Profitable Wonders
For all its predictability, spring always comes as a most welcome surprise.
Almost overnight, it seems, the earth is reanimated and suffused with colour; with ranks of golden daffodils, cherry trees blossoming pink and white and the freshly minted greenery of beech and oak unfolding.
So much happens in so short a time, assiduously recorded for more than 60 years by Norfolk landowner Robert Marsham as ‘The Indications of Spring’.
We learn that in 1755 the leaves of both hawthorn and horse chestnut budded on 31st March. A day later, ‘the frogs and toads croaked’. Within a week – on 6th April – the swallows returned. Next it was the turn of the ash, sycamore and elm to bud simultaneously. Then, on consecutive days – 13th and 14th April – he heard first the song of the cuckoo and then the nightingale.
The leafing of the oak – on 10th May – coincided with his first sighting of young rooks, promptly followed by the flowering of the hawthorn. And so on.
The precise timing of this pageant of new beginnings varies a bit from year to year. It was delayed for almost a month by a bitterly cold winter in 1740, when Marsham records that his chamber pot would freeze overnight. In other years, it was advanced. But the sequential pattern remains pretty much the same. That requires explanation.
The rhythmical cycle of the seasons inevitably turns out to be a fascinatingly complex matter. It is predicated, however, on two very different if equally extraordinary phenomena: a fortuitous, catastrophic, astronomical event four billion years ago; and the ability – relatively recently discovered – of plants and animals to measure the passage of time with great precision.
First, that astronomical event. Not long after the formation of our solar system all those billions of years ago, a massive asteroid, it is hypothesised, collided with the young Earth,
knocking it off its kilter. Ever since, our world, rather than spinning round a vertical axis through its poles, has tilted. In the third century BC, the Greek astronomer Eratosthenes – besides calculating the circumference of the Earth – estimated that axial tilt to be 23.4 degrees, equivalent apparently to the angle at which a double-decker bus would topple.
The implications of that tilt for the cycle of the seasons is as follows. Imagine the Earth to be orbiting anticlockwise around the Sun, transposed on to a clockface.
At 9am, with the axis tilted, the Sun’s radiation is oriented preferentially towards the northern hemisphere, whose longest day, the summer solstice, falls on 21st June. Six months later – at 3pm, as it were – the Sun is now oriented towards the southern hemisphere, shining less directly on the northern hemisphere, whose days have shortened, culminating in the winter solstice on 21st December.
Put another way, without that axial tilt, everywhere on Earth would receive the same 12 hours of light and darkness throughout the year – and there would be no seasons.
Next, the biological contribution to spring’s awakening. The first hint that trees and flowers might calibrate the passage of time emerged in the early decades of the 20th century. That’s when plant physiologists Henry Allard and Wightman Garner demonstrated that flowering could be advanced or delayed by an experimental manipulation of the amount of light to which plants are exposed.
Broadly speaking, they identified two main categories of what they called photoperiodism: short-day plants, which flowered in autumn when the daylight is shortening; and long-day plants, which flowered in the spring and summer when it is lengthening.
From this, they inferred that there had to be some unknown ‘signal molecule’ in the leaves. This molecule must be responsive to the duration of the hours of daylight. When transported to the tips of buds, it initiated flowering. Just over ten years ago, a Swedish scientist identified that molecule as a tiny piece of genetic material, known as messenger RNA.
Similar experiments have revealed the photoperiodic response in animals and birds. It initiates the many hormonal and reproductive changes that ensure – depending on the length of their gestation period – that their offspring are born at the time of year most conducive to their survival.
The ‘surprise of spring’ turns out to be very surprising indeed. It is lucidly explored by Oxford neuroscientists Russell Foster and Leon Kreitzman in their Seasons of Life (Profile Books, 2009).
‘The surprise of spring turns out to be very surprising indeed’