Most plants may be sluggards, but some accelerate faster than a rocket. Meet the world’s most explosive flowers
WHAT does it take for a plant ecologist to get into the Guinness World Records book? For it was a borrowed camera and a chance encounter with physicist that led to the discovery of the world’s fastest-opening flower. The blooms of Cornus canadensis, the bunchberry dogwood, open in less than half a millisecond, launching pollen into the air in a third of the time it takes a bullet to leave a rifle barrel. That made it a record-breaker.
We tend to think of plants as stationary, yet some move with spectacular speed as they propel their reproductive particles – seeds and spores, pollen and sperm – as far as they can. But while you might hear the pop of an exploding spore capsule or see a seedpod shooting its contents into the air, how they do it is not obvious. “Because it happens in less than the blink of an eye, the mechanism is invisible to us,” says Edwards.
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Now, through a pioneering combination of ultrafast digital photography and physics, Edwards and Whitaker are uncovering the secrets of the Usain Bolts of botany. “Plants have evolved some elegant biomechanical solutions for moving their genes to new places,” says Edwards. “It is making us look at them in a different light.”
As a self-confessed physics nerd, Whitaker was more at home with Bose-Einstein condensates than botany until, one day in 2002, he passed the lab in Williams College, Massachusetts, where Edwards was filming exploding bunchberry flowers. Could he help, she asked, and trigger the camera while she set off an explosion? “The speed it moved was so impressive, I thought I’d write an equation of motion for it,” says Whitaker, who is now based at Pomona College, California. So they teamed up, and are getting some stunning results with their latest camera, which is able to record up to 500,000 frames per second on a continuous loop, always saving the last few seconds of images. “In the past you had to anticipate an event and synchronise filming to capture it. Now we can wait for something to happen and then hit stop,” he says.
The first revelation was the bunchberry’s remarkable speed. Their footage showed that the petals fly back in 0.2 milliseconds and the stamens spring upwards to reach a speed of 22 kilometres an hour in 0.5 milliseconds – an acceleration of 2400 g, or 800 times that of the space shuttle on lift-off.
Medieval mechanism
These flowers are only 2.5 millimetres tall, yet they fire their pollen 10 times that distance, and always vertically. How could they pack such a powerful punch? “Once we had an equation of motion we could see it had a really clever mechanism for throwing its pollen upwards,” says Whitaker. It turns out each stamen is a trebuchet, a miniature version of the catapult Europe’s medieval armies used to increase the range of their missiles.
A trebuchet maximises throwing distance by having its payload attached to the throwing arm by a hinge or strap. In the bunchberry, the payload (pollen) is attached to the tip of the throwing arm (the filament) by a thin, flexible strip of filament. “You can sort of see the basic movement in the images, but to get the whole picture you need a combination of images and a model of the motion,” says Whitaker. “That gives better resolution of the timing and also allows you to infer motions at the centre of the flower that are obscured in the videos.”
By 2007, the pair finally understood what was going on. As flowers mature, the stamens grow faster than the petals, which remain joined at their tips. Confined within the petal “cage”, the stamen filaments bend, eventually protruding from between the petals like four bent knees. Increasing tension within the growing flower causes a build-up of elastic energy in the stamens and petals. When an insect hits a trigger hair on one of the petals, the petals burst apart and the stamens shoot upwards (see photo).
As the stamens fly up, the pollen-filled anthers remain pressed firmly together, maintaining contact by swivelling on their straps. Only when the stamens reach maximum velocity do the anthers finally fly apart and fling their loads skywards (). “The release is at just the right moment to maximise the upward thrust of pollen,” says Whitaker.
In fact, the bunchberry’s trebuchet could not be any more efficient. “I fed different combinations of length and stiffness of filament, and size and weight of anthers and so on into the model,” says Whitaker. “But no other combination worked as well.”
Bunchberry flowers can only be cross pollinated, and the rapid-fire trebuchet is what makes that possible. Each plant has hundreds of small flowers, so ensuring pollen reaches the flowers of another plant is a challenge. Only larger insects that fly quickly from plant to plant are heavy enough to trigger opening, and when they do the fast-flying pollen hits them so hard it sticks among their hairs, staying put long enough to make it to another plant. As a back-up, bunchberry flowers eventually burst unaided, lofting pollen out of the still air close to the woodland floor into more turbulent regions where it can hitch a ride on the breeze.
If Edwards and Whitaker were astonished by the bunchberry’s ingenuity, they got an even bigger surprise when they investigated the exploding spore capsules of sphagnum moss. These bog mosses are among the world’s most important plants, forming deep mats over 1 per cent of the world’s land surface and locking away immense amounts of carbon. If you visit a bog in summer, you will find it hard to ignore the sound of popping spore capsules. Ripe capsules are spherical, but as they dry the walls shrink, squeezing the sides to form a long, thin cylinder. The air inside is increasingly compressed, until eventually the capsule’s cap flies off and spores shoot out.
The video footage shows the top blows off in less than 0.01 milliseconds. “It’s so fast you can’t really measure it,” says Edwards. The spores blast out with an initial acceleration of 36,000 g, around 10,000 times that of a rocket car, and reach a top speed of 82 kilometres an hour. “It’s hard to put into words how powerful that explosion is,” says Whitaker.
What was odd was the distance the spores travelled. At such speeds, they should experience immense drag forces and come to a halt in less than half a millisecond, travelling only a few millimetres. Yet somehow they kept on moving – and at speed. Even after 5 milliseconds they were still moving at more than 10 kilometres an hour. Some got as far as 16 centimetres away, with an average journey of 11 centimetres. Something special was happening, but the images were too fuzzy to make out what. “Even at 10,000 frames per second you can’t freeze the motion, and everything is blurry,” says Whitaker. So they cut the exposure time to 20 microseconds. “Then we saw beautiful mushroom clouds – the signature of a vortex ring.”
Vortex rings have peculiar propulsive properties: the air within the ring rolls round and round, a motion that propels the ring through the surrounding air more or less intact. Vortex rings aren’t hard to make, says Whitaker. “You need a circular opening and a brief puff of air through the hole. But once you’ve created the ring, it keeps moving at almost constant speed.” The explosive blast of compressed air shooting out of the circular opening in the spore capsule creates a vortex ring that powers upwards carrying thousands of spores along with it (). “This is the first time vortex rings have been seen in a plant,” says Whitaker.
Sphagnum needs its sophisticated dispersal system because many of the 285 species are very exacting about where they grow and must ensure some spores reach the right microhabitat. That is a challenge for plants with spore capsules scarcely a centimetre above ground, where the air is very still. If they are to travel far, spores must break through that still layer into the moving air around 10 centimetres beyond.
Like many movie-makers, Edwards and Whitaker have a string of sequels in the pipeline. Their next fast-action hero is likely to be the sperm-squirting liverwort.
“With a string of sequels in the pipeline, the next fast-action hero is the sperm-squirting liverwort”
Liverworts, the earliest land plants, are generally confined to damp habitats where sperm from male plants can swim short distances to female plants through a surface film of water. However, some discharge sperm explosively into the air. In 2008, Masaki Shimamura and colleagues at Hiroshima University in Japan captured images of a liverwort firing sperm up to 15 centimetres into the air. In the field they found female plants that were fertilised despite growing more than a metre from the nearest male.
“We have finished filming but we haven’t worked out all the details of the mechanism yet,” says Edwards. “It is completely different from sphagnum, although you wouldn’t know that without analysing the footage.” What’s clear is that pressure builds inside the male antheridia until they pop open. The sperm blast out not in a puff of air but in a cylinder of water – a sort of high-pressure sperm hose. “There are different ways of optimising the movement of water through air and that’s what we are looking at now,” says Whitaker.
Like Hollywood action heroes, there seems to be no shortage of plant stars for Edwards and Whitaker to choose from. Look out for the stinging nettle’s pollen catapult – a completely different device from the bunchberry’s – and the wood sorrel’s flip-action seed cannon “We’ve never seen anything like it,” says Edwards. “What’s exciting is the diversity of mechanisms,” she adds. “It’s quite remarkable. The technology has opened up a whole new window on the lives of plants.”
