Plants can think? Is this wisdom, or just whacko? Well-known author Michael Pollan had a recent article in The New Yorker which reports on the latest research on plant “neurobiology”, a provocative term liable to get many a plant scientist hot under the collar.
I’ve read Pollan’s piece and also listened to his very interesting interview on the fabulous Science Friday (as an aside, if you don’t subscribe to SciFri, you should), so this is my take on his take on the evidence supporting the notion of plants being slightly more clever than we’ve given them credit for.
Remember that silly book from the early 1970s ‘The Secret Life of Plants‘, where researchers supposedly found that plants could think, feel and remember? Where dracaena allegedly exhibited their horror on a polygraph machine when witnessing another plant being trampled? No researcher has ever been able to replicate these experiments, now widely known to be ridiculous tosh. Pollan contends that scientists interested in this field have had to deal with the fall-out ever since, limiting their ability to have papers published in reputable journals or to gain funding for their work.
But he says there’s a growing body of evidence that while plants obviously don’t have brains, their ability to sense, react, adapt and even predict can be likened to a type of “intelligence”. He also postulates that our narrow human-centric view of what constitutes intelligence blinds us to the complex networks among plants that allow them to behave in intelligent ways – behaviours that rally defence mechanisms, share nutrients and even protect offspring.
Some scientists liken it to the herd “intelligence” of ants, bees and birds, where separate but related organisms use networks to achieve beneficial results, such as birds flying very closely together in flocks, or bees swarming.
A plant’s sessile lifestyle, where it can’t get up and move to avoid danger means it must find other ways to deal with predators and nutrient shortages, which include adaptations like the ability to regenerate from only 10% of its original mass, something an animal could never do. The huge number of chemical compounds that plants develop to deal with predators is something we ourselves have used to our own advantage, developing all sorts of life enhancing or even saving drugs from them.
A plant’s own well developed sensory systems means that it can smell, taste, sense, “see” (react to light and shadow) and even hear. Heidi Appel at the University of Missouri’s recent experiment found that by playing the sound of a caterpillar chomping away on a leaf to another plant, that plant would produce defensive chemicals. Unpublished research from Stefano Mancuso at the University of Florence suggests that plant roots can even “hear” water flowing in a nearby pipe and will grow toward it in search of water.
Darwin himself was endlessly fascinated with the developing root, or radicle, of young plants, and how it could sense pressure and gravity, as well as moisture and light. Newer research adds to that sensitivity list with volume, nitrogen, phosphorous, salt and some toxins as well as signals from other nearby plants. By putting together all this information, a plant then will grow in a particular way or direction, such as away from a nearby competitor. Whether this is a ‘decision’ as such is an inflammatory word in plant science.
But can plants also learn? A scientist from the University of Western Australia, currently working in Florence, believes that they can. Monica Gagliano used the sensitive plant, Mimosa pudica, to demonstrate a plant’s habituation to a particular threatening stimulus. The sensitive plant folds up its leaves when they are touched, a defence mechanism that probably limits insect browsing. Gagliano found that when she repeatedly dropped small potted specimens 15cm, their leaves reacted at first, but then the plants stopped folding their leaves in response, unlike when the plant was shaken. The same plant would not respond to being dropped in this way even some weeks after the original ‘learning period’. Gagliano concludes that “there is some unifying mechanism across living systems that can process information and learn”. Gagliano’s paper has produced a barrage of criticism from other plant scientists.
Pollan suggests that this criticism is based on our belief in our own human superiority in the animal and plant kingdom, based on the power of our brains. To suggest that other forms of behavioural intelligence could rival our own in complexity and originality is just way too threatening. However, the networking system used by plants could have profound implications for understanding our own brain processing. Sure, we have a brain ‘command centre’ that directs the way we react to external stimuli, but what happens when we start to examine the command centre, the brain, itself? There’s no boss part of the brain that controls the rest – all the separate sensory, thinking and processing parts just somehow work together in a “leaderless network” as Pollan describes it.
Even memory is a slippery term. In animals like us, it involves the development of new neurological networks, but even immune cells can ‘remember’ a virus and how to react to it in the future.
So can plants also choose, or exhibit an intention? Mancuso says he has time lapse photography evidence that a bean plant doesn’t just wave around randomly seeking a support on which to grow but that it will sense and grow toward a metal pole that it can’t touch, even if it’s half a metre or more away – that somehow it ‘knows’ that the support is there. The concept that plants are conscious of their own environment and can even feel pain is now all getting a bit far fetched for me, and I bend myself towards an alternative explanation from scientist Lincoln Taiz that this is a ‘tropism’ ie a plant’s response to a stimulus, even if we can’t yet see or measure what it is.
So the argument starts to become anthropocentrism (that as humans we think we understand everything about thinking and feeling) versus anthropomorphism – that we look for and explain what we find in nature in human terms that are not always appropriate. Pollan proposes that if we substitute the words “plant intelligent behaviour” for “plant intelligence”, then more plant scientists would be prepared to examine the supporting evidence without prejudice.
Other research by Suzanne Simard at the University of British Columbia quoted by Pollan looks at the ability of trees to use mychorrizal networks between their root systems to exchange information, create nutrient banks where plants, even in different genera, will give and take limited resources when each needs it the most, send warnings about insect attack, and nourish seedlings. Simard used geiger counters to follow radio isotopes injected into selected fir trees, discovering a maze of underground networks throughout the surrounding forest. Tolkein’s Fangorn sounds very real indeed.
So what does it all mean? Where should this lead us? I can’t accept plants as sentient beings, nor can we give up eating them and survive. I suppose it reminds me of what Hamlet says to Horatio – ‘There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy”. So I’m prepared to be open minded until the next instalment, which may well debunk these experiments and theories as comprehensively as happened to The Secret Life of Plants. However, I also think that the words of the late ethnobotanist Tim Plowright, quoted by Pollan, sum up my thoughts on the argument. Whatever else you decide about plants apropos their thinking and feeling and choosing, plants are (literally) awesome because they “can eat light, isn’t that enough?”