanche, reducing the elevation of the mountain's summit(산 정상) from 9,677ft(2,950m) to 8,365ft(2,550m), and replacing it with a mile-wide (1.5km-wide) horseshoe-shaped crater. The debris avalanche was up to 0.7 cubic miles (2.3 km³) in volume.
As with most other volcanoes in the Cascade Range, Mount St. Helens is a large eruptive cone consisting of lava rock interlayered with ash, pumice, and other deposits. The mountain includes layers of basalt and andesite(안산암) through which several domes of dacite lava have erupted. The largest of the dacite domes formed the previous summit and off its northern flank(측면) sat the smaller Goat Rocks dome. Both were destroyed in the 1980 eruption.
3. Distance to Galaxy Before 1920s, we did not know that galaxies are objects far away. We thought they were in our galaxy and were called spiral nebulae(나선형 성운). Today, we mainly use two methods to measure the distances to other galaxies. The first one is the Cepheid variables that we have talked about in Chapter 13. The second one is the Type I supernova. Recall that Type I supernova is the strong explosion of a nova that even the white dwarf is destroyed. We found that the luminosities(빛을 발하는 물체) of Type I supernovae are about the same. Therefore, we can compute the distance to a galaxy by measuring the apparent brightness of a Type I supernova in that galaxy, if we can find one.
Cepheid variables are dimmer(분명하지 않다) than the Type I supernova. As a result, the supernova allows us to measure the distances of galaxies further away, but it happens much less often. Each one has its advantages and disadvantages. Both of them are called the distance indicators.
Interacting and Active Galaxies
The typical distance between galaxies is only about 20-40 times the sizes of a galaxy. (The ratio of typical distance between stars to the size of a star is in the order of tens of millions.) Galaxy collisions are very common.
When galaxies collide, the stars never collide with each other. Just the distribution of the stars are distorted(변형되다) by the mutual gravity. Streams of stars may be ejected out to form something like antennae and the two galaxies will merge(병합하다) together. Some galaxies with multiple nuclei may be the end result of the merging of two galaxies long time ago. Galaxy collision will also trigger star births.
Galaxy collision may also lead to the formation of an active galaxy. An active galaxy is a galaxy that radiates unusual large amount of energy in the form of infrared, ultraviolet and X-rays; always from the nucleus of the galaxy. So, it is also termed the active galactic nuclei (AGN). We are not completely sure how an active galaxy is formed and how it generates such a large amount of energy. One of the most popular hypotheses(가설) is the supermassive black hole theory. When materials surrounding the black hole fall into it, large amount of energy is released. Galaxy collision may create the supermassive black hole at the center.
4.
5. Termite
*Diversity (다양성)
Termites are soil or wood inhabiting eusocial insects(진사회성 곤충) which generally have soft, white bodies and secretive habits. Most termites each dead plant material, which is digested with the help of bacterial or protozoan symbiosis(공생) in their gut. Globally, termites play an important role in reducing dead plant material, but they can be quite destructive to human-built structures.
In many species of termite the nest is simply the cavities created in the wood as the termites eat, but in African and Australian grasslands some termites construct large nests of soil which is cemented with feces and saliva. In tropical rainforests Nasutitermes species attach their nests of chewed plant material and feces to trees, fence posts, and other aboveground locations. Nasutitmermes workers construct covered walkways from their nest to foraging areas.
*Reproduction
-At the appropriate season for establishing new colonies, winged females and males (alates) leave their nest and join in a mass mating flight which is composed of many (thousands or even millions) of alates from that species' colonies. Males and females form pair bonds, and you may see pairs of males and females running, with the male closely following the female (this is called tandem running) in search of nesting location. Once the male and female have paired, they break off their wings, and spend the remainder of their life flightless.
These mass mating flights are easy pickings for predators--frogs, lizards, birds, and spiders all may benefit greatly from the easy availability of termite alates as food. This may be an example of predator saturation; the termite colonies produce far more alates than could possibly find nesting sites in order to insure that at least a few survive. Unlike ants, bees and wasps, termite workers may be male or female. The king continues to live after his initial mating with the queen and lives in the nest; the king and queen may remote occasionally.
*Caste(계급제도)and division of labor
Termites have incomplete metamorphosis. This means that, like cockroaches and grasshoppers, immatures look very much like adults, lacking only wings. Termite workers are essentially immatures, and in the "lower" termites, workers may ultimately develop into reproductives.
At hatching termite immatures lack the intestinal symbionts(공생자) which enable them to digest cellulose. They gain their initial infection of symbionts by feeding on feces from other termites in the colony. Usually such feeding is directly from the anus of the other individual--this is called proctodael feeding.
In order to grow, insects must shed their exoskeleton by molting. When a termite molts it also loses the linings of its foregut and hindgut, as well as the symbionts living in the gut. Termites rely on proctodael feeding in order to reinfect themselves with their symbionts after they molt; the symbionts are regained from the feces of another termite.
Studies of caste in termites are based on measurements of workers. The measurements allow the scientist to determine to which molt the worker belongs. Both males and females serve as workers, and in some species there are sex differences among the workers in their role in the colony. In general, termite workers can be divided into nest workers, who construct the nest and care for the eggs, foragers, who are equipped for chewing wood or other plant material, and soldiers, whose large heads, strong jaw muscles, and sharp jaws enable them to defend the colony from attackers such as ants. Nest worker and foragers have smaller heads. Head size, gender, and behavioral role can be combined in a diagram of termite caste
*Communication
-Termites often work in the dark, and their best-known modes of communication are pheromones. Trail pheromones guide foragers to food. Other pheromones may regulate how many members of each caste are produced, or may inhibit workers from becoming reproductives.
Some species also produce vibrational signals by striking a surface with their heads. Thousands of termites simultaneously bashing their heads produces a noise that is audible to humans at a distance of several meters. Head bashing communicates alarm, alerting the entire nest to a threat. Termites
6. Desertification
Desertification is the process which turns productive into non- productive desert as a result of poor land-management. Desertification occurs mainly in semi-arid areas (average annual rainfall less than 600 mm) bordering on deserts. In the Sahel, (the semi-arid area south of the Sahara Desert), for example, the desert moved 100 km southwards between 1950 and 1975.
WHAT CAUSES DESERTIFICATION?
* Overgrazing is the major cause of desertification worldwide. Plants of semi-arid areas are adapted to being eaten by sparsely scattered, large, grazing mammals which move in response to the patchy rainfall common to these regions. Early human pastoralists living in semi-arid areas copied this natural system. They moved their small groups of domestic animals in response to food and water availability. Such regular stock movement prevented overgrazing of the fragile plant cover.
In modern times, the use of fences has prevented domestic and wild animals from moving in response to food availability, and overgrazing has often resulted. However, when used correctly, fencing is a valuable tool of good veld management.
The use of boreholes and windmills also allows livestock to stay all-year round in areas formerly grazed only during the rains when seasonal pans held water. Where not correctly planned and managed, provision of drinking water has contributed to the massive advance of deserts in recent years as animals gather around waterholes and overgraze the area.
* Cultivation of marginal lands, i.e lands on which there is a high risk of crop failure and a very low economic return, for example, some parts of South Africa where maize is grown.
* Destruction of vegetation in arid regions, often for fuelwood.
* Poor grazing management after accidental burning of semi-arid vegetation.
* Incorrect irrigation practices in arid areas can cause salinization, (the build up of salts in the soil) which can prevent plant growth.
When the practices described above coincide with drought, the rate of desertification increases dramatically. Increasing human population and poverty contribute to desertification as poor people may be forced to overuse their environment in the short term, without the ability to plan for the long term effects of their actions. Where livestock has a social importance beyond food, people might be reluctant to reduce their stock numbers.
WHAT ARE THE EFFECTS OF DESERTIFICATION?
Desertification reduces the ability of land to support life, affecting wild species, domestic animals, agricultural crops and people. The reduction in plant cover that accompanies desertification leads to accelerated soil erosion by wind and water. South Africa losing approximately 300-400 million tonnes of topsoil every year. As vegetation cover and soil layer are reduced, rain drop impact and run-off increases.
Water is lost off the land instead of soaking into the soil to provide moisture for plants. Even long-lived plants that would normally survive droughts die. A reduction in plant cover also results in a reduction in the quantity of humus and plant nutrients in the soil, and plant production drops further. As protective plant cover disappears, floods become more frequent and more severe. Desertification is self-reinforcing, i.e. once the process has started, conditions are set for continual deterioration.
HOW WIDESPREAD IS DESERTIFICATION?
About one third of the world's land surface is arid or semi-arid. It is predicted that global warming will increase the area of desert climates by 17% in the next century. The area at risk to desertification is thus large and likely to increase.
Worldwide, desertification is making approximately 12 million hectares useless for cultivation every year. This is equal to 10% of the total area of South Africa or 87% of the area of cultivated lands in our country. In the early 1980s it was estimated that, worldwide, 61% of the 3257 million hectares of all productive drylands (lands where stock are grazed and crops grown, without irrigation) were moderately to very severely desertified. The problem is clearly enormous.
DESERTIFICATION IN SOUTHERN AFRICA
About half of southern Africa is semi-arid and thus at risk of desertification. The area already transformed into desert-like conditions is not accurately known because uncertainty surrounds the precise definition of a desert, and what the original state of the vegetation was in the semi-arid areas of southern Africa.
The areas which are known to have deteriorated this century are mainly on the edges of the southern Kalahari. The deterioration of the Karoo is less well established. It is possible that desertification of the Karoo began in the last century, when sheep were first introduced, and before good records were available for the area.
In recent years the introduction of artificial water points into the Kalahari within Botswana, together with the widespread erection of veterinary fences, has led to the rapid desertification of huge areas. Similar schemes have had the same effect in the southern Kalahari within South Africa and Bophuthatswana.
HOW CAN DESERTIFICATION BE HALTED?
To halt desertification the number of animals on the land must be reduced, allowing plants to regrow. Soil conditions must be made favourable for plant growth by, for example, mulching. Mulch (a layer of straw, leaves or sawdust covering the soil) reduces evaporation, suppresses weed growth, enriches soil as it rots, and prevents runoff and hence erosion. Reseeding may be necessary in badly degraded areas. Mulching and reseeding are expensive practices. However, the only realistic large-scale approach is to prevent desertification through good land management in semi-arid areas.
7. Feudalism
Feudalism was the system of loyalties and protections during the Middle Ages. As the Roman Empire crumbled, emperors granted land to nobles in exchange for their loyalty. These lands eventually developed into manors. A manor is the land owned by a noble and everything on it. A typical manor consisted of a castle, small village, and farmland.
During the Middle Ages, peasants could no longer count on the Roman army to protect them. German, Viking and Magyar tribes overran homes and farms throughout Europe. The peasants turned to the landowners, often called lords, to protect them. Many peasants remained free, but most became serfs. A serf was bound to the land. He could not leave without buying his freedom, an unlikely occurrence in the Middle Ages. Life for a serf was not much better than the life of a slave. The only difference was that a serf could not be sold to another manor.
Serfs would often have to work three or four days a week for the lord as rent. They would spend the rest of their week growing crops to feed their families. Other serfs worked as sharecroppers. A sharecropper would be required to turn over most of what he grew in order to be able to live on the land.