CAE Reading and Use of English Part 5
You are going to read a newspaper article about trees and leaves. For questions 31-36, choose the answer (A, B, C or D) which you think fits best according to the text.
Those brilliant autumn outfits may be saving trees
As trees across the northern areas of the globe turn gold and crimson, scientists are debating exactly what these colours are for. The scientists do agree on one thing: the colours are for something. That represents a major shift in thinking. For decades, textbooks claimed that autumn colours were just a by-product of dying leaves. ‘I had always assumed that autumn leaves were waste baskets’ said Dr. David Wilkinson, an evolutionary ecologist at Liverpool John Moores University in England. ‘That’s what I was told as a student.’
During spring and summer, leaves get their green cast from chlorophyll, the pigment that plays a major role in capturing sunlight. But the leaves also contain other pigments whose colours are masked during the growing season. In autumn, trees break down their chlorophyll and draw some of the components back into their tissues. Conventional wisdom regards autumn colours as the product of the remaining pigments, which are finally unmasked.
Evolutionary biologists and plant physiologists offer two different explanations for why natural selection has made autumn colours so widespread. Dr. William Hamilton, an evolutionary biologist at Oxford University, proposed that bright autumn leaves contain a message: they warn insects to leave them alone. Dr. Hamilton’s ‘leaf signal’ hypothesis grew out of earlier work he had done on the extravagant plumage of birds. He proposed it served as an advertisement from males to females, indicating they had desirable genes. As females evolved a preference for those displays, males evolved more extravagant feathers as they competed for mates. In the case of trees, Dr. Hamilton proposed that the visual message was sent to insects. In the autumn, aphids and other insects choose trees where they will lay their eggs. When the eggs hatch the next spring, the larvae feed on the tree, often with devastating results. A tree can ward off these pests with poisons. Dr. Hamilton speculated that trees with strong defences might be able to protect themselves even further by letting egg-laying insects know what was in store for their eggs. By producing brilliant autumn colours, the trees advertised their lethality. As insects evolved to avoid the brightest leaves, natural selection favoured trees that could become even brighter.
‘It was a beautiful idea’ said Marco Archetti, a former student of Dr. Hamilton who is now at the University of Fribourg in Switzerland. Dr. Hamilton had Mr. Archetti turn the hypothesis into a mathematical model. The model showed that warning signals could indeed drive the evolution of bright leaves – at least in theory. Another student, Sam Brown, tested the leaf-signal hypothesis against real data about trees and insects. ‘It was a first stab to see what was out there,’ said Dr. Brown, now an evolutionary biologist at the University of Texas.
The leaf-signal hypothesis has also drawn criticism, most recently from Dr. Wilkinson and Dr. H. Martin Schaefer, an evolutionary biologist at the University of Freiburg in Germany. Dr. Wilkinson and other critics point to a number of details about aphids and trees that do not fit Dr. Hamilton’s hypothesis. Dr. William Hoch, a plant physiologist at the University of Wisconsin, argues that bright leaves appear on trees that have no insects to ward off. ‘If you are up here in the north of Wisconsin, by the time the leaves change, all the insects that feed on foliage are gone’ Dr. Hoch said. In their article, Dr. Schaefer and Dr. Wilkinson argue that a much more plausible explanation for autumn colours can be found in the research of Dr. Hoch and other plant physiologists. Their recent work suggests that autumn colours serve mainly as a sunscreen.
Dr. Hamilton’s former students argue that the leaf-signal hypothesis is still worth investigating. Dr. Brown believes that leaves might be able to protect themselves both from sunlight and from insects. Dr. Brown and Dr. Archetti also argue that supporters of the sunscreen hypothesis have yet to explain why some trees have bright colours and some do not. ‘This is a basic question in evolution that they seem to ignore’ Dr. Archetti said. ‘Idon’t think it’s a huge concern,’ Dr. Hoch replied. ‘There’s natural variation for every characteristic.’
Dr. Hamilton’s students and their critics agree that the debate has been useful, because it has given them a deeper reverence for this time of year. ‘People sometimes say that science makes the world less interesting and awesome by just explaining things away’ Dr. Wilkinson said. ‘But with autumn leaves, the more you know about them, the more amazed you are.’
31 What is stated about the colours of autumn leaves in the first two paragraphs?
There has previously been no disagreement about what causes them.
The process that results in them has never been fully understood.
Different colours from those that were previously the norm have started to appear.
Debate about the purpose of them has gone on for a long time.
32 The writer says that Dr Hamilton’s work has focused on
the different purposes of different colours.
the use of colour for opposite purposes.
the possibility that birds and insects have influenced each other’s behaviour.
the increased survival rates of certain kinds of tree.
33 Dr Hamilton has suggested that there is a connection between
the colours of autumn leaves and the behaviour of insects.
the development of brighter leaves and the reduced numbers of certain types of insect.
the survival of trees and the proximity of insects to them.
the brightness of leaves and the development of other defence mechanisms in trees.
34 What is said about the work done by former students of Dr Hamilton?
Neither of them was able to achieve what they set out to do.
Mr Archetti felt some regret about the outcome of the work he did.
Both of them initiated the idea of doing the work.
Dr Brown did not expect to draw any firm conclusions from his work.
35 Critics of Dr Hamilton’s theory have expressed the view that
it is impossible to generalise about the purpose of the colours of autumn leaves.
his theory is based on a misunderstanding about insect behaviour.
the colours of autumn leaves have a different protective function.
his theory can only be applied to certain kinds of insect.
36 In the debate between the two groups of people investigating the subject, it has been
something regarded as a key point by one side is in fact not important.
further research will prove that Dr Hamilton’s theory is the correct one.
both sides may in fact be completely wrong.
the two sides should collaborate.