Another Life: Plants capable of adapting behaviour to survive

A slight frisson of savoir faire attended my planting of the polytunnel’s first spring seedlings. If you’re going to grow mangetout then their variety has to be Carouby de Moussane, wherever that is in France. Otherwise they’re just common “sugar snap” peas that you eat, pods’n’all.

Stroking the floppy little seedlings towards the wire netting they have to climb, the word that popped up, however, was one coined by Charles Darwin as he reclined on his sick couch in 1860-something, surrounded by climbing plants in pots – runner beans, cucumbers, clematis and more.

As they spiralled up their sticks, he tied weights on their tips to slow them down. And vines became covered with paint marks as he timed their twisting movements. Plants, as they grow, respond to the pull of warmth and light, the push of wind, the weight of gravity. What Darwin discovered was the spiralling, bending movement he called “circumnutation” (Latin for circle or sway).

All plants, he found, wave about a bit around their axis, but climbers actively reach out for support. As a tendril brushes a nearby stick or stem, the cells immediately in contact stop growing, while those on the opposite side continue to elongate, pushing the tendril round into a spiral, a process that can take hours.

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One of my favourite plants in the garden is a golden hop, brought from a nursery in west Cork. Planted first beside a woodshed pillar, it duly burst into life each spring, and, reaching the roof, went on vertically a couple of extra feet to crown the yard-light in a wreath of leaves.

Later renovations demanded that the hop be moved round the corner of the woodshed. It survived realignment and later revived on cue, climbing a trellis towards the roof. As the first shoot passed the tin, to probe the empty air, I kept a morning eye on it. From its new position, the shaft of the yard-light was now some 50cm (20in) to its right. What would it do?

I may have missed a day or two of looking, thinking of other things, but at sunrise one morning there it was, swung across some 50cm to make its first firm curl around the vertical pipe. Within a couple of hours it had grown another 8cm or so, curving to tighten its grip.

Modern biologists still differ on whether circumnutation (now often shorn of circum) is solely determined by the plant or is also an effect of external stimuli, such as gravity. This is the “gravitropic overshoot hypothesis”, though I don’t see how gravity could support the hop’s sudden thrust for the light.

The possibility of conscious volition in plants is now a contentious scientific issue attracting new research from a team of Spanish scientists, led by Dr Paco Calvo of the University of Murcia.

Using time-lapse photography, they documented the cellular behaviour of 20 potted bean plants, some with a nearby pole to climb and some without. They advanced new ways to study the “nutation” or bending of the plants. Their observations are held to “support the hypothesis that patterns of nutation are influenced by the presence of a support to climb in their vicinity”.

That plants could "know" if a pole is there is the gist of a technically painstaking paper. The message is more direct in others, such as "Plants are intelligent, here's how", an extensive discussion published in Oxford's Annals of Botany. It finds Calvo and others teamed with Prof Anthony Trewavas of Edinburgh University, an established pioneer of such studies.

Plant intelligence

The common objection to plant intelligence is that plants have no brains or neural systems. But nor have single-celled slime moulds and bacterial biofilms, which explore their surroundings and adapt their shapes to take advantage. “Even phages, bacterial viruses,” as the Trewavas paper notes,” have been shown to have a remarkable social life.”

The universal test for intelligence, he argues, lies in changes of behaviour to survive in fiercely competitive environments. Other researchers have offered the idea of “extended cognition”, in which plants respond to organisms from different kingdoms, such as the mycorrhizal fungi that cluster around their roots.

Meanwhile, my mangetout seedlings, nudging each other for space and light, reach out to grab the netting with tight-coiling tendrils and hoist themselves up, some 2m high, to produce their lovely lavender flowers and the flat, finger-length peapods among the leaves you have to train your eyes to see.

Why are the runner beans of my drawing coiling around the Dublin Spire? Given the rush to grow things that has swept through the locked-down city this spring, it seemed a nice, symbolic notion.