If you were interested in my last blog post on plant learning and memory, have a read of this article I wrote on the topic for Aeon Magazine, which was published today. I didn’t choose the title (I think “mind” has popular connotations related to consciousness that I would want to avoid) but the body of the essay and the subsequent discussion (a comments section that actually opts for reflection and reasoned dialogue!) get at some important issues in thinking about how to theorise plant learning.
“To make effective progress, we need to pay careful attention to plant mechanisms. We need to be clear about when, how and why we are using metaphor. We need to be precise about our theoretical claims. And where the evidence points in a direction, even when it is away from common consensus, we need to boldly follow where it leads. These research programmes are still in their infancy, but they will no doubt continue to lead to new discoveries that challenge and expand human perspectives on plants, blurring some of the traditional boundaries that separated the plant and animal realms.”
Greetings phytophilosophy enthusiasts! Today marks the beginning of National Science Week here in Australia so I’ll be sharing a little plant science with you.
To begin with, I want to have a look at how plants learn. Learn? I hear you thinking, is that really something plants can do at all? If you’re anything like me, the concept of behavioural learning is something you might have wandered into while studying for introductory psychology exams, or perhaps stumbled upon after googling whether it’s possible to train your cat to open doors. At any rate, you might have thought, learning is surely something only animals are capable of – I mean, doesn’t learning require a brain? To the surprise of many though, recent research answers this question with a resounding no! While we might not generally think to associate learning with plants, we now know that plants are nonetheless out there associatively learning! However, the rigorous scientific study of plant learning is relatively new; plant physiologists have been interested in the topic for decades but robust demonstration of behavioural learning in plants has only occurred in the last few years. So, if you haven’t come across plant learning before, don’t worry – you aren’t too far behind the cutting-edge science!
Before we examine what learning looks like in plants, let’s take a step back: what is learning, anyway? At core, learning is a process whereby particular experiences lead to lasting changes in behaviour or knowledge. Some forms of learning, such as social learning, observational learning, and the kind of learning that comes about through activities such as play, require particularly complex kinds of nervous systems and social organisation. Other types of learning are more ubiquitous, and inform the basic ways that organisms interface with the world around them. For any organism born into an environment that varies from place to place or changes over time, the capacity to learn which things around them are dangerous, safe, nutritious, and so on – basically anything that relates to surviving and thriving – is an advantage. Given how this ability is likely to affect reproductive success, it is not surprising that many different sorts of organisms can learn – including plants!
One of the most basic kinds of learning, non-associative learning, was first studied in the giant sea slug Aplysia californica, work which later earned neuroscientist Eric Kandel his Nobel Prize in Physiology or Medicine. Aplysia has an involuntary reflex known as the gill and siphon withdrawal reflex (GSWR) – when stimulated by a curious neuroscientist or exposed to some other environmental disturbance, Aplysia’s siphon and gill retract defensively, as shown in the diagram below. If a harmless stimulus is repeatedly presented to Aplysia, there is a gradual decrease in GSWR for that stimulus, which is known as habituation. Habituation learning is useful so that organisms do not continuously respond to all the things they repeatedly encounter in their environments. Rather, they can ignore innocuous stimuli and put their time and energy to better use, foraging for food, looking for mates, or writing blogs.
Notwithstanding, if an organism suddenly encounters a different stimulus, it still wants to be responsive in case of danger. True habituation, then, needs to be stimulus-specific rather than just a general unresponsiveness. When a new stimulus is introduced and the organism starts responding to the novel stimulus as it did before habituation (so in the Aplysia case, responds with the GSWR), this is called dishabituation. Dishabituation distinguishes genuine learning from other kinds of changes in responsivity, such as that due to fatigue.
This sort of learning, then, basically occurs in environments where an organism must learn to ignore innocuous stimuli. In contrast, if our benign neuroscientist takes a sinister turn and repeatedly administers a harmful stimulus (such as an electric shock) to Aplysia, instead of a decrease we see a learned increase in responsiveness to the noxious and also neutral stimuli over time. This other type of behavioural learning is known as sensitisation.
So how does all this relate to plants? In 2014, biologist Monica Gagliano and colleagues put a little plant known as Mimosa pudica, the sensitive plant, to the test to see whether it could learn. The plant is so-named because, like our shy sea slug Aplysia, when stimulated, Mimosa’s little leaflets close up defensively (see picture).
Gagliano’s research group was not the first to apply behavioural learning paradigms to plants or to Mimosa particularly, but earlier studies were not always well-controlled and findings were inconsistent. The rigour and comprehensiveness of this recent study make it the first to conclusively demonstrate true habituation (to dropping) and dishabituation (to shaking) in the plant. Mimosa plants learn that dropping is harmless and therefore do es not require a folding response, while still maintaining their capacity for responsiveness upon introduction of a novel, unexpected stimulus.
Moreover, these researchers found that not only did Mimosa plants display habituation learning, but that they did so in a context-sensitive way. Plants use their leaves to gather the sunlight they need to produce energy through photosynthesis. Therefore, in low-light environments, where this resource is scarce, there is a greater potential cost to a plant if it responds unnecessarily (leaf closure reduces the rate of photosynthesis by about 40%). By testing plants in both high and low light conditions, Gagliano’s team demonstrated that Mimosa plants learn faster in (low-light) environments where it the potential consequences are greater. Even when plants were initially germinated in high-light environments then transferred to a low-light context they displayed habituation much more rapidly than they had previously. It doesn’t take them long to learn and adapt!
Plants then, certainly seem capable of the simple sorts of non-associative learning that have previously been studied in animals like Aplysia. But what about more complex learning? Most animals are also capable of what is known as associative learning, which involves learning associations between different sorts of stimuli. No doubt many of you will at some stage have come across Pavlov’s famous experiments with dogs that demonstrate a kind of associative learning called classical conditioning. Meat powder naturally causes dogs to salivate. In Pavlov’s experiments, dogs that heard a bell paired with the presentation of meat powder eventually began to salivate when hearing the bell alone, even when meat powder was no longer present.
More generally then, in classical conditioning, organisms make associations that cause them to generalise their usual response to a stimulus onto a neutral stimulus with which it is paired. While the neutral stimulus would not usually elicit that sort of response, after an organism learns the association between the stimulus that would usually generate the response and the neutral one, the neutral stimulus alone becomes enough to elicit a response. Classical conditioning is important for organisms because it underlies the ability to make behavioural choices based on predicted outcomes. For instance, if a bell predicts when and where food will appear, it is adaptive to respond to the bell because it is a reliable predictor of reward.
In a study published late last year (2016), Gagliano and colleagues demonstrated that plants are capable of this sort of associative learning too. Moreover, they demonstrated this not with unusual, fast-moving plants like Mimosa, but this time with rather ordinary garden pea plants (Pisum sativum). One of the challenges of applying behavioural learning paradigms developed for animals to plants is in figuring out experimental designs that use environmental signals and plant responses that are ecologically relevant and can test the things we want them to. In this experiment the researchers used a Y-maze as shown in the picture. Plants learned that air movement in one arm of the Y-maze (caused by a fan) predicted subsequent light availability (in one experimental condition the fan was in the same arm as the light and in the other the opposite arm – plants were capable of learning either).
When seedlings in the test condition were examined for growth direction at the crucial junction of the Y-maze, they demonstrated a robust conditioned response to the fan. In contrast, plants in the control condition demonstrated only the innate blue light trophic response. This learning also turned out to be dependent upon signals relating to time of day – learning about light foraging cues occurred reliably only during subjective “day time” for the plant (which was manipulated experimentally). This makes sense as under usual conditions this would be when light might be available to plants and therefore the time to be looking out for cues that could predict its availability and location – attempting to do so at other times might incur too hefty an energetic cost.
Where to from here?
Plants, it seems, are smarter than we thought – they certainly display a pretty robust capacity to learn! What, then, does the current state of research into plant learning look like? The short answer is that it’s still very early days. This is an exciting time in the field as there are still so many questions! In the absence of neurons, what sorts of mechanisms enable this sort of learning in plants? How is information stored? Did the capacity for learning evolve several times in different lineages, or was it present before the point of evolutionary divergence? What sorts of directions will this research travel in? How can we improve, extend, and diversify our experiments? How can we foster cross-pollination between the fields of comparative psychology and plant biology? What sorts of things can plants learn? Does the capacity for learning vary across different members of the plant kingdom as it does with animals? To what extent do other non-neural organisms also display the capacity to learn?
Over the next few decades no doubt this research programme will expand and develop in unanticipated ways to answer some of these questions. In the meantime, it is uniquely exciting watching little pea seedlings in my garden emerging this season – as I spend my time learning about them, I wonder what they are learning about, too!
This is Fred. Fred is a potted rescue plant that I found one wintry Friday night in Rundle Mall (Adelaide CBD) lying upended, snapped in half, and looking very miserable underneath a bench. It was after business hours and as the city was ramping up for a frenetic Friday night, I was walking to the train station with a friend, heading to a his house for a quiet evening watching movies after a busy day at the office. While oblivious passers-by swarmed to pubs and clubs, the flash of wilted yellow petals and browned leaf edges glimpsed in my peripheral vision caught my attention – it was obvious that this plant needed hydration and some urgent medical attention. I extracted Fred from beneath the bench and tried to pile as much of the scattered soil back into the pot as I could. Seeing that the stem, though snapped, was still partially attached on one side, I propped it up as best I could with a little stake that remained in the pot. My friend and I then made our way to the train station with our new plant companion.
Very soon Fred, newly named and bearing splint bound with electrical tape in the hopes that the wound would heal, came to live on my office windowsill. The wilted flowers dropped away first, followed by most of the leaves and secondary stems, and I watched with anxiety over the next couple of weeks as it looked more and more likely that my eager attempts to save Fred’s life had been in vain. I grew despondent as Fred was reduced to what looked like just a green stick with a single broad leaf remaining. Nonetheless, I faithfully and regularly maintained the watering regime and Fred stubbornly clung to life.
A couple of weeks after the initial rescue, I arrived at the office and was overjoyed to find little budding leaves at some of the nodes in Fred’s stem. Over the next few weeks, these developed into larger leaves and bright yellow flowers also started blooming. I removed the tape and it appeared that the remaining tissue from the partially snapped node had been enough to sustain Fred and the reattachment had been successful. It has been a month and a half since I first found Fred, and my little begonia friend is still going strong, keeping me company in the office as I head into the final stages of my PhD.
Rescue plants and plant blindness
The other day I posted a photo of Fred to Instagram with the hashtag #rescueplant, thinking it a little play on the usual concept of a rescue animal – I adopted Fred in much the same way as my sister adopted the stray kitten Penelope she found in her garden, and the way my cousin adopts unwanted pets from her local shelter. Fred too had been left unwanted in the mall by a little pop-up flower shop, unsellable and of no use to them because of the injury (I often go back there at night now and collect slightly damaged but still pretty flowers that get left behind, on the look-out for more plants like Fred that might need rescuing).
To my surprise, when I posted the photo I found quite a few other rescue plants on Instagram under that hashtag, including one account, @lawrencetheplant, which documents the plight of a Croton Petra (Codiaeum variegatum var. pictum “Petra”) called Lawrence who was rescued from a dumpster. This account posts pictures of Lawrence’s rehabilitation and documents the amusing trials of his “new mum” learning about her plant and trying to take care of him. There are family photos of Lawrence with his (human) siblings, posts where Lawrence finds out about his heritage, as well as information about things that go wrong (overwatering, etc), all told as an extended Instagram story from the (hilarious and alarmed!) perspective of the plant.
The warmth of attention focused on this plant and other Instagram rescue plants got me thinking about some of the literature on plant blindness that I have encountered in writing my thesis. The term was introduced a couple of decades ago to describe a number of related phenomena:
“(a) the inability to see or notice the plants in one’s environment; (b) the inability to recognize the importance of plants in the biosphere and in human affairs; (c) the inability to appreciate the aesthetic and unique biological features of the life forms that belong to the Plant Kingdom; and (d) the misguided anthropocentric ranking of plants as inferior to animals and thus, as unworthy of consideration” (Wandersee & Schussler, 1999, p. 84)
Since this concept was defined, a literature has grown up around the topic suggesting that it is a robust and widespread phenomenon (at least in the western world). People have difficulty noticing, naming, and attending to plants in the first place, have difficulty recognising and recalling plants from memory, conservation efforts are biased against plants and toward animals, students in a variety of age groups prefer to learn about animals over plants, children and high school students have trouble recognising and attributing “aliveness” to and in plants, and have limited knowledge of and many false beliefs about plant biology and ecology relative to animals.
Given the importance of plants in the world’s (often threatened) ecosystems and cultures, and plants’ value as fascinating organisms in themselves, there are many reasons why plant blindness is a problem. The questions therefore naturally arise: what causes plant blindness, and what can be done about it?
Plant blindness partly arises and persists because of our own perceptual and cognitive biases. Plants often move at a timescale too slow for us to notice, and many of their responses are not even motor responses, but involve physiological changes more suited to dealing with the threats and opportunities that come with their more sessile lifestyles. This means we are less likely to attribute interesting behaviour or even “aliveness” to them and less likely to view them as individual organisms with their own goals and directedness. The way we characterise or identify their behaviour is therefore often fundamentally different to the way that we view, say, individual dogs or ants who display overt motor behaviours that are in many ways similar to our own behaviour.
Other causes of plant blindness are philosophical and historical, having to do with the way that the “nature” of plants has been theorised and discussed over the centuries by philosophers and botanists (which I discussed briefly in an earlier blog post). Partly, too, plant blindness is learnt over the course of our lives. We are taught to pay attention to certain things over others, and taught how to think about and value those things. How and what we learn is influenced by our perceptual and cognitive systems and by the cultural values and philosophical beliefs about plants to which we are exposed. However, there is plenty space to change the way that people learn about, think about, and value plants, and practical ways to work towards this.
Plant storytelling and invested engagement
This is where I think plants like Fred and Lawrence can come in. Emotive, happy-ever-after videos about rescue pets are often shared widely by my friends on social media. These are usually accompanied by the personal histories of the animals involved. We are encouraged to empathise with their plight and personally invest in their stories and well-being. Children also often learn about the needs of animals by helping care for their pets, and develop interest in other animals by being taught about their behaviour in the classroom, or watching them at zoos or animal parks. My local zoo has blogs that you can follow to observe the lives of some of the animals there and learn about the life stories of new arrivals. Plants, on the other hand, are often discussed in the abstract or passively, as representatives of a type rather than as individual living beings, more akin to herbarium specimens than living organisms.
In the literature, many partial remedies to plant blindness are proposed: redesigning curricula to include more plant examples, time-lapse photography to visualise plant movement, age-appropriate plant experiments to learn about plant structure and physiology, actively directing children toward ‘plant mentors’ (significant people in children’s lives that introduce them to interaction with plants), and the use of ‘charismatic plant’ species to generate interest in the botanical domain and conservation (a good example of how this can work involves the recent interest in the Titan Arums at the Adelaide and Mount Lofty botanic gardens, which unexpectedly attracted thousands of visitors when they bloomed). Working towards defeating plant blindness can and should involve all of these elements.
Here though, I wish to discuss and draw attention to the important role that telling the stories of individual plants can play. Storytelling is an important way that we learn how and what to value, and it can give insight into perspective taking where perspectives differ from our own. Introducing plants as individuals (including naming them!), getting people involved in their stories, lifestyles, and needs, getting people to care about a specific plant is, I believe, an often neglected route to changing their concepts associated with the plant realm. Philosophers and educators often talk about conceptual change as though it were just a matter of learning more or better facts and theories about a subject matter. However, getting people to conceptualise plants differently is, I think, as much a matter of changing their ways of seeing and modes of engagement with plants as their acquiring new facts and knowledge.
In practice, these routes to conceptual change often work synergistically. In one direction, knowing more about plants – what they are doing and why – increases their fascination and can draw our attention and interest to them. In the other direction, storytelling about concrete, individual plants (with names!) that encourages valuing and investment of care in them, provides a context for adult engagement with botanical knowledge, or for child-directed active and collaborative learning. If a plant is struggling and we care about it, we want to know why so that we can help. If we are invested in our plant we want to know about its history and what helps it thrive. This helps provide a context of active engagement that facilitates learning about plant biology and ecology, which then drives further interest and engagement.
Reading blogs and stories about individual plants like Lawrence and Fred can be helpful in this respect (for engaging adults, as well as children, particularly if done humorously as in Lawrence’s case). In a pedagogical context, helping children find their own plant to propagate or rescue, care for, and personally invest in can be even more motivating. Encouraging children to find out (actively research) and write about their plant, what it is doing and why (particularly from the plant’s perspective), can be a great way to help them start to overcome plant blindness. With younger children, this can be done through collaborative storytelling and by talking about their plant. In the literature, one well-documented contributing cause of plant blindness is simply the failure (particularly in urban environments) to promote or provide opportunities for children to engage with living plants. Enabling such opportunities may help children start seeing plants in a different way, as active, behaving, and developing individuals to be valued and cared for. This can be done at home, or in a classroom learning context. As a fortunate side-effect, it turns out that fostering a habit of tending to plants also has lifetime mental and physical health benefits.
So what can Fred teach us?
Helping a child look after a plant like Fred could provide numerous opportunities for learning. Finding out why Fred’s partly severed stem was detrimental, as well as why and when a stem can be put back together, could be a way to learn about how plant vascular tissue transports the nutrients, water and signals that help Fred thrive and respond. Finding out why Fred dropped leaves after wounding and drought stress, as well as how they regrew, would be a good way to introduce some basic plant physiology and information about stress responses. Figuring out whether Fred is a male or a female or both could be used to learn about plant reproduction, pollination, different types of flowers, and how these vary in different plants. Discussing why most of Fred’s leaves grew towards the light could start a conversation on plant tropisms generally (growth movements away from or toward various environmental stimuli). Wondering about Fred’s evolutionary ancestry, what other sorts of plants there are, and how they are like or unlike Fred in various ways could help provide a situation to learn about where Fred sits evolutionarily, and the adaptive value of some of Fred’s features.
As well as providing active care, a child could write a story about Fred, draw pictures of significant events or processes in Fred’s life, create a comic with Fred as the main character, or anything else s/he could think of. As an adult, caring for Fred equally provides opportunities (as demonstrated through Lawrence’s life story on Instagram) for amusing family photos, biologically informative descriptions of life events and anecdotes, and problem solving plant ailments. Botanic gardens could, in the manner of zoos and their animals, introduce new plants or document seasonal goings on via blogs and websites (as many have already started to do). These could inform readers not only about plant species, but by following individual plants and their life histories, could attract people to view particular plants, foster awareness of conservation efforts, and give urban dwellers more of an awareness of the seasons, cycles and biological rhythms from which they are so often quite disconnected.
Invested engagement with plants through caring for them, telling their stories, and understanding their individual histories and their behaviour can provide a context in which acquiring this knowledge is no longer an abstract, boring or disengaged exercise. It becomes, instead, a concrete and dynamic process grounded in valuing, caring for, and learning to see particular plants like Fred in a new light. Facilitating this new lens is one point of change-inducing intervention in the self-reinforcing collection of systemic factors that continue to perpetuate plant blindness. Hopefully, through the creative and collaborative efforts of botanists, educators, parents, policymakers, and the public, imaginative methods of promoting attentiveness to plants will continue to expand and eventually render plant blindness a thing of the past.
Hi phytophilosophers – given my increased teaching and other responsibilities this semester I unfortunately haven’t had a lot of time recently for this blog! I will be back writing eventually, but in the meantime, if any of you are in Adelaide and enjoy thinking about plants and philosophy, feel free to come along this Wednesday evening to my talk on the topic of metaphor use and conceptual reform in the plant sciences. All welcome – there are drinks and nibbles, doors at 7pm, talk starts about 7.30pm.
Hope you’ve all had a nice weekend and Mothers’ Day!
This week we’re usual time, usual place: Napier 210, doors @ 7pm. Laura Ruggles is presenting this week on some of her awesome research. She is a current PhD candidate in philosophy here at Adelaide and some of you may remember her as our former Philosophy Club president.
PLANT BEHAVIOUR AND INTELLIGENCE: ANTHROPOCENTRIC METAPHORS OR A CALL FOR CONCEPTUAL CHANGE (OR BOTH!)?
In the last decade or so there have been an increasing number of botanists, plant physiologists, philosophers and science communicators questioning the traditional conceptualisation of plants as passive, simple and reflexive organisms. Increasingly it is common to hear theorists use concepts like plant intelligence, cognition, perception, communication, memory and behaviour. Regularly theorists discuss what plants do as though they are agentive organisms with goals and desires that take action to fulfil these aims. In response, there has…
I wander through quiet green,
stillness of humid air
like sticky sheets on a sleepless night
to my hair,
sweat drenching me in a glowing sheen
mirroring the colours playing in the
lichen coated leaves
that brush past my eager face.
upturned eyes survey
trees, towering above
from gathering armies of cloud.
the world seems waiting, breath indrawn –
only my footfall and the occasional bird
defiant in the muted surrounds
dare challenge the hush.
beneath earth that delights naked toes,
borne on air,
inhaled in gulps by hungry lungs,
so recently reacquainted with its fullness,
in each dripping frond, each tendril,
a flurry of activity
stimulating growing things, the
of gene, protein, particle,
for the coming deluge.
heavy drops break through the leafy fortress.
like cathartic release of some great, pent-up grief
the sky opens, sudden explosion
drenching my ears and my clothes
with its thunderous outburst.
but no forest stillness breaks.
that waiting has only ever been my own –
mine the sole indrawn breath.
the ceaseless chatter,
oscillatory pulse of life
pausing neither for me
neither waiting, nor caring
for poetic sentiment
or dramatic effect.
This poem was written a while ago after getting caught in a storm in a Queensland rainforest. I had been recently reading a little on electroculture, the practice of stimulating growth in plants through the application of various types of electricity. There was a flurry of research activity on the topic back in the early 1900s, with contradictory findings. We know now that plants seem to use the electrostatic fields that often accompany thunderstorms as an early signal that allows them to prepare for and make the best use of the coming rain. This is perhaps not of greatest use in already damp rainforests, but in other places, because of the time it takes to synthesise new proteins, chlorophyll and so on, if plants are to make best use of the available water they need to respond and prepare in advance of its actually permeating the soil. Electrostatic activity is a good predictor of a coming storm that allows them to prepare in this manner, which is probably why in some contexts we see increased growth after mild electrostimulation of regularly watered plants.
Goldsworthy, A. (2006). ‘Effects of Electrical and Electromagnetic Fields on Plants and Related Topics’ (Ch. 11) in A.G. Volkov (Ed.) Plant Electrophysiology: Theory and Methods. Springer, Berlin.
For many of us the ideas and images that this word conjures up are of a particular kind. We may imagine a forest, a single tree in a park, a garden, a field of daisies, a handful of picked flowers, a crop. What springs to mind may be a thing that grows beside the road, still and leafy, perhaps gently swaying in the breeze. Relaxing, splashes of colour on a green backdrop, unconcerned if we snap off a small branch, unconcerning as we go about our daily business. We know they aren’t just there to look at and enjoy, though, plants are also very useful; they are sources of food and of medicine for many animals, including us. Throughout history humans have written countless classificatory manuals and amassed thousands of specimen collections that help us identify and describe species for these and other purposes.
Plants are things we appreciate for their aesthetic, metaphoric, and sensory qualities too – the beauty of a rose, the cheerfulness of a sunflower, the sweet, sharp scent of crushed mint, the alarming rafflesia flower in bloom, larger than a child, spattered with a shock of reds and leopard-spots, emanating a stench of decaying flesh. They feature in our art, have inspired many a poem with their beauty and their shade, with their capacity to create for us respite, healing, calm, delight, a sense of place and memory. For us plants form a large part of the landscape, a beautiful and varied backdrop against which our dramas and activities and those of the rest of the animal kingdom play out. They form for us too a continuously regenerating food source that enables these activities.
For thousands of years in the minds of many people, those involved in agriculture and in the western philosophical tradition and its scientific offshoots, this is the way plants have been viewed. They are the background; they are a resource we can use. Plants form the base of the pyramid of life, or the ‘great chain of being’ upon which we humans sit at the pinnacle. “Plants have the form of life that is least developed,” wrote Thomas Aquinas in the late 1200s, “The highest degree of life, however, is in man.”[i] This perspective is only very recently in the history of ideas, starting to have its foundations forcefully shaken.
There have been many colourful and fascinating folk and philosophical ideas about plants throughout the last few millennia and throughout the world. However, to begin the story about plants that was set to dominate early scientific thinking and continues to shape modern assumptions, we must travel back (as is the case for so many of the stories behind western ideas), to Aristotle. Writing in ancient Greece in the century before 300BC, the view of plants that Aristotle painted set the stage for many centuries of dismissive and exploitative assumptions about plant life to come but also kick-started the development of the science that would eventually begin to unravel these same assumptions.
Plants, according to Aristotle, are basically defective animals. They are like animals in being alive, but only capable of nutrition and growth, not sensation, appetite or movement and certainly not cognition or “rationality”, which he saw as solely the domain of man (though animals needn’t feel too bad – he also viewed this last, rational element of the ‘soul’ as completely lacking in slaves, incomplete in children and defective in women, whose rational faculty ‘lacked authority over their irrational soul’!). His view of the living world was a hierarchical one and his view of plants was strictly utilitarian; “Plants,” Aristotle claimed, “exist for the sake of animals, the brute beasts for the sake of man.”[ii]
For hundreds of years after the ancient Greeks, progress in European botany stalled. Outside of herbals during the early to mid-medieval period, there were almost no original writings or new insights, much misinformation about plants was spread and reproduced and empirical study of the natural world ran up against severe sanctions from the church. However, in the late 1100s Aristotle entered the stage once more. His writings were amongst the many ancient texts reintroduced to the western world from both Islamic sources and the original Greek and were translated and popularised throughout Europe in the early renaissance. Thomas Aquinas in the mid 1200s, in his efforts to reconcile traditional theology with Aristotle’s natural philosophy, resolved many perceived tensions between these approaches and enshrined the general Aristotelian approach to the natural world within the curriculum at institutes of higher education, endorsed by the then-powerful Catholic church.
The influence of Aristotle’s views on the subsequent development of early botany and western science in general cannot be overstated. Ironically, although the reconciliation of natural philosophy with theology meant that science could now be pursued without immediate risk of heresy charges and excommunication from the church, Aristotelian ideas on nature subsequently became enshrined dogma that substantially impeded scientific progress in the studies of plants as well as the rest of the natural world. To quote Bertrand Russel rather dramatically on the topic, “throughout modern times, practically every advance in science, in logic, or in philosophy has had to be made in the teeth of the opposition from Aristotle’s disciples”.[iii]
This can certainly be said of advances in the study of plant behaviour and intelligence. As a representative example, as late as 1911, over two thousand years after Aristotle, philosopher Henri Bergson in his Creative Evolution was still making claims along the old lines: “hence the world of plants with its fixity and insensibility, hence the animals with their mobility and consciousness”.[iv] This pervasive juxtaposition of, on the one hand passive, reflexive, immobile plants and on the other active, intelligent, purposive animals, is an incredibly exaggerated one that within today’s philosophical and general circles we still find bandied about. There are of course many differences between plants and animals but this simplistic contrast does those differences no justice.
Later renaissance thinkers such as Francis Bacon, René Descartes and John Locke challenged many problematic parts of the general Aristotelian philosophy and curriculum. However, although the frameworks and arguments supporting Aristotle’s claims about plants were rejected, the specific assumptions about them remained. Plants’ lack of sensory capacity, lack of active movement and their general backgrounding and status as just a resource rather than purposive organisms in their own right remained a framework that persisted well beyond the renaissance into modern times, despite the mounting body of evidence that suggested it was incorrect. “Just as men lived in the firm belief that human destinies depended upon the stars,” writes the botanist Kerner von Marilaun in the introduction to his 1894 textbook, “so they clung to the notion that everything upon the earth was created for the sake of mankind”.[v]
In the late 1800s most of Europe’s renowned plant physiologists still assumed along the same lines as renaissance thinkers several centuries earlier that plant movement was mechanical and insensitive. Julius von Sachs, for example, was a great German scientist credited with establishing plant physiology as a distinct discipline and pioneering approaches to reliable laboratory methods. For him, phototropism (movement of plants toward light) was the mechanical result of sunlight hitting cells in plant stems, causing osmotic changes in turgor that resulted in an overall bending. He quite scathingly criticised those amongst his peers who suggested that it might instead be a matter of sensation and response.[vi]
Real challenges to this view began in 1880, when Charles Darwin published a book called The Power of Movement in Plants. One of the many things he described in this book involves a now-famous set of ingeniously simple experiments on plant movement in response to various environmental signals. These showed that in plants the shoot tip (in the case of responding to light) and of the root tip (in the case of responding to gravity) is the primary site of sensation, while the parts of the plant that respond are distal, leading him to conclude that there is a “localisation of their sensitiveness, and the transmission of an influence from the excited part to another which consequently moves”.[vii] Darwin’s studies of movement in plants and his conclusions were radical for their time. Initially criticised and rejected by many of his contemporaries (including Sachs [viii]), reproduction of his results and further experimental refinements confirmed his findings and the extension of this research program lead to the discovery of the first plant hormone, auxin.[ix]
Darwin compared plant responsivity to the effects of stimuli on nerves in animals, and in a previous book on climbing plants had commented that “the power of moving, both spontaneously and from various stimulants, is far more common with plants than is generally supposed to be the case by those who have not attended to the subject”.[x] It was Charles Darwin too, who in 1873, organised the event in which Sir John Burdon-Sanderson (a medical physiologist) undertook the first known recording of a plant action potential. It is well known, now, that neurons in animal nervous systems use action potentials to communicate with one another during information processing. It is not well known outside of the community of plant physiologists that plants also use them.[xi]
Without exhausting the reader with an extensive run-down on the history of plant physiology after Darwin (or the complexities of thought on the matter of plant sensation and movement preceding him, only briefly touched on above), suffice it to say that we are, in this day and age, now well aware that plants certainly are actively monitoring and sensing their environments, receiving input from dozens of biotic and abiotic stimuli, combining and multiplexing these signals to produce complex and flexible behavioural responses according to the context and their needs. Their many movements, tropisms and some of the mechanisms underlying these are starting to be well understood. We know plants use a huge variety of hormones as well as electrical signalling for internal coordination of their behaviour, and a wide repertoire of chemicals for signalling and communication with other plants, animals, fungi and bacteria. We know they can recognise self and non-self, that they can discriminate in perception and response between not only plants of different species but can tell kin from non-kin plants, and cooperate more effectively with plants to which they are closely related. We know they use fungi as underground networks not only to share resources but also pick up useful information from their neighbours. We know that they actively form memories, learn and unlearn new behavioural patterns.[xii]
There are many reasons today why the activities of plants remain at the fringes of our thinking such that we fail to appreciate them for what they are. The legacy of Aristotelian thinking lingers implicitly, the timeframes (relative to the speed of animals) of the majority of plant movements are incredibly slow so as to be almost imperceptible to unassisted human perception as movements at all, we have our own cognitive biases when it comes to ascribing ‘aliveness’ to things less like us, or describing ‘behaviour’ as anything but movement, along with the many other historical and philosophical factors at play. Plants still, for many people, are a passive, largely inert (albeit beautiful) background or a useful resource. Many people are simply not aware of the rich diversity of plant behaviours and lifestyles, the incredible complexity of their sensory systems and the ingenuity of their life strategies.
For others, the inability to meet plants where they are at stems from the human tendency to anthropomorphise other beings or to attribute to them almost magical powers – the avid fans of The Secret Life of Plants who believe they can communicate psychically with their tomatoes and that plants have internal conscious worlds structured much like our own (even after decades of research debunking the claims and methods set out in that book and failure to reproduce their results) offer a modern example of this, though there are also plenty of examples from the mythology and plant-lore between and before Aristotle and Darwin too.[xiii] Fascinating and insightful as these myths and stories are, then as now, this tendency is another reason many theorists shy away from discussing plant intelligence or cognition – they are seen as either crude anthropomorphic metaphors or signs of magical thinking, and they prevent us from appreciating plants as they are rather than as we want them to be.
I strongly believe we need to get away from requiring that plants in their behaving be likeus to be intelligent, and instead appreciate the complexity, adaptability and suitability of their behaviour relative to their own requirements and lifestyles. Notwithstanding the usefulness of some plant/animal comparisons and metaphors, we need to explore plants on their own terms, in their own contexts and environments, as their own beings, rather than as set against animals with their capacities and described in terms of what they lack. We need to stop viewing plants as only a resource to be used by us and appreciate them also as beings with their own intrinsically valuable and interesting lives and goings on, actively pursing strategies relevant to their own wants and needs. We need to stop viewing ourselves, and animals in general, as the benchmark against which the rest of the biological world – plants, fungi, bacteria and everything else – is measured. If we can do this it will stretch our philosophies [xiv], stretch our ideas of what it is to be cognitive, to be intelligent, to be an ‘individual’, to behave, to be alive. Appreciating plants in their own right stretches our own sense of place in the world, too, what it is to be human on this planet, sharing it with the rest of life as we do.
Thinking about plants in this way, looking at what they are, what they are doing, why and how they are doing it, exploring their diversity and their quirks, where they sit in and how they contribute to their ecosystems, looking at how we think about and depict them, how we relate to them and how these forms of relating developed and may need to change, how plants relate to one another and to the world around them – these are fascinating explorations that give me endless delight!
In this blog I hope to share with you some of this delight and some of these perspectives and provide you with some food for thought in an informal way, through musings on related topics, through exploration of plants in art and literature, through short summaries of scientific findings and exciting little factoids about plant behaviour, through descriptions and my own sketches of the seasonal changes in the plants around me, links to other people’s writings on the topic and whatever else ends up becoming a part of the process.
Hopefully upon exploring these topics more, this word will gradually grow to conjure up in your mind images and ideas, beings and processes altogether different and far richer that what it did at the outset. This has certainly been the process for me, and I hope that you enjoy the journey as much as I do!
[i] Aquinas, T. (1967). Summa Theologiae (Vol. X): Cosmogony. Blackfriars, Great Britain.
[ii] Aristotle, (1952). Politics, in The Works of Aristotle (Vol. 2), W.D. Ross (Ed.) and B Jowett (Trans.). The University of Chicago, Illinois.
Also see Aristotle (1936). On Plants, in Minor Works of Aristotle, W.S. Hett (Ed., Trans.). Heinemann, London.
[iii] Russell, B. (1972), A History of Western Philosophy, Simon & Schuster, New York.
He is even more scathing in The Scientific Outlook (1949, p.43) where, writing on Darwin, Russell comments that “Like every other innovator of modern times, he had to combat the authority of Aristotle. Aristotle, it should be said, has been one of the great misfortunes of the human race.”
[iv] Bergson, H. (1911). Creative Evolution, A. Mitchell (Trans.), MacMillan & Co Ltd, London.
[v] Kerner von Marilaun, A. (1894). The NaturalHistory of Plants: Their Forms, Growth, Reproduction, and Distribution, F.W. Oliver (Trans.). Blackie, London.
[vi] Sachs, J. (1883). Text-book of Botany: Morphological and Physiological (2nd ed.). Clarendon Press, Oxford.
[vii] Darwin, C. (1880). The Power of Movement in Plants. John Murray, London.
[viii] Sachs comments on Darwin’s work are situated in the context of the establishment of a laboratory-based discipline of plant physiologists that viewed the methods of gentlemen naturalists such as Darwin as suspect “Charles Darwin and his son Francis…on the basis of experiments which were unskillfully made and improperly explained, came to the conclusion, as wonderful as it was sensational, that they growing point of the root, like the brain of an animal, dominates the various movements in the root.”
For some excellent reviews of this historical context see:
De Chadarevian, S. (1996). Laboratory science versus country-house experiments. The controversy between Julius Sachs and Charles Darwin. The British Journal for the History of science, 29(1).
Hutschera, U. & Briggs, W.R. (2009). From Charles Darwin’s botanical country-house studies to modern plant biology. Plant Biology, 11(6).
[ix] A comprehensive modern book on the role of plant auxin:
Zažímalová, E., Petrášek, J., & Benková, E. (2014). Auxin and its Role in Plant Development. Springer, Dordrecht.
[x] Darwin, C. (1875). The Movements and Habits of Climbing Plants (2nd ed.). John Murray, London.
[xi] For a comprehensive introduction to plant electrophysiology see:
Volkov, A.G. (Ed.). (2006). Plant Electrophysiology: Theory and Methods. Springer-Verlag, Berlin.
[xii] For a good introductory and popular book on plant perception see:
Chamovitz, D. (2013). What a Plant Knows: A Field Guide to the Senses. Scientific American, New York.
For more comprehensive reviews see anything by Anthony Trewavas. He popularised the notion of plant intelligence in a 2003 paper, has written many papers since and published a fantastic textbook in 2014, Plant Behaviour and Intelligence.
Monica Gagliano from the University of Western Australia does some fantastic work into plant learning, memory and acoustic emissions and perception – her 2014 paper Experience teaches plants to learn faster and forget slower in environments where it matters is a wonderful study on plant habituation learning.
For a really readable and thorough recent book on plant tropisms, check out:
Gilroy, S. & Masson, P.H. (Eds.). (2008). Plant Tropisms. Blackwell Publishing, Australia.
[xiii] Tompkins, P. & Bird, C. (1973). The Secret Life of Plants: A fascinating account of the physical, emotional, and spiritual relations between plants and man. Harper and Row.
The contentious claims and findings continue to hold sway in popular culture but have been disputed by scientists and have failed tests of repeatability:
Galston, A.W. (1979). The not-so-secret life of plants: In which the historical and experimental myths about emotional communication between animal and vegetable are put to rest. American Scientist, 67(3).
Horowitz, K., Lewis, D. & Gasteiger, E. (1975). Plant Primary Perception: Electrophysiological Unresponsiveness to Brine Shrimp Killing. Science, 189. pp. 478-480.
Kmetz, J. (1975). An Examination of Primary Perception in Plants. Parapsychology Review, 6. p. 21.
Schwebs, U. (1973). Do Plants Have Feelings?. Harpers. pp. 75-76.
Also see for other examples and discussion: Whippo, C.W. & Hangarter, R.P. (2009). The sensational power of movement in plants: a Darwinian system for studying the evolution of behavior. American Journal of Botany, 96.
[xiv] Other really interesting recent approaches to plant philosophy include Marder’s phenomenological approach to plants and Hall’s plant ethics:
Marder, M. (2013). Plant Thinking: A Philosophy of Vegetal Life. Columbia University Press, New York.
Hall, M. (2011). Plants as Persons: A Philosophical Botany. State University of New York Press, NY.