Free IELTS Academic Reading Practice Test 2

Life without rain

How desert plants find water, store it and make every drop last

To a visitor, a desert can seem almost empty of life. Rain may fall only a few times a year, and in some places several years can pass without any at all. During the day, the surface of the ground can become hot enough to burn bare skin, while at night the temperature may drop close to freezing. Yet most deserts are home to a surprising variety of plants, from tiny flowers to cacti taller than a house. All of them face the same basic problem: how to obtain water, how to keep it, and how to use as little of it as possible.

Plants lose water mainly through tiny openings in their leaves called stomata. These openings allow carbon dioxide, which plants need in order to make food, to enter the leaf, but every time they open, water escapes into the air as vapour. For a plant growing in a damp forest, this is not a serious difficulty, since the lost water is easily replaced from the soil. In the desert, however, a plant that kept its stomata open throughout a hot day could lose more water in a few hours than it could take in over several weeks.

One of the most familiar solutions is to store water inside the plant itself. Plants that do this are known as succulents, a group that includes cacti as well as many other kinds of plant. Their stems or leaves contain a soft tissue that holds water rather like a sponge. After heavy rain, a large cactus may contain several hundred litres of water. Many species also have deep folds running down the stem, similar to the folds of a closed umbrella. These allow the stem to swell as it fills with water and to shrink again during dry periods without splitting or tearing.

Succulents also save water by changing the time of day at which they take in carbon dioxide. Most plants open their stomata in daylight, when they use the energy of the sun to make sugars. Many succulents do the opposite. They open their stomata only at night, when the air is cooler and more humid, and store the carbon dioxide in the form of an acid. During the day, with their stomata firmly closed, they use this stored supply to make food. The process is less efficient than the one used by most plants, which is one reason why many cacti grow so slowly, but it reduces water loss dramatically.

The shape and surface of desert plants help in other ways. Many cacti have no ordinary leaves at all: their spines are leaves that have changed, over millions of years, into hard, narrow points. Spines lose almost no water, discourage thirsty animals from feeding on the plant and, where they grow thickly, provide a little shade for the stem. Other desert plants are covered in a layer of wax, which gives them a shiny appearance and reduces the amount of water that can escape. Some have pale or silvery hairs, which reflect sunlight and keep the plant cooler than the air around it.

Roots matter just as much as stems and leaves. Some desert shrubs send a single main root deep into the ground to reach water far below the surface, and roots of this kind have occasionally been found tens of metres underground. Cacti generally take the opposite approach. Their roots are shallow but spread widely, sometimes several metres in every direction, so that they can collect as much water as possible from a brief shower before it evaporates. After rain, some species grow new fine roots within hours, which then die back once the soil has dried out.

Not all desert plants attempt to survive the dry season at all. Many small annual plants spend most of their lives as seeds lying in the soil. When enough rain falls, they germinate, grow, flower and produce new seeds within a few weeks, before the ground dries out again. Their seeds often contain chemicals that prevent them from germinating after light rain, which might not provide enough water for the plant to complete its life cycle. In years of unusually heavy rain, this strategy can turn a brown landscape into a carpet of flowers almost overnight, an event that draws large numbers of visitors to some desert regions.

Scientists are paying increasing attention to these adaptations. As droughts become more frequent in many farming areas, plant breeders are studying how desert species control their use of water, in the hope of developing food crops that need less irrigation. Engineers, too, have examined the surfaces of desert plants for ideas on how to collect water from fog and dew. The plants that seem to struggle in some of the harshest places on Earth may yet help people to cope with a drier future.

From luxury to everyday: the story of glass

How a material made largely from sand took thousands of years to find its way into every home

AGlass is so common today that it is easy to forget how remarkable it is. It is made largely from sand, one of the cheapest and most plentiful materials on Earth, yet it can be shaped into objects as delicate as a wine glass or as tough as the windscreen of a car. For most of its history, however, glass was anything but ordinary. For thousands of years it was a precious material, and the story of how it became part of everyday life is also the story of the slow build-up of practical knowledge, much of it passed from one craftsman to another in secret.

BNobody knows exactly when or where glass was first made. Natural glass, formed when lightning strikes sand or when volcanoes erupt, was used for cutting tools long before people learnt to produce glass themselves. The earliest man-made glass objects, mostly small beads, appear to date from around 4,500 years ago, in the region that is now Iraq and Syria. An ancient story claims that glass was discovered by accident when merchants lit a cooking fire on a sandy beach, but this is generally regarded as a legend, since an ordinary fire does not get hot enough to melt sand. Early glassmakers learnt to lower the melting point by adding other substances, such as the ash of burnt plants, and for many centuries they produced only small, brightly coloured items that were valued almost as highly as precious stones.

CThe most important change in the ancient world came in the first century BC, somewhere on the eastern coast of the Mediterranean, when craftsmen discovered that a lump of hot glass could be inflated by blowing air into it through a long metal tube. Until then, glass vessels had been formed around a core of clay or poured into moulds, and a single bowl might take days to complete. Blowing allowed a skilled worker to make a bottle or a cup in minutes. The technique spread rapidly through the Roman Empire, and glass, once a luxury, became cheap enough to be used for storing and serving food. Roman glassmakers even produced the first glass windows, although these were thick and uneven and let through only a little light.

DIn later centuries, the most celebrated glass in Europe came from Venice. In the late thirteenth century, the city’s rulers ordered its glassmakers to move their furnaces to the nearby island of Murano. The official reason was to reduce the risk of fire in a city of closely packed wooden buildings, but the move also made it easier to stop the craftsmen’s methods from reaching rival cities. Glassmakers enjoyed a high social position, yet they were forbidden to leave the republic, and those who did so risked severe punishment. Even so, some did escape, and by the sixteenth century Venetian techniques were being copied in workshops across Europe.

EIn England, the industry was transformed by a shortage of fuel. Glass furnaces burned enormous quantities of wood, and in 1615 a royal order banned the use of wood for glassmaking, partly to protect the forests that supplied timber for shipbuilding. Glassmakers were forced to switch to coal, which burned hotter but gave off smoke that discoloured the glass. To keep the smoke away, they began to melt the glass in covered pots. Later in the century, an English glassmaker found that adding lead to the mixture produced a heavy, brilliant glass that was easier to cut and engrave. This ‘lead crystal’ allowed English producers to compete with Venice for the first time.

FFor most of history, flat glass for windows was difficult to make. One method involved blowing a large bubble, cutting it open and spinning it until it formed a flat disc; another involved blowing a long cylinder, splitting it and flattening it while it was still soft. Both methods produced glass with an uneven surface, and windows remained expensive. In Britain, a tax on windows, first introduced in 1696, led many householders to block up some of their windows, and the results can still be seen in older buildings. Large sheets of glass became affordable only in the nineteenth century, when improved methods allowed factories to produce glass in quantities that would have astonished earlier craftsmen. In 1851, an enormous exhibition hall made almost entirely of iron and glass was put up in a London park, a striking demonstration of what the material could now do.

GThe last step towards the perfectly flat windows of today came in the 1950s, when a British company developed a process in which molten glass is poured onto a bath of liquid tin. The glass floats on the metal and spreads out into a sheet with two smooth, even surfaces, so that no grinding or polishing is required. This ‘float glass’ method is still used to make most of the world’s flat glass. Meanwhile, the material continues to find new roles: thin glass fibres carry much of the world’s internet traffic as pulses of light, and researchers are developing windows that darken automatically to control how much heat enters a building. Because glass can be recycled again and again without losing quality, some argue that this ancient material may have an important part to play in a more sustainable future.

The psychology of the queue

Why does ten minutes in one line feel like nothing and ten minutes in another feel like an hour? Researchers who study waiting say the answer lies more in the mind than on the clock.

Few experiences are as universally disliked as waiting in a queue. Estimates of how much of a lifetime the average person spends standing in line vary so widely that they are probably of little value, but nobody doubts that the figure is substantial. What is less obvious is that the frustration a queue produces bears only a loose relationship to the time it actually takes. Two people who wait exactly the same number of minutes may emerge with entirely different impressions: one scarcely aware of the delay, the other convinced that the whole morning has been wasted. For the growing number of researchers who study waiting, this gap between the clock and the mind is the heart of the matter.

The mathematical study of queues has a long history. It originated in the early twentieth century, when engineers tried to calculate how many lines a telephone exchange would need to cope with its busiest hours. Queueing theory, as it became known, treats waiting as a problem of flow: customers arrive at a certain rate and are served at a certain rate, and the length of the line follows from the relationship between the two. The approach has proved enormously successful in the design of everything from road junctions to computer networks. Yet the operations researcher Ruth Harding, who spent much of her early career building such models for banks, came to regard them as answering only half the question. The models, she has written, could predict to the second how long customers would wait, but not whether they would ever come back.

Harding’s later work focused on what she calls ‘occupied’ and ‘empty’ time. In a series of studies carried out in post offices and clinics, she found that people who had something to do while they waited – a form to fill in, a screen to watch, even a mirror to look into – consistently underestimated the length of their wait, whereas those with nothing to occupy them tended to overestimate it, sometimes considerably. Uncertainty had a similar effect. Customers who were told how long they could expect to wait reported less irritation than those left to guess, even when the estimate they had been given turned out to be somewhat too low. A known end point, it seems, allows people to settle into a wait rather than spend it in a state of anxious alertness.

The social psychologist Daniel Leclerc approaches the queue from a different angle. For him, a queue is less a technical system than a society in miniature, governed by an unwritten rule of fairness: first come, first served. His observations of queues at railway ticket offices and football grounds suggest that people are far more tolerant of a long wait than of an unjust one. Someone who has stood for twenty minutes in an orderly line may remain perfectly calm, while the same person, having waited for ten, can become furious on seeing a stranger push in ahead. Strikingly, Leclerc found that objections to queue-jumping were often voiced most forcefully not by the person immediately behind the intruder but by people several places further back, who lost almost nothing by it. The anger, he argues, is moral rather than practical.

Fairness also helps to explain the spread of the single ‘serpentine’ queue, in which customers wait in one long, winding line that feeds several counters. This arrangement can be slightly more efficient than a separate line for each counter, but its chief advantage is psychological. With multiple lines, every customer is exposed to the possibility that a neighbouring line will move faster, and the sight of someone who arrived later being served first is a dependable source of resentment. The single line removes this comparison altogether. Mira Halvorsen, who studies service design, has found that customers frequently judge a single queue to be quicker even when it is visibly longer, simply because it never appears to be treating them unfairly.

Not every attempt to manage the experience of waiting has been so well received. Many businesses now offer priority lanes to customers who pay extra or belong to loyalty schemes, and these have proved commercially successful. The economist Samuel Brandt maintains that such schemes are an efficient way of distributing a scarce resource: those who value their time most highly can pay to save it, while everyone else benefits from lower basic prices. His critics reply that this overlooks the social meaning of the queue. Once advantage can be bought, the principle that everyone waits on equal terms gives way to one in which wealth determines treatment, and the indignation that Leclerc observed may simply be redirected towards the business itself. Brandt accepts that visible priority lanes can annoy other customers, and suggests that the firms which profit most from them are those that keep them discreetly out of sight.

Technology now promises to make the physical queue unnecessary. Virtual queuing systems, which send customers a message when their turn approaches, allow people to wait wherever they like, and early evidence suggests that most customers prefer them. The psychologist Aiko Tanabe, however, cautions against assuming that the problem of waiting has thereby been solved. In her view, removing the visible line also removes some of the reassurance it provided: customers can no longer see that others are waiting too, or confirm for themselves that they are making progress, and complaints of having been ‘forgotten’ are common. Waiting, she suggests, is bearable largely because it is shared and visible. If she is right, the designers of tomorrow’s virtual queues may find themselves rebuilding, on a screen, many features of the very line they set out to abolish.