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READING PASSAGE 15
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.
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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.
