Few of us above a certain age can forget Shakespeare's memorable description of this time of year as that . . .
When yellow leaves, or none, or few, do hang
Upon those boughs which shake against the cold,
Bare ruined choirs where late the sweet birds sang.
Contrary to popular legend, it is not Jack Frost who brings about this autumn desiccation, having first painted the leaves in their autumn motley of various shades of yellow, brown and red.
The precise trigger for the yearly colour-change varies with the different species, being a combination of declining day-length and falling temperature, and influenced by individual genes and the prevailing climate. In Europe, for example, the horse chestnut, aesculus hippocastanum, is among the first to turn yellow as early as mid-September, while oaks and alders may remain green for another month or more.
A late spring or a summer drought, by slowing growth, may delay the onset of the autumnal change for several weeks, while a moist, mild spring, combined with vigorous growth throughout the summer months, can often promote an early fall of leaves.
The green summer foliage owes its colour to a substance known as chlorophyll. Its most important role is as a vital catalyst for photosynthesis, the process whereby hydrogen and carbon, obtained respectively from water and from CO 2 are converted by the energy obtained from sunlight into the tissues that provide for growth.
But the abundant chlorophyll acts also as a pigment, and the resultant green dominates all the other colours that the vegetation might otherwise display.
As the days grow short, however, and the temperature begins to fall with the approach of winter, deciduous trees act on the reasonable assumption that growth for several months is likely to be much curtailed. They decide, as it were, that growing in the wintertime is just not worth the effort, and begin an orderly retreat from life.
They draw in the sugar and the proteins that normally reside in foliage, and cease producing chlorophyll. The existing chlorophyll disintegrates with great rapidity; its green predominates no more, and other pigments - the oranges and reds and browns - are left for a brief but glorious period to reign supreme. Meanwhile, hormones stimulate a layer of cells at the base of every leaf to die and form a seal between the leaf and parent twig. In due course the dead cells form a protective, corky layer of tissue, from which even the lightest breeze may separate the leaf, and send it spiralling to join the countless others on the ground.