Wednesday, April 25, 2007

Plant Ballistics

Long before armies devised means of hurling projectiles at one another, plants had already devised catapults, guns and other mechanisms to help them disperse seed.

One such is the seed capsule of sphagnum moss, which, as it ripens, contracts to about a quarter of its size, and compresses the air within it to about double the pressure of a car tyre. Towards the end of the contraction, the lid blows off and the spores within are fired out with a pop, as though by an air gun, 1 ft (30 cm) or so into the air - no mean achievement for a 2 mm gun.

The Mediterranean squirting cucumber, Ecbellium elaterium, is recorded as shooting forth a stream of juice and seeds under pressure to a range of 12 yd (11 m) at a muzzle velocity of 20 mph (32 km/h) or more. A rather different method has been adopted by an American form of pumpkin, Cyclanthera explodens. Its fruit consists of two shells clamped together about a curved lever, held down under pressures that are at least ten times greater than that within a car tyre. When ripe, the shells spring apart and the lever flies out like a sling, projecting the seeds up to 3 yd (2.7 m).

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