Tendrils Alive
I have accidentally come across a very remarkable plant. Cobaea scandens is a climbing annual from Mexico that grows to three metres from seed. I first saw it two years ago on the TV programme Gardener’s World and was intrigued enough to buy some seeds. It grows in an ingenious way which I will explain in a moment but imagine my joy when I found out that Darwin, who wrote about it in his book On the Movements and Habits of Climbing Plants (1875), also thought that the plant is special. My intuition was confirmed.
The plant has the tendrils with which it grabs other plants or a support (in the garden). These tendrils are very lively, responsive to touch (Darwin: “every part of every branchlet is highly sensitive on all sides to a slight touch, and bends in a few minutes towards the touched side”). The tendrils wave in the breeze, looking for something to hold on to.
Tendrils are indeed the most striking feature of the plant — until it flowers, that is. Darwin again:
About every inch to an inch and a half very fine tendrils split off the main stem which branches into two and then two again. The four termini develop little hooks that cling to anything they encounter, including human skin. They readily catch soft wood, or gloves, or the skin of the naked hand.
The hooked prongs are very striking in appearance, like little toasting forks that catch on anything they encounter. In function, these little devices resemble the hooks on the cocklebur that inspired the fastener Velcro®. This interests me because I have written about other examples of adhesion by small structures in my book The Gecko’s Foot (2005). The hooks are perfectly regular and identical across the whole plant, giving the plant a technological aspect.
The tendrils are very active, searching laterally at first, looking to grasp their neighbours and clamber over them. If they don’t find anything they return to the main stem and will then grasp the bamboo stick you’ve provided for them. Whatever they touch they start to coil around. They wrap themselves many times round the stick making a messy but tough bond.
Meanwhile, the main stem climbs up, throwing off the next tendril. which makes a new connection 25–50 mm further up. The most interesting phenomenon is what happens between these nodes. Six opposed leaves form between the main stem and the point where the tendril meets the stick. The end of the tendril that has wrapped itself around the stack then adapts its coiling growth to grow up to the next tendril grasping point, kinking as it grows like a telephone cord. It thickens as it does so, creating a strong flexible cable just like its telephonic analogue.
The end result is two parallel stems, linked by tough flexible spacers. It reminds me of the slim Italian Maber scaffolding towers that adorn so many major building rejects now. The parallel isn’t exact because a Maber tower is adjoined to a lift which ferries men and materials up the building. On the other hand, you could argue that the coiled cable is the support that allows the plant to ferry water and nutrients up to the leaves,
The sticky ends of the tendrils are also reminiscent of the footpads of the gecko. The Gecko’s Foot tells the story of how the secret of the gecko’s extraordinary adhesion was discovered (as recently as 2000). The sole of each foot ends in about half a billion small hairs at the tip. The force that allows the gecko to climb even upside down on the ceiling is the strength of the molecular Van der Waals force, which operates only on the nano scale. In other words, this is nature’s nanotechnology in action.
By comparison with the gecko, with its half a billion hairs, Cobaea’s two prongs constitute a macrotech monster (although it’s possible that the hooks are subdivided into many hooklets) but if nature gets a good idea she uses it at every scale possible. Take the rule of six, the hexagon. You’ll find this all the way up from graphene, a sheet of hexagonal carbon atoms one atom thick (that’s about 0.6 of a nanometer0, through the bees’ honeycomb, to the great hexagonal blocks of rock in the Giant’s Causeway.
The plant is fast growing, although if you sow the seeds early in the Northern Hemisphere you wouldn’t know this. I sowed in early February; they took a month to germinate and then grew so painfully slowly for almost another month I thought they would fail. But then in late March they suddenly kicked into life and were ready to plant out, being around 18 inches high, by the end of the first week in April.
The combination of aggressive fastening and rapid growth means that they are highly successful, not to say predatory, climbers clambering over rival vegetation. Of course it’s the flowers that induced me to grow them in the first place: white in var. Alba or purple in the standard version, and beautiful they are, but I’ve never seen a plant that was so rewarding to watch in the seedling stage.
Regarding Darwin’s book on the subject, the man himself had a low opinion of it, calling it a “horrible bore”. So why did he write it? Darwin was interested in all of the big problems of biology, but was constantly frustrated by the lack, at the time, of a way into the deep genetic and biochemical mysteries of nature. Cobaea is a plant that exemplifies some of the most interesting questions in biology: that of form and inherited behaviour: these are questions that even the mind of Darwin had no purchase on at the time. Enquiry had to start with his kind of minute observation. It is curious though that he doesn’t comment on the kinking behaviour of the tendrils; he, of course, had never seen a telephone cable, but even without the visual simile its behaviour is very striking.
Ken Thomson, the author of Darwin’s Most Wonderful Plants: A Tour of His Botanical Legacy, reinforces the poor opinion of Darwin’s book: Darwin encouraged readers to jump to the table of contents (“good advice,” notes Thompson).
The history of biology has been haunted by such patronising attitudes. In Citizen of the World (1762), Oliver Goldsmith derided the alleged pedantry of all those who study small creatures. Commenting on naturalists such as Abraham Trembley (1710–84), who wrote an article about that familiar creature of school biology, the hydra, Goldsmith wrote: “their fields of vision are too contracted to take in the whole … Thus they proceed, laborious in trifles, constant in experiment, without one single abstraction, by which alone knowledge may be properly said to increase.”
But Cobaea is a fine, fairly simple example of one of biology’s greatest mysteries: the form of any organism is encoded in DNA yes, but how does the linear string of DNA bases in Cobaea instruct a tendril to fork into two, repeat it, and then the two hooks to form on the ends? But that is the relatively easy bit. Cobaea has behavioural strategies. The tendrils respond to touch, curling around a stick when they meet it. Then that kinking habit: what tells the tendrils to curl up into make that phone cable?
We now know some of these answers. There are instructions in DNA telling dividing cells to change their strategy but what about the behaviours. That is controlled by chemical responses to the environment (light, heat, touch, chemical signals from the soil, air and other plants etc) by plant hormones (their chemical composition coded by the genes of course), which function as a kind of plant nervous system.
Darwin’s friend Henry Walter Bates (1825–1892) was also interested in form. His subject was butterflies; he reasoned that the wing patterns of butterflies might yield clues to the mystery of form before more complex creatures because they are two-dimensional: “on these expanded membranes nature writes, as on a tablet, the story of the modifications of species”. The science of evolutionary developmental biology (see Sean B. Carroll’s Endless Forms Most Beautiful (2011) and the gene editing technique CRISPR (see my article on Medium ‘The Butterfly Wing Colouring Book’) are now revealing many of the mechanisms behind pattern and form. It is precisely in such a detailed investigation of small things that science proceeds. Never having heard of Oliver Goldsmith, little things rule the natural world: they are Giants of the Infinitesimal.
