In the search for precious sunlight, instead of growing sturdy trunks to reach towards the light, climbing plants such as honeysuckle and grapevines cling to their surroundings and then heave themselves upwards. Scientists have now cracked how some plants do this, and in the process they have created a new kind of spring.
Climbing plants have been puzzling biologists since the 19th century – including Charles Darwin. The technique the plants use to winch themselves upwards is well known, but the underlying mechanism has been a mystery until now.
The new research, published in the journal Science, investigates in unprecedented detail the supporting tendrils of the cucumber plant. When first formed, a tendril is almost straight, and while growing it slowly waves around in a poorly understood process called circumnutation. When it encounters a foothold, the end of the tendril wraps around it, securing a support.
The tendril then shortens by coiling up into a corkscrew-like helix, pulling up the rest of the plant. But rather than twisting only in one direction – impossible without twisting the plant at the other end – the two halves of the coiled section curl up in opposite directions, separated by an uncoiled stretch called a perversion, so there’s no net twist. How this coiling occurs wasn’t understood.
A group of scientists led by Sharon Gerbode and Josh Puzey, who carried out the work while at Harvard University, investigated the nature of recently discovered specialised cells that form a stiff ribbon of material inside each soft, fleshy tendril.
This ribbon controls a tendril’s shape, and the team suspected that to coil, cells on one side of the ribbon are stiffened and shortened more than those on the other side, causing a turn towards the stiffened side. Because neither end of the tendril can twist, a double-coil separated by a perversion naturally results.
The team confirmed their hypothesis by creating tendril models from silicone rubber strips. However, a new puzzle arose. Rather than unravelling when stretched, as their model did, the coiled cucumber tendrils wound even more.
“We couldn’t believe our eyes!” says Gerbode. “In fact, we spent an entire afternoon sitting at a microscope, pulling on a fibre ribbon over and over, doubting that it could really be winding further. It was exciting and utterly baffling.”
The team then created a more sophisticated model, using stiff fabric on the inside of the coil and copper wire on the outside. This reproduced the real tendril’s behaviour – and created the first artificial spring that doesn’t twist at either end when pulled.
Their “twistless spring” was a “delightful surprise”, says Gerbode. The researchers have applied for a patent on their design.
Several plants appear to have developed similar springs. “Overwinding” has now also been found in the passion flower which, says Puzey, must have evolved independently because details of the mechanism differ from that seen in cucumbers.
Life and style
Video: how plant tendrils ‘overwind’ when pulled
Researchers working at Harvard University have created artificial springs that mimic the tendrils of plants such as cucumbers