Can plants hear? Can trees talk? Can they think? Until recently, all these possibilities belonged to the realm of fantasy and science fiction.
For millennia, we have bestowed human attributes to trees, but it’s only recently that science has found indisputable evidence of the similarities of plants and humans in the way they collaborate, communicate, and respond to external stimuli.
According to the Aristotelian ladder of life, man is at the top of the pyramid while plants are located at the very last echelon, next to small pluricellular and unicellular organisms.
This organization has been used, taught, and remained largely uncontested for centuries.
Botanical species are regarded as unintelligent, passive, unfeeling creatures, whose only function is to produce oxygen through photosynthesis and serve as food for or fodder the rest of the food chain.
We seem so certain of our human supremacy that, in a recent survey asking participants to describe what they saw in the picture of a man standing in a forest, 98% completely ignored the predominant vegetation.
This “plant blindness” is an evolutionary trait - it’s highly improbable that we’ll be devoured by a tree or a bush, therefore we keep our attention clear for predators and food - but it reflects the arrogance that prevails up to this day.
Although in the past there have been experiments trying to prove plants’ awareness and sensitivity, these lacked enough scientific rigor to be conclusive.
Fortunately, the new century and the new available technologies have heralded a kinder, more inclusive perception towards trees and plants.
Botanists experts in plant physiology - the study of the function and behavior of plants, which include the dynamic processes of growth, metabolism, reproduction, defence, and communication- have conducted interesting experiments that explore plants’ senses and reactions. And their results are defying our old conceptions about the abilities and sensitivity of plants, opening our eyes to new possibilities.
But first, we must put things into an adequate perspective.
Plants are not even close to being human or even animal-like. Plants and trees have no mouths or digestive system, yet they feed and drink. Plants don’t have vocal cords, yet they produce sounds. Plants don’t have eyes, yet they perceive light, something indispensable to their photosynthesis process. Plants have no noses or lungs, but they breathe. They have no nervous system, but they feel. They have no muscles, but they move.
In fact, if a science fiction author were in need of inspiration to come up with an alien life form, he would only need to look at any garden.
In this order of things, is it so far-fetched to think that trees and plants might be able to communicate? Several experiments are now affirming that they do, and in more ways than once thought possible.
As explained by Dr. Stefano Mancuso, plants are far more sensitive than animals. A single plant is able to detect at least 20 different chemical and physical parameters at the same time, some of which are well far from our experience, such as sensitivity to magnetic fields, electrical gradient, radiation, acidity and heavy metals in water, air, and soil.
Plants and trees are highly specialized organisms which have adapted to the environment in way very different from us, and these differences also apply to the way they communicate.
In a 2012 research, plant scientist Monica Gagliano and her colleagues reported the detection of clicking noises from plant roots. The research team used a laser vibrometer trained on the roots of the plant while they were submerged in water in a controlled lab setting, thus verifying that the sounds were emanating from the roots and no other source.
The experiment has been successfully replicated by other scientists around the globe, even using low-tech equipment, with the anecdotal addition that the plant’s clicking ceased as soon as someone else entered the room.
Dr. Lilach Hadany at the University of Tel-Aviv posits that plants may be able to produce sounds due to the air bubbles in their xylems, the tubes that transport water and nutrients from the roots to their stems and leaves.
Drought stress may cause bubble formation in the plant’s xylems, producing a sound like the one we make by sucking remaining liquid with a straw.
These sounds are ultrasonic - about 20-100kHz- far beyond the hearing human capability, but some scientists propose that some animals and insects may be able to hear them, which makes sense in the plant world.
However, the question about the precise function of such noises still remains open.
Thinking Olive Tree”, in Italy
Some scientists posit that plants use sounds to communicate their hydric stress, to lure insects, or to alert other plants of surrounding dangers.
But in addition to producing sounds, plants also seem to be highly sensitive to sound waves. Gagliano has observed that the growth of roots in plants responds to sounds by changing their direction of growth, an experience also repeated successfully by Dr. Mancuso.
All of these make us wonder - Can plants hear?
It appears that the old advice of talking to plants might be true - in spite of lacking ears, plants appear to be highly sensitive to sounds.
Apparently, plants might perceive sounds through their flower petals, their leaves, or even their roots.
In a study published in 2017, Monica Gagliano found that plants seemed to be able to sense the sound of water through their roots, thus guiding them to locate their source underground.
In 2019, a
study led by Dr. Hadany found that beach evening-primroses (
Oenothera drummondii) increased the amount of sugar in their nectar when they were exposed to the sound of a buzzing bee (their main pollinator).
Other plants (such as the Nepenthes hemsleyana) seem to have even evolved in order to better capture sound waves of its pollinators.
The Qingdao Physical Agricultural Engineering Research Center has taken a practical approach to this sensitivity to sound, designing a special module that broadcasts sounds to plants, testing it on rice fields. As a result, they have reported that its use has increased the rice production, lowering the need for fertilizers.
If this is surprising, it is even more so that plants appear to have memory and can even be trained.
Among other botanists who have performed memory experiments with plants, Dr. Stefano Mancuso successfully trained a Mimosa pudica - a plant whose reflex is to wrap its leaves at any contact- not to close its leaves at the contact of a non-threatening touch, and recorded that the plant remembered this training for as long as 40 consecutive days.
In another experiment, plants exposed to the sounds of a munching caterpillar produced more chemicals to deter their feeding when later attacked by real caterpillars, a result that echoes a Pavlovian response, something only thought possible on animals.
But perhaps the most interesting form of communication in plants is the one provided by the Mycorrhizal network found in forests, the so-called “wood-wide-web.”
This is a complex symbiotic system of interconnections between the roots of plants, trees and every other living organism in the soil, including fungi and bacteria (the word mycorrhizal comes from the Greek Myco, “fungi”, and Rhiza, “root”).
In this network, plants, trees, fungi, and bacteria not only help each other by exchanging vital nutrients (fungi provide nitrogen, phosphorus, and other minerals to trees and plants, and trees provide sugars to fungi), but they also allow its members to communicate with the surroundings through the net of roots and fungal threads known as mycelium.
This net -which uncannily resembles the neural connections in the brain- can cover wide expanses of the forest, allowing trees and plants to communicate with each other despite being miles away.
Mycorrhizal network (Photo: National Forest Foundation)
All interconnected plants, fungi, trees, and microorganisms in this net exchange information that would impact their survival, such as the presence of predators and toxins, environmental stressors, water availability, etc.
Trees and plants use chemical exchanges to connect and contact with other trees and plants, and by leveraging the differences and similarities in their microbial makeup, they are able to recognize their own kin, very much how we would recognize the features of our relatives.
But if kin recognition is surprising, it is also interesting to notice that trees and plants privilege their exchanges with their own kin, sharing their nutrients with the young saplings or sick individuals of their own family before those individuals in need from other groups. Some studies have even reported hierarchies within each community.
However, cooperation across species is at the core of the plant world.
Scientists have found that the interspecies communication between individuals sharing the same mycorrhizal network -for instance, between fir and birch trees- increase their fitness and resilience, and is vital for the survival of the forest.
As Peter Wohlleben aptly stated: “A tree can be only as strong as the forest that surrounds it”.
Cooperation, collaboration, communication...Can trees really decide on these mechanisms, or are they merely automatic evolutionary adaptations?
Dr.
Paco Calvo at the University of Murcia in Spain focused on this subject, and managed to doze off a
Mimosa pudica plant by using a common anaesthetic. When electrodes were attached to the leaves, these registered a suppression of electrical activity in them, proving that the plant had effectively been “put to sleep”.
Since anesthetics are meant to be used on animals (creatures with a neural system), the experience brings forth the question - does this result indicate that plants are conscious?
As impossible as this possibility may seem, other experiments also show that plants as simple as beans modify their behaviors depending on what they sense in their surroundings, whether it’s a pole to provide support, or a competitor for food and light.
All of which provide striking evidence of plants’ awareness.
The concept for the Tree of Souls in “Avatar” was based on the Mycorrhizal network
Do all these mean that plants are intelligent?
The answer will depend on the concept of ‘intelligence’ that we use. If we describe ‘intelligence’ as being able to articulate thoughts and emotions and make calculations, we’ll be circumscribing it solely to the human race.
Professor Tony Trewavas at the University of Edinburgh once declared: “under a broad definition, plants can be considered intelligent because they clearly respond to stimuli in ways that improve their odds of survival,” adding that “all life is intelligent because if it wasn’t, it simply wouldn’t be here.”
However, some scientists still reject the idea of ‘plant intelligence’, arguing that plants have no brain or neurons and therefore lack the machinery for thinking. However, it is
well documented that the slime mold (
Physarum polycephallum), an organism without brain or nervous system, can successfully solve even complex mazes and other challenges.
A more broad and generous concept defines intelligence as ‘the ability to solve problems’.
In this case, and as proven both in nature and in multiple controlled experiments, plants can be effectively described as ‘intelligent’.
Another concept of intelligence, such as the one used by Dr. Michio Kaku, establishes several conditions and several layers of intelligence, including a creature’s awareness of its position in space, its consciousness of hierarchy, etc. And yet, even under this rigorous conceptualization, plants qualify as intelligent beings - even plants as simple as rice.
Since all the botanical species that surround us are the result of millions of years of successful evolution, should we be surprised? Probably not.
The eminent French botanist, Dr. Francis Halle, described that, after American scientists sequenced the human DNA (at 26,000 genes), his own research group sequenced the genome of rice, coming up with a startling 50,000 genes.
This finding seemed to shatter the concept that man - a creature much more complex than a rice plant, and at the top of the evolutionary pyramid- had to have the most genes. However, his colleague, Dr. Axel Kahn, explained that their result was correct - the more a living creature evolves, the more genes it has, and this simply proved that the rice plant was more evolved than humans.
“How can that be?”, you may think. And the answer is: success in nature is not measured in levels of technological achievement but on adaptation and survival rates.
Plants originated well before humans and have been on the planet for much longer - some of them were already here when dinosaurs roamed the face of the Earth. All the botanical species we see around us have adapted to millennia of changes and challenges, succeeding through cooperation with other living organisms.
In fact, as expressed by eminent astrophysicist Carl Sagan, “Prokaryotes and Protista are our ancestors”. And we should not forget that we share 50% of our DNA with trees.
These discoveries have shaken the followers of the Darwinian theory. But we should not forget that even the noted British scientist manifested a great interest in plants and their similarities to animal responses.
In 1880, Charles Darwin published a volume about his detailed studies on plants, “The Power of Movement in Plants”, which many consider the first modern study in plant growth.
In this book as well as in previous research, Darwin drew attention to the similarities between animals and plants, remarking at least three: the sensitivity to touch (thigmotropism), sensitivity to light (phototropism), and sensitivity to gravity (geotropism).
In addition, Darwin studied sleep in plants (nyctinasty), and determined that this occurred to protect the leaves from injury due to radiation. (If you doubt that plants sleep, take a look at zinnias and how they shut their petals at dusk, only to reopen them at the touch of sunlight).
Through millennia of evolution, plants and trees have developed mechanisms to communicate among themselves and with the insects and small animals that populate their world. Trees and plants move at a much slower rate than humans, and they may not understand our words any more than we can understand their chemical messages, but this doesn’t mean that they can’t communicate.
Although it’s tempting to bestow human qualities to plants and trees, this would be as incorrect as denying that they possess their own unique methods of response.
We should not expect plants to process thoughts or do human things any more than we expect a human to transform CO2 into oxygen. Using human parameters to measure the responses of a different species can only lead us to error.
If we descend from our Aristotelian pedestal and begin to connect with all living species with the same level of respect, we may start to heal the broken balance we have caused to the planet.
Nature is ready to speak to us - Are we ready to listen?
To Explore More...
* From the Ents in the Lord of the Rings saga, to the Little Shop of Horrors, and Grandmother Willow in “Pocahontas”, films and literature offer no few stories and examples about talking trees and plants. Some great books featuring sentient, talking trees are: “The Giving Tree”, by Shel Silverstein, “The Two Towers”, by J.R.R. Tolkien, “The Three Talking Trees”, by Francis Timoney, “The Oak King and the Ash Queen”, by Ann Phillips, and “The Last Tree”, by Sophie Lipton.
To Learn More...
* Are you curious to learn more about the behavior of plants and trees from the point of view of science? Then read any of these books, or better still - read them all!
- “Brilliant Green: The surprising history and science of plant intelligence” by Stefano Mancuso and Alessandra Viola.
- “The Hidden Life of Trees” by Peter Wohlleben.
- “Finding the Mother Tree” by Suzanne Simard
* Would you like to know more about the mycorrhizal network? Then watch this brief but entertaining and thorough video, based on the research of Dr. Suzanne Simard:
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