IELTS Academic Reading Practice Test 31

Silk has been prized for thousands of years for its lustre, strength and softness. Chinese tradition credits its discovery to Leizu, the wife of the legendary Yellow Emperor, who is said to have noticed a cocoon unravelling in her cup of hot tea. The story cannot be taken literally, but archaeology confirms that silk production in China is extremely ancient: fragments of woven silk dating from the fourth millennium BCE have been found in Henan province, and chemical traces of silk proteins have been detected in burials that are older still.

Almost all of the world's silk comes from the domesticated silk moth, Bombyx mori, which is descended from a wild species still found in China and neighbouring regions. Thousands of years of selective breeding have made the domesticated insect entirely dependent on human care. The adult moths cannot fly and do not feed, and the larvae, known as silkworms, eat only the leaves of the mulberry tree. Over a period of roughly four weeks, a silkworm eats almost continuously, shedding its skin four times as it grows and increasing its weight many thousands of times. Several wild moth species are also used for silk, particularly in India, but they generally produce coarser and less regular fibres.

When it is fully grown, the silkworm stops eating and begins to spin a cocoon around itself, a process that takes two to three days. The silk is produced by a pair of glands in the insect's body and emerges from an opening below its mouth as a single continuous filament. This filament consists of two strands of a protein called fibroin, held together and coated by a gummy protein known as sericin. One cocoon may contain a filament with a usable length of several hundred metres. Inside the cocoon, the silkworm transforms into a pupa and, if left undisturbed, emerges as a moth after two or three weeks.

For silk producers, the emergence of the moth is a problem, because the insect damages the filament as it escapes, leaving it in short pieces. Most cocoons are therefore heated, by steam, hot air or boiling water, to kill the pupa before it can emerge. The cocoons are then soaked in hot water to soften the sericin, and workers locate the end of each filament with a brush.

Because a single filament is far too fine to be used on its own, the filaments of several cocoons are reeled together to form one thread, the sticky sericin helping them to adhere. Later, the thread may be boiled in soapy water to remove the remaining sericin, a process known as degumming, which gives silk its characteristic shine and soft feel.

For many centuries China kept the techniques of sericulture to itself, while exporting finished silk along the network of overland routes later called the Silk Road. Along these routes, silk served not only as a trading good but at times as a form of payment and a diplomatic gift. Silk reached the Roman Empire, where it was so expensive that some Roman writers condemned it as a wasteful luxury. Knowledge of silk production nevertheless spread gradually to Korea, Japan, India and Central Asia. According to the Byzantine historian Procopius, in the mid-sixth century two monks smuggled silkworm eggs to Constantinople for the Emperor Justinian, hidden inside hollow canes. Whatever the truth of the story, the Byzantine Empire did establish its own silk industry at around this time.

From the Byzantine world and from Muslim Spain and Sicily, silk-making spread to Italy, where cities such as Lucca and Venice became famous for luxurious fabrics in the later Middle Ages. In the sixteenth and seventeenth centuries, France built up a major industry centred on Lyon, encouraged by royal policies that promoted the planting of mulberry trees. By the nineteenth century, raising silkworms had become an important source of income for rural families in southern France and northern Italy.

In the 1850s, however, European silk production was devastated by a disease of silkworms known as pébrine, which caused dark spots on the insects and killed them in vast numbers. In 1865 the French government asked the chemist Louis Pasteur to investigate, although he had never studied silkworms. Pasteur and his assistants showed that the disease was caused by microscopic organisms passed from the female moth to her eggs, and they devised a simple method of control: each female was examined under a microscope after laying, and if she was found to be infected, all her eggs were destroyed. The method helped the industry to recover, but European silk never regained its former importance, partly because of competition from Asian producers.

Today China produces the large majority of the world's raw silk, followed by India, which is unusual in producing all of the main commercial types, including several from wild or semi-domesticated moths. Synthetic fibres, beginning with nylon, which replaced silk in women's stockings from the 1940s, have taken over many of silk's former uses. Yet demand for the natural fibre persists, and researchers are now exploring new applications for silk proteins in medicine, from surgical sutures to scaffolds for repairing damaged tissue. Some producers also make so-called peace silk, in which the moths are allowed to emerge before the cocoons are processed, producing a softer but less even yarn.

To a plant, a desert presents a formidable combination of challenges. Rain is scarce and unpredictable, often falling in a few heavy storms separated by months or even years of drought. Temperatures at the soil surface can far exceed those of the air, while nights may be bitterly cold, and strong sunlight and drying winds draw moisture from any exposed tissue. Yet deserts are far from lifeless. Over millions of years, plants have evolved a remarkable range of strategies for coping with drought, which botanists often group into three broad categories: escaping it, storing water to survive it, and enduring it.

The simplest strategy is to avoid drought altogether by living only when water is available. Desert annuals, sometimes called ephemerals, spend most of their existence as seeds lying dormant in the soil. Many of these seeds contain chemicals that inhibit germination and are washed away only by a substantial fall of rain, which prevents them from sprouting after a light shower that would not provide enough moisture for the plant to complete its life. When a heavy storm does arrive, the seeds germinate, and the plants grow, flower and set seed within a few weeks. In exceptionally wet years, this can produce spectacular displays known as superblooms, when large areas of desert are carpeted with flowers.

Succulent plants take a different approach, storing water in fleshy tissues when it is available and using it slowly during dry periods. The saguaro cactus of the Sonoran Desert, which can grow taller than ten metres, has a pleated stem that expands like an accordion as it absorbs water after rain and contracts again as the water is used. Most of its roots lie only a few centimetres below the surface, spreading outwards for a distance roughly equal to the plant's height, so that they can capture rainwater quickly before it evaporates. In a mature saguaro, stored water accounts for most of the plant's total weight. The spines of cacti are modified leaves; they lose far less water than broad leaves would, and they also shade the stem and deter thirsty animals.

Storing water is of little use if it is quickly lost, and most of a plant's water loss occurs through stomata, the tiny pores that open to take in carbon dioxide for photosynthesis. Many succulents, including cacti and agaves, use a modified form of photosynthesis known as crassulacean acid metabolism, or CAM, named after the stonecrop family in which it was first studied. CAM plants open their stomata at night, when the air is cooler and more humid, and store the carbon dioxide they absorb in the form of an acid. During the day the stomata stay closed, and the stored carbon dioxide is released inside the plant for photosynthesis. As a result, CAM plants use only a fraction of the water that conventional plants need to fix the same amount of carbon, although most of them grow slowly.

Other plants rely on reaching water that lies far below the surface. Mesquite trees, common in the deserts of North America, have roots that can penetrate tens of metres into the ground, and roots have reportedly been found at depths of more than 50 metres during mining excavations. Such plants, known as phreatophytes, tap underground water that remains available even in years without rain, and they often grow along dry riverbeds where water collects beneath the sand. Their survival depends on the level of that groundwater, however; where it is pumped out for farms or cities, they can decline rapidly.

Some plants simply endure drying out. The so-called resurrection plants can lose almost all the water in their tissues, curling into a brown and apparently dead ball, and then revive within hours when rain returns. One such species from the Chihuahuan Desert is sold as a curiosity, because it unfurls and turns green soon after being placed in water. The creosote bush, widespread in the deserts of the south-western United States and Mexico, tolerates extreme drought, partly thanks to small leaves coated with resin that reduces water loss. It is also among the longest-lived organisms known: one ring of creosote bushes in the Mojave Desert, nicknamed King Clone, is thought to have grown outwards from a single original plant for about 11,700 years.

In a few coastal deserts, plants live alongside an unusual source of moisture. The Namib Desert of south-western Africa receives very little rain but is regularly covered by fog drifting in from the Atlantic Ocean. Its most famous plant, Welwitschia mirabilis, produces only two leaves in its entire life, which grow continuously from the base and become split and tattered at their tips. Individual plants are believed to live for many centuries, although estimates for the largest specimens vary widely. Whether the plant can absorb fog through its leaves has long been debated, and the question is still not fully settled.

Scientists are increasingly interested in these adaptations as climate change brings more frequent droughts to farming regions. Researchers are investigating whether the genes responsible for CAM photosynthesis could be introduced into conventional crops to reduce their water requirements, and plants such as agaves are being studied as potential sources of biofuel that could be grown on land too dry for food crops. Such projects are still at an early stage, but they suggest that plants which have spent millions of years adapting to scarcity may hold lessons for a warming world.

For most of the twentieth century, the effect of minimum wages on employment seemed to many economists a settled question. According to the standard model of a competitive labour market, a wage floor set above the level the market would otherwise produce makes workers more expensive, so employers hire fewer of them. Minimum wages might raise the pay of those who kept their jobs, but at the cost of unemployment among the least skilled, often the young. Introductory textbooks presented this conclusion with considerable confidence. Over the past three decades, however, empirical research has unsettled it, and I believe the evidence now supports a more nuanced position than either supporters or opponents of minimum wages usually admit.

The turning point came with a study by the economists David Card and Alan Krueger, published in 1994. In April 1992 the state of New Jersey raised its minimum wage from $4.25 to $5.05 an hour, while neighbouring Pennsylvania did not. Card and Krueger surveyed fast-food restaurants on both sides of the state border before and after the increase, treating Pennsylvania as a comparison group that would show what would have happened in New Jersey without the change. Contrary to the textbook prediction, they found no evidence that employment in New Jersey's restaurants had fallen relative to Pennsylvania's; if anything, it had slightly increased. Fast-food restaurants were a natural choice for such a study, because they employ large numbers of low-paid workers and their wages closely follow the legal minimum.

The study provoked fierce controversy. The economists David Neumark and William Wascher re-examined the episode using payroll records obtained from restaurants, rather than the telephone survey data Card and Krueger had collected, and reported that the increase had in fact reduced employment. Card and Krueger replied with an analysis of official employment statistics that supported their original finding. The exchange illustrated a difficulty that has dogged the field ever since: results can depend heavily on the data and methods chosen, and well-qualified researchers examining the same episode can reach different conclusions.

Why might a higher minimum wage not destroy jobs? One explanation draws on the idea of monopsony, a term introduced by the economist Joan Robinson in the 1930s to describe a market in which a single buyer, or a small number of buyers, has the power to set prices. If employers have some power to set wages, perhaps because workers cannot easily move between jobs, they may pay less than the value of what workers produce. In that case, a moderate minimum wage can raise pay without reducing employment, and might even increase it by drawing more people into work. Higher wages may also reduce the costly turnover of staff and encourage greater effort.

Subsequent research has largely, though not universally, supported the view that moderate minimum wages have small effects on employment. When the United Kingdom introduced a national minimum wage in 1999, many predicted significant job losses, but studies commissioned by the Low Pay Commission, the independent body that advises the government on the rate, found little evidence of them. A 2019 study by Doruk Cengiz, Arindrajit Dube, Attila Lindner and Ben Zipperer examined 138 increases in state minimum wages in the United States between 1979 and 2016. They found that the number of jobs paying below the new minimum fell, but that this was almost exactly offset by an increase in jobs paying at or slightly above it, implying little overall loss of employment for low-wage workers. Germany, which introduced a national minimum wage of 8.50 euros an hour in 2015, offers a further test: most studies there have found that large-scale job losses did not materialise, although the number of small, part-time jobs known as mini-jobs fell.

Not every study points in the same direction, however. When Seattle raised its minimum wage in stages towards $15 an hour, a team at the University of Washington found that the increase to $13 in 2016 led employers to cut the hours worked by low-wage employees, so that some workers' total earnings fell. A separate team at the University of California, Berkeley, studying the city's restaurant industry, found no significant effect on employment. The Seattle increases were large relative to local wages, and the conflicting results are a reminder that the effects of a very high minimum wage remain genuinely uncertain.

It is important to be clear about what the evidence does and does not show. Most of the increases studied were moderate in relation to local pay, and the finding that such increases cost few jobs cannot simply be extrapolated to much larger ones. There must be some level at which a minimum wage would cause substantial unemployment; the real debate is about where that level lies. Nonetheless, the research has transformed the discipline. In 2021 David Card was awarded a share of the Nobel prize in economics, partly for his use of natural experiments such as the New Jersey study.

In my view, the sensible conclusion is that governments can raise minimum wages with more confidence than the old textbooks suggested, but should do so gradually and by reference to typical local earnings. The United Kingdom's approach of setting a target linked to median earnings, and asking an independent commission to monitor the effects, offers one model. Minimum wages are not a cost-free way of eliminating poverty, but nor are they the job-destroying measure that economists once almost unanimously assumed them to be.