IELTS Academic Reading Practice Test 437

Vanilla is one of the most familiar flavours in the world, yet few people would recognise the plant that produces it. It comes from an orchid, a climbing vine that in the wild grows up the trunks of trees in the forests of Mexico and Central America. The vine clings to its host with aerial roots and can climb high into the canopy. Each flower lasts a single day, and only a small proportion of them ever produce the long, dark pods from which the spice is taken, which must then be cured for months before the aroma develops.

The people who first cultivated the plant were the Totonacs, who lived on the Gulf coast of Mexico. They came to regard the vine as sacred, and used the pods to perfume tobacco and to flavour a drink made from cacao. When the Aztecs conquered them in the fifteenth century, they demanded vanilla as tribute and added it to their own chocolate drink. They gave the spice a name meaning 'black flower', a reference to the appearance of the cured pod.

Vanilla reached Europe in the sixteenth century. The Spanish conquistador Hernán Cortés, who arrived in Mexico in 1519, is usually credited with bringing both cacao and vanilla back to Spain, where the two were combined in a fashionable drink. The Spanish called the pod vainilla, meaning 'little sheath', and it is from this word that the English name derives. For the next three centuries, however, vanilla remained a Mexican monopoly. Attempts to grow the vine in other tropical colonies nearly always failed, and the reason was not understood for a long time.

The problem was pollination. In its native forests the vanilla flower is traditionally said to be pollinated by a small stingless bee of the genus Melipona, though other native bees may also do the work. When cuttings were taken to islands such as Réunion, Mauritius and Madagascar, the vines grew well but produced almost no pods, because the insects that did the work had been left behind. Growers tried transferring pollen by hand, but the flower's structure is delicate and the early attempts rarely succeeded.

The breakthrough came in 1841 on Réunion, then a French colony known as Île Bourbon, and it was made by an enslaved twelve-year-old boy named Edmond Albius. Working on a plantation where his owner, a botanist, had been trying to fertilise vanilla flowers, Albius found that the membrane separating the pollen from the stigma could be lifted with a small stick or the thumb and the two pressed together. The technique took seconds and could be taught to anyone; it is essentially the method still used today. Réunion soon became a major producer, and the 'Bourbon' vanilla sold today takes its name from the island's old colonial title.

Albius received little reward, and for decades the technique was credited to his owner rather than to him. He was freed when slavery was abolished in the French colonies in 1848 and took the surname by which he is now known, but he spent much of his later life in poverty and died in 1880. A statue stands in his honour on Réunion, and the method he devised made possible the vanilla trade as it exists today.

After pollination, the pod takes eight or nine months to ripen and is harvested while still green. The flavour develops during curing, which traditionally involves killing the pod by plunging it into hot water or exposing it to strong sun, then sweating it in blankets and drying it slowly in the shade for several weeks. Chemical changes during this period produce vanillin and hundreds of other compounds, and the skill of the curer is one reason why the best vanilla is so expensive.

Vanilla is, by most measures, the second most costly spice in the world after saffron. The vines take three or four years to begin producing, the flowers must be pollinated by hand one at a time, and the curing is slow. Mexico, where the plant originated, now grows only a small share of the world crop. Madagascar and the neighbouring islands of the Indian Ocean supply the great majority of it, while the vanilla of Tahiti comes from a different species, with a fruitier scent, and is correspondingly rare. The crop is also vulnerable to cyclones, which can strip a season's flowers in a few hours.

Vanillin, the compound chiefly responsible for the smell and taste, was first produced artificially in the 1870s by German chemists, who made it from a substance found in pine bark. Other routes followed: vanillin was extracted from clove oil, and later from the waste liquid of paper mills. Today most of the flavouring used in chocolate, ice cream and soft drinks is synthetic, made from petrochemicals, and costs a fraction of the natural product. For buyers, the difference matters: the pods contain a complex mixture that synthetic vanillin cannot reproduce.

Small farmers who grow vanilla are exposed to violent swings in price, since a good harvest in one year can depress the price the next, and theft from plantations is a serious problem in Madagascar, where growers sometimes harvest the pods early to guard against it. Attempts to insure crops and to shorten the long gap between planting and income have had limited success. For all its familiarity, vanilla remains an uncertain livelihood for those who produce it.

Most people treat an optical illusion as a curiosity, a trick played by a picture on an eye that is easily deceived. Vision scientists see it differently. For them an illusion is an experiment that nature has already performed: a case where the visual system produces a systematic error, and where the pattern of the error reveals the rules the system follows. Illusions are therefore not failures of vision but one of the most direct routes into how ordinary seeing works.

The starting point is the eye. Light entering the pupil is focused by the lens onto the retina, a thin sheet of tissue at the back of the eye. Only a small central region, the fovea, gives sharp detail; towards the edges, resolution falls away steeply, and at the point where the optic nerve leaves the eye there are no receptors at all. The retinal image is also flat, upside down and much smaller than the scene it represents. From this impoverished signal the brain produces a stable, coloured, three-dimensional world, and it does so by guessing.

The best-known example is the Müller-Lyer illusion, in which two horizontal lines of identical length appear to differ because the ends of one carry inward-pointing fins and the ends of the other outward-pointing ones. The most widely taught explanation is that the brain reads the fins as cues about depth and then applies a correction for distance. In the 1960s a celebrated study compared city-dwelling Europeans with San hunter-gatherers of the Kalahari and found the town dwellers considerably more susceptible. Yet later work has detected the illusion in a wide range of populations, including groups with little experience of rectangular buildings, so the cultural account cannot be the whole story.

A different puzzle is set by the Hermann grid, a pattern of black squares separated by white lines. Most viewers notice grey smudges at the intersections of the lines, and the smudges disappear as soon as they look straight at them. The textbook account invokes lateral inhibition: cells in the retina that respond to bright light suppress their neighbours, so the intersections are signalled as darker than the surrounding lines. Elegant as this is, experiments have shown that the effect depends on the size and spacing of the grid in ways the simple version does not predict, and several rival explanations now compete.

Colour shows the same constructive work more strikingly. In a famous demonstration devised by Edward Adelson of the Massachusetts Institute of Technology, a chessboard is shown with a shadow falling across it. Two squares, one inside the shadow and one outside, are identical in brightness, yet the one in the shadow looks far lighter, because the brain allows for the dimming effect of the shadow and reports the colour of the surface rather than the light reaching the eye.

The tendency is called colour constancy, and it is not identical in everyone: in 2015 a photograph of a dress divided viewers, some of whom saw its colours as blue and black and others as white and gold. The image had not changed; the assumptions about lighting had.

In the peripheral drift illusion, a static pattern of repeated shapes appears to rotate or drift when it is seen out of the corner of the eye, and stops the moment the viewer looks at it directly. The patterns that work best contain a repeated sequence of dark and light edges arranged so that brightness changes more abruptly in one direction than the other. Motion detectors in the visual cortex are tuned to precisely this kind of asymmetry, and they signal movement where none exists. It is strongest in peripheral vision, where the brain is most sensitive to motion and least able to check it against detail.

Expectation supplies another family of errors. When a hollow mask is rotated, viewers almost invariably see a solid, convex face turning towards them, even though the light and the shading plainly indicate a hollow interior; the brain's model of what a face should be overrides the evidence. A similar preference for the familiar explains the Ames room, a distorted chamber that looks rectangular from a particular viewpoint. A person walking from one corner to another seems to shrink dramatically. The brain has interpreted the scene using the assumption that rooms have right-angled corners.

Why the brain should be fooled at all, and whether one theory can cover every case, remain open questions. The most ambitious accounts treat perception as a form of inference, in which the brain combines noisy signals from the eyes with prior expectations and settles on the most probable interpretation of the scene. Such theories explain a great deal, but they are hard to test directly, and some illusions have several competing explanations that predict the same experience. Attempts to link susceptibility to illusions with particular clinical conditions have produced inconsistent results, which suggests that the underlying mechanisms are more varied than was once assumed.

Illusions matter beyond the laboratory. Designers of roads, aircraft cockpits and control rooms take account of them, because a display that misleads the eye can mislead the hand; the painted markings that make a road appear narrower deliberately exploit the same perceptual shortcuts. Artists have exploited them for centuries. And for the rest of us they are a reminder that what we see is not a photograph of the world but a working model of it, assembled quickly from incomplete evidence.

Graffiti is as old as writing, but the word itself is Italian, derived from graffiare, to scratch, and it was first used by archaeologists for marks incised into plaster and stone. In everyday use today it usually means something narrower: spray-painted names and images on walls, trains and street furniture. The argument about whether such work should be regarded as art or as vandalism has been running for half a century and has generated more heat than light.

Marking public surfaces is not a modern habit. The walls of Pompeii carry thousands of inscriptions: election slogans, declarations of love, insults, notices of gladiatorial games and complaints about prices. Medieval churches contain ships, hands, initials and prayers scratched into the stone by masons and pilgrims, and soldiers in the Second World War left the figure of Kilroy wherever they went. What is new is not the impulse to leave a mark but the technology: the aerosol can, cheap and portable, allowed a name to be written large and in colour, and in a few seconds.

Modern graffiti grew out of New York in the late 1960s and 1970s. Teenagers began writing their names, or tags, on walls and then on subway cars, and the practice spread rapidly through the city's hip-hop culture. A tag was a signature, valued for its style and its placement, and the most admired writers were those whose names travelled furthest on the trains. Transit officials spent heavily on cleaning cars and by the 1980s had largely cleared the system, but the practice had already been carried to other cities and continents.

For property owners and city authorities the case against graffiti is straightforward. It is made without the consent of the person who owns the wall, and removing it costs public money. In 1982 two American criminologists, James Wilson and George Kelling, published an argument that gave such concerns a theoretical shape: visible disorder, they suggested, signals that nobody is in charge and so invites more serious offending. The idea, later known as broken windows, influenced policing in New York and elsewhere. The authors themselves later complained that it had been applied in a cruder form than they had intended, and the evidence that cleaning up graffiti reduces violent crime is much weaker than the theory implies.

Art historians and critics make a different case. They point to the skill involved, the invention shown in adapting a design to a particular wall, and the long tradition of artists working outside official institutions. Jean-Michel Basquiat began by painting cryptic slogans on the streets of New York before his work reached galleries, and Banksy's stencils, made illegally in Bristol and London, now sell for large sums while the artist remains anonymous. Museums collect what was once scrubbed away. The weakness of this defence is that it selects the few hundred practitioners who succeeded and passes over the thousands of scribbled initials that nobody would call art.

Empirical support for the disorder argument has been sought in field experiments rather than in crime statistics. In one widely cited study, researchers in the Dutch city of Groningen left an envelope with a five-euro note protruding from a letterbox and observed what passers-by did. In a clean, tidy street almost everyone left it alone. In a street where the walls were covered with graffiti, or where litter was scattered, roughly twice as many people took the money. The design was elegant and the result is often quoted, but it tested a small, opportunistic act in a specific setting; it says nothing about how people behave in a street where a mural has been painted.

Law and context are inseparable from the argument. In Britain the Criminal Damage Act of 1971 makes it an offence to damage property without a lawful excuse, and the quality of the image is irrelevant to the charge. Some local councils have responded by designating walls where painting is permitted, and festivals now commission large murals with the owners' agreement. Within the culture, writers distinguish between graffiti, which is chiefly about letters and style, and street art, which is about images; the distinction matters to practitioners even if it is invisible to a magistrate.

Money has complicated matters further. Murals have been used to attract visitors and investment to run-down districts, and a few neighbourhoods have become destinations in their own right. Artists who once worked in the dark now receive commissions, and their work appears on advertising campaigns. Many of them are uneasy about this. They complain that the same authorities which once prosecuted them now use their images to sell apartments, and that the arrival of galleries and expensive cafes drives out the communities that produced the work in the first place.

The dispute, in my view, is badly framed. Two questions are being asked at once, and the answer to one does not settle the other. Whether a painting on a wall has artistic merit is a matter for judgement about skill, originality and meaning. Whether it was made with the consent of the person who owns the wall is a matter of property and law. A work can be genuinely good and still be an offence against the owner, and the law's bluntness tells us nothing about its value. Cities do best when they separate the two: provide legal spaces, punish unauthorised damage in proportion to its cost, and judge each piece on its merits rather than on its category.