|
Few words to say about this bookBog'liq THE-BIBLE-OF-IELTS-READING-BOOKC
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?
|
| |
