IELTS Reading Matching Information: Autumn Leaves

Autumn leaves

Canadian writer Jay Ingram investigates the mystery of why leaves turn red in the fall*

A  Background 

Why do leaves turn red in autumn?

B ‘What we already know’

Summer leaves are green because they are full of chlorophyll, the molecule that captures sunlight and converts that energy into new building materials for the tree. As fall approaches, the amount of solar energy available declines, so the tree saves its resources by dropping its leaves. As chlorophyll runs out, other colours show through. But this does not explain the reds and purples of some trees.

C ‘The question’

The source of the red is widely known: it is created by anthocyanins. What’s puzzling is that anthocyanins are actually newly made in the leaves at the same time as the tree is preparing to drop them. Why should a tree bother making new chemicals in its leaves when it’s about to drop them?

D Theory 1

Some think the new chemicals might act as a defence against attacks by insects or fungi, or that they might attract fruit-eating birds or increase a leaf’s tolerance to freezing. However there are problems with each of these theories.

E Theory 2

It has also been proposed that trees may produce red colours to show insects that they are healthy and robust and would be easily able to mount chemical defences against infestation so that the insects might lay their eggs on a duller, and less resistant host. But there’s no proof to support it. 

F Theory 3

Perhaps the best 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 – that the red pigment is made in autumn leaves to protect chlorophyll from too much light. But why does chlorophyll need protection when it is the natural world’s supreme light absorber?

G Explanation

Chlorophyll can sometimes be overwhelmed by sunlight.  

H  Evidence

There are clues to support this theory. On many trees, the leaves that are the reddest are those on the side of the tree which gets most sun and the red is brighter on the upper side of the leaf. Also, many trees get much redder the more north you travel. It’s colder there, so they’re more stressed, their chlorophyll is more sensitive and it needs more sunblock.

I Conclusion

It is still not fully understood why some trees produce red pigments while others don’t bother, and simply reveal their orange or yellow hues.

NB ‘fall‘ (US) = ‘autumn‘ (UK)