Country: Nature Notes

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The Independent Culture
THE FALL of leaves is a menace to gardeners and park-keepers - a single large oak may have 250,000 to dispose of - yet it is an essential part of the life cycle of deciduous trees.

At this time of year the fall seems to be precipitated by the shortening of daylight hours, which triggers an ageing process. A weak zone, or abscission layer, forms at the base of the leaf's stalk: the cells there decay progressively until the leaf drops away, whereupon a healing layer closes the wound and forms a scar.

The same process may occur prematurely if leaves are damaged by drought, disease or insects. The shedding of leaves enormously reduces the amount of water that a tree loses through transpiration, enabling it to close down most of its activity for the winter.

Autumn colours blaze up in the dying foliage because the green chlorophyll breaks down and bleaches away, revealing other pigments. These include anthocyanin, which turns leaves red or bluish, betacyanin (red), xanthophyll (yellow) and carotene (orange).

The brilliant colours of the New England fall in the US are due to the predominance of species such as red oak and maples, rather than to any particular properties in the soil.