At first glance, the Malvaceae plant Lavatera cretica is just an inconspicuous weed. This mallow has pinkish flowers and broad, flat leaves that follow the sun during the day. However, what the flower does at night has drawn the attention of the scientific community to the humble plant. A few hours before dawn, the plant begins to turn its leaves in the assumed direction of sunrise. Malva seems to remember where and when the sun rose in previous days, and waits for him there.
When scientists in the laboratory try to confuse the mallow by changing the location of the light source, it simply learns a new direction. But what does this statement mean in general - that the plant is able to remember and learn?
The idea that plants can act intelligently, let alone learn and form memories, was a marginal point of view until recently. Memories are considered to be fundamentally a cognitive phenomenon, so much so that some scientists consider their presence a necessary and sufficient indication that the body possesses basic types of thinking. It takes a brain to form memories, and plants don't even have the rudimentary nervous system that insects and worms have.
However, over the past ten years, this view has been challenged. Mallow is no exception. Plants are not just passive organic automata. We now know that they can sense and integrate information about dozens of natural variables, and apply this knowledge for flexible, adaptive behavior.
For example, plants can recognize whether neighboring plants are related or not and adapt their feeding strategies accordingly.
Impatiens pallida is one of several species known to spend most of its resources on growing leaves rather than roots in the presence of outsiders - a tactic apparently aimed at competing for sunlight. Surrounded by related plants, touch-me-not shifts priorities. In addition, plants are capable of building complex targeted defenses in response to the identification of specific predators. A small flowering Tal's gum (Arabidopsis thaliana) can track the vibration of its eating caterpillars and release special oils and chemicals to repel insects.
Plants also communicate with each other and with other organisms, such as parasites and microbes, using multiple channels - this includes, for example, fungal "mycorrhizal networks" that link the root systems of various plants like a kind of underground Internet.
Perhaps not all that surprising, then, that plants are able to learn and use memory to make predictions and decisions.
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What is included in the concepts of "learning" and "memory" if we are talking about plants? The most obvious example in the debate is the vernalization process, during which some plants must be exposed to low temperatures in order to bloom in spring. Winter memory helps plants distinguish between spring when pollinators such as bees are busy and autumn when they are free, and the decision to bloom at the wrong time can be disastrous for reproduction.
In biologists' favorite experimental plant, Tal's reticulatus, a gene called Flowering Locus C (FLC) produces a chemical that prevents its small white flowers from opening. However, when a plant experiences a long winter, the byproducts of other genes measure the duration of exposure to cold temperatures and suppress FLC in large numbers of cells during the cold weather. When spring comes and the days lengthen, a plant that has a low FLC due to the cold may start to bloom. However, the anti-FLC mechanism requires prolonged exposure to cold weather to work effectively, rather than short periods of fluctuating temperatures.
The so-called epigenetic memory is involved here. Even after the return of vernalized plants to warm conditions, the FLC content remains at a low level due to the remodeling of chromatin marks. These are proteins and small radicals that attach to DNA inside cells and affect gene activity. Chromatin remodeling can even be passed on to subsequent generations of separated cells, so that the latter “remember” past winters. If the cold season has been long enough, plants with some cells that have not been exposed to the cold can still bloom in spring because chromatin modification continues to inhibit FLC expression.
But is it really a memory? Botanists studying epigenetic memory will be the first to agree that it is fundamentally different from what cognitive scientists study.
Is this term just an allegorical convention that combines the familiar word "memory" with the unfamiliar field of epigenetics? Or do the similarities between cellular changes and memories at the level of the organism reveal to us unknown depths of what memory really is?
Epigenetic and "brain" memories have one thing in common - constant changes in behavior or state of the system caused by a natural pathogen from the past. Yet this description seems too general, as it also covers processes such as tissue damage and metabolic changes. Perhaps the interesting question here is not whether or not memories are needed for cognitive activity, but rather what types of memory indicate the existence of an underlying cognitive process, and whether plants have these processes. In other words, rather than looking at "memory" itself, it is worth exploring the more fundamental question of how memories are acquired, formed, or learned.
“Plants remember,” behavioral ecologist Monica Galliano said in a recent radio interview. "They know exactly what's going on." At the University of Western Australia, Galliano studies plants using animal-specific behavioral learning techniques. She argues that if plants can show results that suggest that other living organisms can learn and store memories, we must equally consider the likelihood that plants also have these cognitive abilities. One of the forms of learning they have studied in detail is adaptation, during which living organisms exposed to unexpected but harmless pathogens (noise, flash, or light) will later demonstrate a proactive response that will fade over time.
Imagine that you enter a room with a humming refrigerator: at first it is annoying, but as a rule, you get used to it and, most likely, after a while, you will not even notice this noise. A complete adaptation presupposes a specific stimulus, therefore, with the introduction of a different and potentially dangerous stimulus, the animal triggers a new defensive response.
Even in a noisy room, you are more likely to flinch at a loud banging sound. This is called habituation relief and is what distinguishes true learning from other types of change, such as fatigue.
In 2014, Galliano and his colleagues tested the mimosa's learning abilities of a bashful, small, creeping annual. Its leaves curl up in response to a threat. Galliano and his colleagues dropped the mimosa from a height (which, in principle, could not have happened to a plant in its evolutionary history), and the plant learned that it was safe and did not show a folding reaction. However, a response was observed when the plant was suddenly shaken. Moreover, the scientists found that the adaptation of the bashful mimosa was also contextually determined. Plants learned faster in dimly lit environments where closing the leaves was more costly due to the scarcity of lighting and the observer's need to conserve energy. (Galliano's team was not the first to apply a behavioral learning approach to plants such as bashful mimosa,however, previous studies were not always strictly controlled and therefore gave conflicting results.
But what about more complex learnability?
Most animals are also capable of conditioned and associative learning, during which they learn that two stimuli are paired with each other. This is what allows you to teach the dog to approach the sound of the whistle - the dog begins to associate this behavior with a treat or affection.
In another study, Galliano and his colleagues tested whether seed peas could relate air movement to light availability. They arranged the seeds in a Y-maze, one of the branches of which was set in motion by air - it was also the brightest. The plants were then left to grow in the maze, and scientists expected to see if they would master the association. The results were positive: they showed that the plants mastered the conditioned response in a situationally determined manner.
There is growing evidence that plants have some of the inherent learning abilities of animals. Why did it take so long to realize this? You can do a little experiment. Take a look at this picture. What is depicted here?
Most will either name the general class of animals in the picture ("dinosaurs") and describe what they do ("fight", "jump"), or - if a dinosaur fan comes across - designate a specific animal ("driptosaurus"). Lichens, grass, shrubs and trees will rarely be mentioned - for the most part they will be perceived as the background of the main event, the "battlefield" of animals.
In 1999, biologists James Wandersee and Elizabeth Schuessler dubbed this phenomenon plant blindness - a tendency to ignore the potential, behavior, and uniquely active role of plants in nature. We treat them as a background element and not active agents of the ecosystem.
To a large extent, this blindness is due to history; we are talking about the philosophical remnants of long-abolished paradigms that continue to influence our understanding of the natural world. Many scientists are still influenced by the famous Aristotelian concept of scala naturae, the “ladder of beings,” where plants are at the bottom of the hierarchy of abilities and values, and humans are at the top. Aristotle emphasized the fundamental conceptual division between the immobile, insensitive plant life and the active and sensitive animal kingdom. In his opinion, the difference between the animal kingdom and humanity is just as significant; he did not believe that animals have any kind of full-fledged thinking. After the spread of these ideas in Western Europe in the early 1200s and during the Renaissance, this position of Aristotle remained consistently popular.
Today, this systematic prejudice against non-animals can be called zooshavinism. It is ubiquitous in the educational system, biology textbooks, trends in scientific publications and in the media. In addition, children growing up in cities rarely interact with plants, rarely care for them, and generally do not understand them well.
The way our bodies function - our systems of perception, attention, and cognition - contributes to herbal blindness and related prejudices. Plants do not jump at us, do not pose a threat, and their behavior does not affect us.
Empirical research suggests that they are not noticed as often as animals, they do not attract attention as quickly as animals, and we forget about them more easily than about animals. We perceive plants as objects or even do not pay attention to them at all. In addition, the behavior of plants is often caused by chemical or structural changes that are so small, rapid or slow that we cannot observe them without special equipment.
Also, since we ourselves are animals, it is easier for us to recognize animal behavior. Recent discoveries in the field of robotics indicate that research participants are more willing to attribute properties such as emotions, intentionality, and behavior to systems that mimic human or animal behavior.
We rely on anthropomorphic prototypes to try to determine if behavior is sane. This explains our intuitive reluctance to attribute cognitive abilities to plants.
But prejudice may not be the only reason we shrugged off the cognitive potential of plants. Some scholars have expressed concern that concepts like "grass blindness" are just confusing metaphors. When cognitive theory is applied to plants in a less abstract and vague way, they say, one gets the impression that plants function very differently from animals. Plant mechanisms are complex and amazing, they admit, but they are not cognitive mechanisms. It is believed that we give memory such a broad definition that it loses its meaning, and that processes such as adaptation, in fact, are not cognitive mechanisms.
One way to explore the meaning of the cognitive process is to examine whether the system uses representations. A set of colored lines can form a picture of a cat, a representation of a cat, just like the word "cat" in this sentence.
The brain creates representations of elements of the environment and thus allows us to navigate in this environment. When the process of forming representations fails, we can begin to form in the mind images of objects that are not really near us, for example, to see hallucinations. And sometimes we perceive the world a little wrong, distort information about it. I may misheard in the lyrics of the song - or shudder, thinking that a spider is crawling along my hand, when it is just a fly.
The ability to misinterpret incoming information is a sure sign that the system is using information-laden representations to navigate the world. This is the cognitive system.
As we form memories, we are likely to hold onto some of this displayed information so that we can later use it offline. Philosopher Francisco Calvo Garson of the Spanish University of Murcia stated that for a physical property or mechanism to be called representative, it must "be able to represent temporarily inaccessible objects or events." It is the ability of representation to reflect something that does not exist, he claims, that allows memory to be considered a sign of cognitive activity. A property or mechanism that cannot function offline cannot be considered truly cognitive.
On the other hand, some scholars admit that some representations can only function online, that is, they represent and track elements of the environment in real time. The mallow's nocturnal ability to predict where the sun will rise, long before it appears, seems to involve offline representations; other heliotropic plants, which only follow the sun as it moves across the sky, obviously employ some kind of online representation. And yet organisms using only online representation, scientists say, can also be considered cognitive. However, offline processes and memory are more convincing evidence that the body does not just reflexively respond to the environment. This is especially important in relation to the study of organisms that we are not intuitively inclined to regard as cognitive, such as plants.
Is there evidence that plants display and store information about the environment for later use?
During the day, mallow turns its leaves towards the sun using the motor tissue at the base of the stem - this process is actively controlled by changes in water pressure inside the plant, this is called turgor. The scale and direction of sunlight are encoded in light-sensitive tissues distributed over the geometric pattern of the veins of the mallow leaves, and information about them is stored until the morning. The plant also keeps track of the cycles of day and night with its internal circadian clock, which is sensitive to natural signals of sunset and sunrise.
At night, by looking at information from all of these sources, mallow can predict where and when the sun will rise the next morning. It may not operate with concepts like "sun" or "dawn", but it stores information about the vector of the sun and the cycles of day and night, which allow it to reorient its leaves before dawn so that their surface is facing the rising sun. It also allows the plant to learn a new position when physiologists fool its head by changing the direction of the light source. In artificially created darkness, the anticipatory mechanism can also function offline for several days. It's about optimizing the available resources - in this case, sunlight.
Can this mechanism be considered a "representation" - replacing the elements of the surrounding world that determine the behavior of the plant? I think so.
Just as neuroscientists seek to identify the mechanisms of the nervous system in order to study memory in animals, plant researchers seek to understand the mechanisms of memory that allow plants to store and use information, and also use this memory to customize their behavior.
We are just beginning to comprehend the unique abilities of this flexible and diverse group of organisms. As we broaden our horizons of curiosity beyond the animal kingdom and even the plant kingdom to study fungi, bacteria and protozoa, we may be surprised to find that many of these organisms employ the same basic behavioral strategies and principles that we ourselves, including the ability to kind of learning and forming memories.
For progress to be made, particular attention must be paid to mechanisms. We need to clearly understand when, how and why we resort to allegory. You should be precise in your theoretical statements. And if evidence points us in a direction that is at odds with conventional wisdom, we must boldly follow where it leads. Such research programs are still in their infancy, but they certainly continue to generate new discoveries that undermine and expand the human understanding of plants, blurring the usual boundaries that separate the plant kingdom from the animal kingdom.
Of course, trying to think about what thinking in general can mean in the case of these organisms is rather a flight of fantasy, since they actually do not have a division into brain (mind) and body (movement).
However, with some effort, we can ultimately go beyond the existing concepts of "memory", "learning" and "thinking" - which originally drove our request.
We see that in many cases the reasoning about the processes of learning and memory in plants is based not only on allegorical images, but also on dry facts. And the next time you come across a roadside mallow quivering in sunlight, slow down, look at it with new eyes and remember that this inconspicuous weed is fraught with extraordinary cognitive abilities.