Sentence completion

Reading Passage 1 – The Humble Pencil

A Few objects are as ordinary, or as overlooked, as the pencil. Billions are produced every year, and most of us have used one since early childhood without giving it a moment's thought. Yet the pencil has a surprisingly long and complicated history, shaped by geology, warfare, chemistry and commerce.

B The story usually begins in the sixteenth century in Borrowdale, a valley in the north-west of England, where an unusually pure deposit of graphite was discovered. Local shepherds are said to have used lumps of it to mark their sheep. At the time, nobody understood what the substance was. Because it left a dark grey mark rather like lead, it was called 'plumbago', from the Latin word for lead, and the misunderstanding has survived in the language ever since: we still talk about pencil 'lead', even though pencils have never contained the metal.

C Borrowdale graphite was so pure that it could be sawn into thin sticks and used directly for writing. These sticks were brittle and left marks on the hand, so users began wrapping them in string or sheepskin, and later inserting them into hollowed-out wooden holders. The graphite also had a more valuable use: it was used to line the moulds in which cannonballs were cast, producing smoother, rounder shot. The mine became so important that the English Crown took control of it, and at times it was flooded or guarded to prevent theft. Smuggling was common nonetheless.

D Elsewhere in Europe, pencil makers had to make do with impure graphite, which they ground into powder and mixed with substances such as sulphur or glue. The results were often scratchy and unreliable. The decisive breakthrough came in France in the 1790s. At war with Britain, France was cut off from English graphite, and the government asked the engineer and inventor Nicolas-Jacques Conté to find an alternative. His solution was to mix powdered graphite with clay, shape the mixture into rods and fire them in a kiln. The method had a further advantage that would prove enormously important: by changing the proportion of clay, the maker could control how hard or soft the pencil was. More clay produced a harder pencil and a lighter line; more graphite produced a softer pencil and a darker one.

E Conté's process is still the basis of pencil making today, and the grading systems used around the world grew out of it. In much of Europe, pencils are marked with letters: H for hard, B for black (that is, soft), and F, which is said to stand for 'fine point', for a grade between the two. Numbers alongside the letters indicate the degree, so a 4H is harder than a 2H, and a 6B softer than a 2B. In the United States, a simpler system of numbers is widely used, in which a 'number 2' pencil is roughly equivalent to the European HB.

F The wooden casing has a history of its own. For much of the nineteenth century, the preferred wood was red cedar from North America, which was soft, straight-grained and easy to sharpen. Demand was so great that by the early twentieth century supplies were running low, and manufacturers are reported to have bought up old cedar fences and barns simply to obtain the wood. Eventually most of them switched to incense cedar, which grows in the western United States and is still widely used.

G The rubber eraser arrived separately. In 1770 the English engineer Edward Nairne is credited with noticing that a piece of natural rubber could rub out pencil marks; before that, people often used pieces of bread. Rubber at that stage tended to become sticky and smelly in warm weather, and it was only after the development of vulcanisation in the 1840s, a process which made rubber far more durable, that erasers became common. In 1858 an American, Hymen Lipman, received a patent for a pencil with an eraser attached to one end, a design that remains familiar in many countries, although in much of Europe pencils are still usually sold without one.

H In an age of screens, it might be expected that the pencil would be disappearing. In fact, it continues to be manufactured in enormous numbers. It needs no power, works in extreme temperatures, and its marks can be erased – qualities that have kept it in use among artists, engineers, carpenters and schoolchildren alike.

The development of the pencil

8. Borrowdale graphite was used to coat the inside of (8) ______ for making cannonballs.

9. To stop theft, the mine was sometimes guarded or (9) ______.

10. Other European makers mixed powdered graphite with sulphur or (10) ______.

11. Conté heated rods of graphite and clay in a (11) ______.

12. Before erasers, people often rubbed out pencil marks with (12) ______.

13. Erasers only became common after a process called (13) ______ made rubber last longer.

Reading Passage 1 – Living Partnerships: The Hidden World of Lichens

A Walk through almost any landscape on Earth, from a city pavement to a windswept mountain summit, and you are likely to pass lichens without noticing them. They appear as grey-green crusts on rocks, leafy patches on tree bark or tiny branching tufts hanging from twigs. For centuries, naturalists classified them as a single kind of plant, and it was not until the second half of the nineteenth century that scientists began to understand that a lichen is not one organism at all, but a partnership.

B At the heart of every lichen is a fungus. Fungi cannot make their own food, and most obtain it by breaking down dead material or by living on other organisms. The lichen fungus has taken a different route: it houses a population of microscopic photosynthetic partners, either green algae or cyanobacteria, within its tissues. These partners use sunlight to produce sugars, a proportion of which pass to the fungus. In return, the fungus provides a sheltered structure that protects its partners from intense light and from drying out, and it absorbs water and minerals from rain, dust and the surface on which it grows. The body that results from this cooperation looks and behaves quite unlike either partner growing alone. Recent studies have suggested that some lichens also contain additional partners, such as yeasts, which makes the picture more complicated still.

C This arrangement allows lichens to survive in places where few other living things can. They are found on bare rock in hot deserts, on the frozen soils of Antarctica and on the exposed bark of trees in the tropics. The key to their resilience is an ability to tolerate almost complete drying out. When water is scarce, a lichen simply shuts down: its metabolism slows almost to a halt, and it can remain in this dormant state for long periods. As soon as it is moistened by rain, dew or even humid air, it resumes photosynthesis, sometimes within minutes. Experiments carried out on spacecraft have even shown that certain species can survive a period of exposure to the vacuum and radiation of space.

D The price of this toughness is extremely slow growth. Because lichens are active only when they are both wet and exposed to light, many crust-forming species expand by no more than a millimetre or two a year, and some grow far more slowly. Scientists have turned this characteristic into a practical tool. By measuring the diameter of the largest lichens on a surface such as a gravestone, a rock fall or ground left behind by a retreating glacier, and comparing it with lichens on surfaces of known age, researchers can estimate how long the surface has been exposed. The technique, known as lichenometry, has been used to date glacial deposits formed over the last few centuries, although its accuracy depends on careful calibration for each region, since growth rates vary with climate.

E Lichens are also remarkably sensitive to the quality of the air around them. Having no roots and no waxy outer layer of the kind that protects the leaves of many plants, they absorb much of what they need directly from rain and the atmosphere, and pollutants are taken in along with nutrients. Sulphur dioxide, released when coal and oil are burned, is particularly damaging to many species. In the industrial cities of nineteenth-century Europe, observers noticed that lichens were disappearing from trees and walls, and later studies linked this loss to polluted air. Today, surveys of which species are present in an area are widely used as an inexpensive way of monitoring air quality, and in several European cities some lichens have returned as levels of sulphur dioxide have fallen.

F People have found other uses for lichens too. In the Arctic, some species form a major part of the winter diet of reindeer and caribou, which dig through the snow to reach them. For centuries, lichens were also an important source of dyes: certain species were fermented to produce purple and red colours for wool, and the chemical indicator litmus, which turns red in acidic and blue in alkaline conditions, was originally obtained from lichens. In addition, lichens produce a wide range of chemical compounds, many of which are not known from any other group of organisms, and some of these are now being investigated for possible medical use.

G Despite their resilience, lichens face new pressures. Although many species have recovered from sulphur pollution, others are harmed by nitrogen compounds from traffic and intensive farming, which favour a small number of nitrogen-tolerant species at the expense of more sensitive ones. Because lichens grow so slowly, communities that are damaged may take decades to recover. Ecologists argue that these modest organisms deserve more attention, not only because of their value as indicators, but also because of the many roles they play in the ecosystems they inhabit, from beginning the slow process of turning rock into soil to providing food and nesting material for animals.

11. When water is not available, a lichen can stay in a (11) ______ state for long periods.

12. Lichenometry has been used to find the age of (12) ______ produced over the last few centuries.

13. Because lichens absorb substances directly from rain and air, they take in (13) ______ as well as nutrients.