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READING PASSAGE 17 
Autumn leaves
 
Canadian writer Jay Ingram investigates the mystery of why leaves turn red in the fall 
A 
One of the most captivating natural events of the year in many areas throughout North America is the 
turning of the leaves in the fall. The colours are magnificent, but the question of exactly why some trees turn 
yellow or orange, and others red or purple, is something which has long puzzled scientists. 
B 
Summer leaves are green because they are full of chlorophyll, the molecule that captures sunlight converts 
that energy into new building materials for the tree. As fall approaches in the northern hemisphere, the amount 
of solar energy available declines considerably. For many trees – evergreen conifers being an exception – the 
best strategy is to abandon photosynthesis* until the spring. So rather than maintaining the now redundant 
leaves throughout the winter, the tree saves its precious resources and discards them. But before letting its 
leaves go, the tree dismantles their chlorophyll molecules and ships their valuable nitrogen back into the twigs. 
As chlorophyll is depleted, other colours that have been dominated by it throughout the summer begin to 
be revealed. This unmasking explains the autumn colours of yellow and orange, but not the brilliant reds and 
purples of trees such as the maple or sumac. 
C 
The source of the red is widely known: it is created by anthocyanins, water-soluble plant pigments reflecting 
the red to blue range of the visible spectrum. They belong to a class of sugar-based chemical compounds also 
known as flavonoids. What’s puzzling is that anthocyanins are actually newly minted, made in the leaves at the 
same time as the tree is preparing to drop them. But it is hard to make sense of the manufacture of 
anthocyanins – why should a tree bother making new chemicals in its leaves when it’s already scrambling to 
withdraw and preserve the ones already there? 
D 
Some theories about anthocyanins have argued that they might act as a chemical defence against attacks by 
insects or fungi, or that they might attract fruit-eating birds or increase a leafs tolerance to freezing. However 
there are problems with each of these theories, including the fact that leaves are red for such 
a relatively short period that the expense of energy needed to manufacture the anthocyanins would outweigh 
any anti-fungal or anti-herbivore activity achieved.* photosynthesis: the production of new material from 
sunlight, water and carbon dioxide. 
E 
It has also been proposed that trees may produce vivid red colours to convince herbivorous insects that they 
are healthy and robust and would be easily able to mount chemical defences against infestation. If insects paid 
attention to such advertisements, they might be prompted to lay their eggs on a duller, and presumably less 
resistant host. The flaw in this theory lies in the lack of proof to support it. No one has as yet ascertained 
whether more robust trees sport the brightest leaves, or whether insects make choices according to colour 
intensity. 
F 
Perhaps the most plausible suggestion as to why leaves would go to the trouble of making anthocyanins 
when they’re busy packing up for the winter is the theory known as the ‘light screen’ hypothesis. It sounds 
paradoxical, because the idea behind this hypothesis is that the red pigment is made in autumn leaves to protect 
chlorophyll, the light-absorbing chemical, from too much light. Why does chlorophyll need protection when it 
is the natural world’s supreme light absorber? Why protect chlorophyll at a time when the tree is breaking it 
down to salvage as much of it as possible? 
G 
Chlorophyll, although exquisitely evolved to capture the energy of sunlight, can sometimes be overwhelmed 
by it, especially in situations of drought, low temperatures, or nutrient deficiency. Moreover, the problem of 
oversensitivity to light is even more acute in the fall, when the leaf is busy preparing for winter by dismantling 
its internal machinery. The energy absorbed by the chlorophyll molecules of the unstable autumn leaf is not 

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immediately channelled into useful products and processes, as it would be in an intact summer leaf. The 
weakened fall leaf then becomes vulnerable to the highly destructive effects of the oxygen created by the 
excited chlorophyll molecules. 
H 
Even if you had never suspected that this is what was going on when leaves turn red, there are clues out 
there. One is straightforward: on many trees, the leaves that are the reddest are those on the side of the tree 
which gets most sun. Not only that, but the red is brighter on the upper side of the leaf. It has also been 
recognised for decades that the best conditions for intense red colours are dry, sunny days and coo nights, 
conditions that nicely match those that make leaves susceptible to excess light. And finally, trees such as 
maples usually get much redder the more north you travel in the northern hemisphere. It’s colder there, they’re 
more stressed, their chlorophyll is more sensitive and it needs more sunblock. 
I 
What is still not fully understood, however, is why some trees resort to producing red pigments while others 
don’t bother, and simply reveal their orange or yellow hues. Do these trees have other means at their disposal 
to prevent overexposure to light in autumn? Their story, though not as spectacular to the eye, will surely turn 
out to be as subtle and as complex. 

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