Networks of plants can achieve surprising things. For example, one study has shown that the trees in a forest are interconnected through the fungi that grow around their roots. A radioactive chemical injected into one tree soon appears in another. This data also showed that trees of one species seem to nurture younger ones, or even share resources with other species – for example between evergreens and deciduous trees when they have lost their leaves.
But you might be tempted to think that a botanist who talks about ‘plant intelligence’ has spent too long in a hot greenhouse. The notorious 1973 book The Secret Life of Plants was based on bogus science that sent too many people scurrying to the cabbage patch to commune – unrequited – with nature. There is in fact a scientifically respectable side to this field. Research in plant physiology has revealed genuinely interesting parallels to other forms of intelligence that are worth investigating.
Definitions of intelligence often focus on animals, and include reasoning and sentience (subjective feelings). Defined more broadly, intelligence simply involves sensing, then processing or retaining knowledge about that sensation, and responding to it. ‘Knowledge’, may simply mean a molecular memory or imprint of the stimulus, for example a change in the configuration, concentration or location of a certain molecule.
Under this broader definition, other types of organisms are intelligent. Bacteria can communicate and cooperate together in biofilms; slime moulds can navigate mazes; and parts of our bodies – like our immune system – can learn and hold memories. None of these types of intelligence involve brains or even single nerve cells. They are decentralised, intelligent networks that can sense altered information flow and remember or respond to it.
A plant has far more than five senses. They can detect light in a number of different ways, the presence of certain substances in the environment, humidity, mechanical pressure, gravity, damage by predators or parasites, and more. Each of these inputs produces an effect on the plant’s growth and development.
Shoots respond negatively to gravity and also grow towards the light as well as away from potential competitors; roots respond positively to gravity and also towards water and concentrations of nutrients; diseased leaves are amputated; and chemicals are released to inhibit the growth of unrelated plants.
Plants can’t just walk off to avoid trouble, so they produce a fantastic array of chemicals that help them make the most of the environment in which they find themselves – some of which we use to treat our own illnesses. Plants also have a modular structure, with permanently ‘embryonic’ regions and tissues, so if one part is chomped away by a sheep the rest survives.
A group of plant scientists began to use the word ‘neurobiology’ – in a somewhat metaphorical sense – to describe this field of study, but some of their colleagues objected. Does it ring alarm bells set in place to guard against the wacky science promoted by that 1970’s book?
In my mind, there is every need to be cautious when speaking about plant neurobiology, but metaphors are there to be played with. They help us to see the world from new perspectives and ask more interesting questions in our science. Are there parallels between plant and animal sensing? Are there possibilities for network-types of intelligence that we have not yet explored?
The living world continually astonishes. Whether or not plants can be classed as ‘intelligent’ in any sense of the word, their remarkable properties are a fabulous example of the wonderful solutions that living organisms find to the challenges of life in the wild. What happens if we leave behind our very human or animal-centred view of the world for a while? This shift in perspective fits extremely well with the picture given in the Bible where ‘all creation praises God’, simply by being itself. The concept of ‘higher organisms’ is perhaps not terribly biblical, except in reminding us of our responsibility to look after the world. So what can we learn from the plants?
The Intelligent Plant, The New Yorker, 2013
“Plant neurobiology: an integrated view of plant signalling”, Brenner et al., Trends in Plant Science, 11 (2006), p413-419
“Plant neurobiology: no brain, no gain?”, Alpi et al., Trends in Plant Science, 12 (2007), p135-6
“Aspects of Plant Intelligence”, Anthony Trewavas, in Conway Morris (Ed.), The Deep Structure of Biology (Templeton Press, 2008)
Martin J. Hodson and John A. Bryant, Functional Biology of Plants, (Wiley, 2012)
A. Trewavas, Plant Behaviour and Intelligence, (OUP, 2014)
With thanks to John Bryant for his help with this article (all remaining errors are the author’s own).