The minds of plants

Laura Ruggles writes: At first glance, the Cornish mallow (Lavatera cretica) is little more than an unprepossessing weed. It has pinkish flowers and broad, flat leaves that track sunlight throughout the day. However, it’s what the mallow does at night that has propelled this humble plant into the scientific spotlight. Hours before the dawn, it springs into action, turning its leaves to face the anticipated direction of the sunrise. The mallow seems to remember where and when the Sun has come up on previous days, and acts to make sure it can gather as much light energy as possible each morning. When scientists try to confuse mallows in their laboratories by swapping the location of the light source, the plants simply learn the new orientation.

What does it even mean to say that a mallow can learn and remember the location of the sunrise? The idea that plants can behave intelligently, let alone learn or form memories, was a fringe notion until quite recently. Memories are thought to be so fundamentally cognitive that some theorists argue that they’re a necessary and sufficient marker of whether an organism can do the most basic kinds of thinking. Surely memory requires a brain, and plants lack even the rudimentary nervous systems of bugs and worms.

However, over the past decade or so this view has been forcefully challenged. The mallow isn’t an anomaly. Plants are not simply organic, passive automata. We now know that they can sense and integrate information about dozens of different environmental variables, and that they use this knowledge to guide flexible, adaptive behaviour.

For example, plants can recognise whether nearby plants are kin or unrelated, and adjust their foraging strategies accordingly. The flower Impatiens pallida, also known as pale jewelweed, is one of several species that tends to devote a greater share of resources to growing leaves rather than roots when put with strangers – a tactic apparently geared towards competing for sunlight, an imperative that is diminished when you are growing next to your siblings. Plants also mount complex, targeted defences in response to recognising specific predators. The small, flowering Arabidopsis thaliana, also known as thale or mouse-ear cress, can detect the vibrations caused by caterpillars munching on it and so release oils and chemicals to repel the insects. [Continue reading…]

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  1. The research alluded to is fascinating, and it clearly required a break from pervasive attitudes to undertake — but I think Ruggles is approaching it from the wrong direction, by about 180 degrees. Completely omitted from her discussion — and I don’t think I’m exaggerating with ‘completely’ — is the development of theoretical and practical computation.

    She writes: “We now know that [plants] can sense and integrate information about dozens of different environmental variables, and that they use this knowledge to guide flexible, adaptive behaviour.” But she could be talking about your smartphone.

    If we wake up to the idea that ‘knowledge’ in this sense can be obtained and used by any physical system which implements certain abstract operations, then it becomes entirely natural to look at plants with fresh eyes to see what they can do and how they manage to do it. “If I only had a brain” is yesterday’s worry. Recall Claude Shannon’s reply when he was asked if machines could think: “I’m a machine and I can think.”