Detained in the polytunnel by the unexpected rattle of a shower, I contemplated the encouraging progress of my mangetout peas. I had planted them around the outside of an upright cylinder of sheep fencing and already, at pencil-height, some were stretching out a first tendril to wrap around a strand. Indeed, one could imagine a good many of the plants were actively reaching for the nearest support. If that were so, how did they know it was there?
Similar thoughts have troubled those of a speculative nature for some centuries. "The motions of the tendrils of plants," ran a typical observation in 1812, "and the efforts they apparently make to approach and attach themselves to contiguous objects, have been supposed by many naturalists to originate in some degrees of sensation and perception . . ."
It was another few decades before Charles Darwin spent a lot of time, much of it lying sick on his couch, watching climbing and twining plants to see how they actually managed their ascents. His study filled up with more than 100 plants: runner beans, wild cucumbers, nasturtiums, clematis, tropical house plants and vines in their pots filled every surface and shelf. As hops spiralled up poles, he tied weights to their tips to try to slow them down. Vines became covered with paint marks as he timed their twisting movements.
The physical activity of plants, now made obvious by time-lapse video (as in the corkscrewing growth of potato plants that sometimes heralds the weather report on RTÉ 1), is usually just below the threshold of human observation. I think of my early surprise, when painting nasturtiums in a vase, to catch the blossoms shifting around for a better share of the light (or rather, not quite catching them, as in those classic Kit-Kat ads).
Darwin, helped by his son Frank, established the wide, sweeping movements of climbing and twining plants as they search for the nearest support, their tips or tendrils bowing to each point of the compass and tracing circles or ellipses in the air. He pictured it as like a man swinging a rope around his head until the end of it hits a branch, whereupon it whips around it in a tight grip.
How it works is that as the end of the tendril brushes against a promising support the cells that are in contact with it stop growing, while those on the opposite side of the tendril continue to elongate, pushing the tendril round in a clutching spiral. Just to know this is to spend silly minutes on one's knees with a bit of twig, trying to tickle a tendril into starting to bend (like offering one's finger to a baby's hand). But only a few, very sensitive, climbers oblige so swiftly; the rest take hours.
The Darwins found that the same mechanical search-and-spiral action applies to plants such as hops, convolvulus or runner beans that don't have tendrils but twine up a supporting pole. They called the movement "circumnutation". Noting that no twining plants in England climb trees, Charles concluded that there must be a maximum thickness to the pole, past which twining is impossible. This rather puts paid to my fantasy of someone growing runner beans up the Dublin spire in a fit of emblematic greenery.
The mathematics involved continue to inspire research, as in a paper presented to the fifth Plant Biomechanics Conference in Stockholm last summer. It concluded that the critical ratio between the radius of the pole and the intrinsic curvature of the plant is R/Rc=3.31, past which the plant collapses in a tangle.
So much for plant movement, but where are the orders coming from? Darwin, again, noting that plants bend towards the light, tried wrapping the tip of the shoot in foil; it still bent. Then he cut the tip off: the bending stopped. The plant's tip is the control tower, but what is in control? In 1926, a Dutch graduate student, Fritz Went, isolated the plant hormone called auxin that governs the growth of cells, the extension of roots and the whole shape and structure of the plant.
Synthetic auxin is the "hormone rooting powder" gardeners use when hopefully sticking a shrub cutting into a pot, and what some growers spray to stop fruit trees dropping their crop prematurely. The natural stuff, needless to say, is of overwhelming interest to GM scientists in their quest to dominate the machinery of plants. Only this month, auxin made the cover of Nature with research on how it acts as a "molecular glue" to govern gene expression in growth and development.
To grow the juicy spring shoots of purple sprouting broccoli that filled another bowl for me this week, I had used the gardener's traditional knowledge: cut out the top shoot and this will let the lower shoots grow. By lopping off the auxin in the leading shoot, I had ended its "apical dominance".