Chinese witch hazels (Hamamelis mollis) expel their seeds at speeds of up to 12.3 meters per second (28 miles an hour) – not that impressive for a baseball pitcher perhaps but extraordinary for a plant. A study of the ballistics of these trees reveals some impressive ballistics to ensure the seeds fly as far as possible.
Sedentary organisms such as plants face the challenge of ensuring the next generation spreads out into the world rather than getting under one's feet (some human parents may identify). Most plants make their seeds so light they're carried on the wind or so tasty animals do the moving.
Chinese witch hazel has evolved a different method. It fires its seeds with such force they can conquer new territory. Dr Simon Poppinga of the University of Freiburg was so intrigued by the distances achieved, he filmed the ejection process with a camera capable of capturing 30,000 frames per second.
In the Journal of the Royal Society Interface, Poppinga and colleagues reveal it all starts with the force applied to the seeds, which on the witch hazel plant they studied was as great as that applied by a 19th-century gun to its bullets. This produced an acceleration around 2,000 times that induced by Earth's gravity. Rather than using explosives, however, the witch hazel squeezes the seeds like a bather holding a bar of soap until it shoots from between their fingers. Naturally, the seed shape has also evolved to minimize air resistance.
To prepare for this, the hard outer portion of the fruit shrinks, while the fleshy inner section undergoes a complex shape change to maximize pressure on the seed as it dries. All this painfully slow drying ends with a sudden powerful release that's accompanied by a loud cracking sound.
Perhaps the most remarkable aspect is the spin imparted to the seed as it leaves, causing it to rotate at up to 430 hertz. Oddly, seeds from a single tree had no consistent direction of spin, half turning clockwise and the other half counter-clockwise while one wobbled ineffectively. Humans have used the stabilizing force of spin in rifles and American football to prevent tumbling and make the projectile travel further, but it seems H. mollis was way ahead of us.
Using the measured values, the team estimate a seed fired from the highest fruit on a tree in the Freiburg Botanical Garden would fly 18 meters (60 feet).
Poppinga is still unsure exactly how the seeds are made to spin, but suggests it has something to do with the substantial ridge on the seed's surface. The source of the cracking sound and the exact mechanism of the “latch” that holds the seed in place until sufficient pressure has built up are also yet to be identified.
