One of the few books I can still recall reading for English at school was The Day of the Triffids (1951) by John Wyndham, a terrifying sci-fi novel about gigantic plants that could move around, communicate with each other, and kill and eat humans. Triffids took over the world after a catastrophic event sent most of the population blind. Whilst upon rereading it recently, it seemed hilariously corny rather than scary, and more an allegory about the Cold War era in which it was written than anything else, it made me think about how our view of plants has changed over time.
The idea that a plant could show signs of 'intelligence' was of course the stuff of science fiction at the time I was reading the book in the early 1970s. The triffids wandered about walking on their three root appendages, and communicated with each other by rattling three leafless sticks at the base of their stems. They seemed to have an unerringly good sense of where to attack humans with their stingers to maximise their venom. Of course, these things seemed far-fetched at the time, because we all know plants can't move, right? Or hear anything? They don't have a brain or a nervous system so they can't possibly show any sign of intelligence.
However, a book called Brilliant Green (2015) by Stefano Mancuso and Alessandra Viola sets out to examine the history and science of plant intelligence, and I found it compelling! Whilst we gardeners love and appreciate plants, we might regard them as fairly passive, insensitive, inert objects. Though we know they do grow, photosynthesise and respire, none of these things is visible to the naked eye in real time - and we might find it hard imagine that they could solve problems or communicate with other plants or with insects and animals! Historically, Western thinking regarded plants as being only slightly higher on the evolutionary scale than rocks - basically because they can't move about like other living things.
Yet advocates for the notion of plant intelligence include Charles Darwin and his son Francis. Charles Darwin spent a lot of his time studying the plant world and how it related to his theory of evolution, and wrote a book called The Power of Movement in Plants (1880), which noted how roots move - and that the tip of the root has the facility for perception of environmental stimuli, and decision-making about the direction of movement, acting almost like a brain. Darwin's work was scoffed at, especially by other leading botanists of the era, but today we know that he was right: in fact, root tips are able to detect many more physicochemical parameters in the environment than he realised.
Of course, I'm not suggesting that you and your potted geranium are ever going to have an intellectual discussion about Renaissance art or the like. Plants differ so fundamentally from us because they do not have individual organs that control various functions, but are instead constructed on a modular design, in which no part of a plant is truly indispensable. 'A plant's functions are not related to organs - which means plants breathe without lungs, nourish themselves without having a mouth or a stomach, stand erect without having a skeleton, and ... make decisions without having a brain' (Mancuso and Viola, 2015, p. 34). A plant is made up of repeating combinations of simple modules like Lego bricks to create branches, stems, leaves and roots. Thus, a plant is more like a colony than an individual entity, which is greater than the sum of its individual parts. Plants evolved like this for their own survival because they live anchored to the soil. They have their abilities for cognition and bodily functions in every cell, not in discrete organs like us: so that they can survive attack where they lose a large part of themselves by being eaten by herbivores! Because of this, they have a regenerative capacity that animals and humans can only dream of. Stick a piece of a plant into the ground and, all going well, it will grow into a new plant. Equally, it can have 90% of itself eaten and can grow back into a normal plant. We gardeners take these abilities totally for granted every time we prune our plants or take a cutting.
Mancuso and Viola argue that plants not only have the same five senses as we do but have 15 others, which they use to solve problems and make decisions regarding food, moisture, competition and threats, and to communicate with other plants and animals - certainly one way to define intelligence. For a start, plants have the ability to sense light in its different wavelengths and perceive visual stimuli via photoreceptors in their leaves, stems and roots - because sunlight is crucial to a plant's existence, as they get most of their energy through it through photosynthesis. Plants grow in the direction of light and move their leaves to maximise their exposure to light. For its sense of 'smell', a plant has receptors for volatile substances that set off a chain of signals, communicating information to the entire plant. Plants also produce volatile compounds which they use to communicate warnings with one another and to other creatures - such as ones produced when the plant is stressed by pests or diseases, cold, heat or pollutants in the soil or atmosphere.
The roots of a plant probe the soil to find nutrients and grow more densely where the greatest amount of nutrients are, including tiny trace elements: indicating a sense of 'taste'. Mechanosensitive receptors that are activated when the plant touches something or when vibration reaches it show that plants have their own sense of 'touch' - seen most obviously in plants such as the 'sensitive plant' (Mimosa pudica), which retracts its leaves when touched as a defence mechanism, being able to differentiate between various stimuli and opening up again once it learns that the stimulus isn't dangerous. Carnivorous plants also shut their traps when the prey has landed on them. Tendrils of climbing plants curl up in a few seconds when they touch something, twining around it to get support.
And, of course, a plant doesn't have ears, but sounds are vibrations that the human ear captures and converts to electrical signal to transmit auditory information to the brain. Plants' mechanosensitive channels can sense vibrations in the earth. Music has been shown to improve plant growth, with certain frequencies, especially lower ones, promoting seed germination, plant growth and root lengthening, whilst higher frequencies can have an inhibiting effect. Roots can also make 'click' sounds possibly due to breaking of cell walls, and these may be used for communication between plants.
The 15 other senses that plants have that we don't have seem to be particularly centred in the root tip: senses to measure soil humidity and identify sources of water even at a great distance, and to detect gravity and electromagnetic fields. They can also locate and recognise trace amounts of chemical elements that are important or harmful to growth even when these are several metres from the roots.
Plant communication is also quite sophisticated. A plant can communicate within itself, sending electrical and chemical and/or hormonal messages through its vascular system, for example to warn leaves when there is a water shortage in its roots so that its leaf stomata can close, to reduce water loss through transpiration. Plants can communicate with one another, using chemical molecules released into the air or water containing information. Many fungi have important symbiotic relationships with plants, especially in the form of 'mycorrhizal symbiosis', where fungi colonises the host plant's roots, in order to access nutrients from the plant and in turn providing vast, intricate underground pathways between the roots of other plants that facilitate the exchange of information through complex chemical and electrical signals.
Plants also communicate with insects and animals. A plant under attack by an insect produces substances that make the leaf indigestible or toxic. Plants have developed complex ways of luring insects or birds in to ensure pollination, such as enticing them with nectar or producing irresistible aromas. This process can also be used to attract beneficial predator insects to devour destructive ones. To help plants disperse their seeds, in a number of cases, colourful, ripe fruit attracts birds or animals to ingest the flesh and the seeds, which are then expelled far from the original plant.
Whether or not people are convinced that plants are actually intelligent after reading this book (or this blog!) is up to the individual. For me, the message was that plants have amazing abilities: they are exquisitely aware of their surrounding environment and perfectly adapted to it. And we are utterly dependent on them for our survival - for food, oxygen, fuels, construction materials and medicines, among other things, including our psychological wellbeing! We ignore, demean, subjugate or destroy them at our own peril! We should in fact worship them ...
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