Are plants intelligent? Discoveries over the past 50 years have challenged the previous idea that plants are unthinking and inert. In Brilliant Green, scientist Stefano Mancuso and journalist Alessandra Viola argue that plants process information, sleep, remember, and signal to one another — showing that they are far from passive machines. This excerpt, from chapter 4, “Communication in Plants,” discusses the role that a plant’s hydraulic system has while communicating, both with itself and with other plants.
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Imagine a planet where plants have learned to communicate. In this imaginary world they can exchange information and even make themselves understood by animals, including the most complex animals, humans. On this planet, plants have learned to “speak” with animals in their language and can argue persuasively to get the help they need.
They use an information network of other plants and certain animals to extend the reach of their explorations beyond their own organism. They know how to obtain small services and, when necessary, intervention from other species, especially when, unable to change their location, they must defend themselves from herbivorous predators. They also get help with reproducing and propagating themselves in the environment.
Can you imagine such a world, where the most silent, passive, and defenseless organisms we know—plants—influence, and in some ways orchestrate, the lives of animals, from the smallest root worm to human beings? This world already exists: welcome to Earth.
Does a plant communicate within itself? First let’s ask a different question: How would having this capacity be useful to a plant? Trying to answer this will help us understand the roots’ ability to communicate with the leaves, and vice versa.
With their senses, plants gather information about their environment and orient themselves in the world. Plants are able to measure dozens of different parameters and process a great many data. But for a living organism, unlike a computer, putting information to practical use is more important than collecting infinite amounts of it.
For instance, what if a plant’s roots detect that there’s no more water in the soil, or what if a leaf is under attack by an herbivore? In such situations, informing the rest of the plant would seem to be essential. Indeed, any delay in transmitting the information could compromise the entire organism’s survival. Passing along this news is truly indispensable, but can we really call it communication?
To answer that, let’s start by defining what we mean by “communication.” Everyone knows the meaning of this word, but sometimes it’s useful to redefine a word, even one in common use, to be sure we’re all using it the same way. In one of its most common definitions, communication refers to the transmission of a message from a sender to a receiver. Communication thus requires three things: a message, a sender, and a receiver. In this elementary communicative model, there’s no mention that the two subjects (sender and receiver) must be located in different organisms, and in fact the functioning of our own body—like that of every other living being—clearly demonstrates that communication occurs between different parts of the same organism. For example, if we bang our foot and we feel pain, this is the result of communication between our foot and our brain. In the same way, if we touch something soft and we feel pleasure, this is made possible by the transmission of the tactile sensation from the hand to the brain. Obviously, in any animal, the different parts of the body are capable of transmitting messages.
Communicating is vital to every living being: it allows us to avoid danger, to accumulate experience, to know our own body and the environment. Is there any reason why this simple mechanism should be denied to plants? Perhaps because they don’t have a brain? In reality, there’s no reason why an organism without a brain shouldn’t be capable of transmitting messages inside itself, and in fact, as we’ll soon see, plants are quite good at it. It’s true that there are some technical obstacles that might seem to make this impossible. Plant organisms aren’t equipped with biological structures normally devoted to the transmission of electrical signals, signals which in animals transmit information from the periphery to the central system. In other words, plants don’t have nerves. And yet, we’ve just said that communicating a message is as fundamental for a plant as it is for an animal, and can have equal urgency.
The information that comes from the roots, like that which comes from the leaves, is essential for the entire organism and has to be transmitted rapidly for the plant to stay alive.
To transport information from one part of its body to another, a plant uses electrical as well as hydraulic and chemical signals. It thus has three independent systems, which at times are complementary, and which function over both short and long ranges, connecting parts of the same plant that are as close as a few millimeters or as far as dozens of meters apart. Let’s briefly look at how these systems work.
The first system, based on electrical signals, is one of the most used, and in practical terms is the same as the electrical system used by animals and humans, though “customized” in certain ways for plants. For example, we’ve already said plants don’t have nerves—that is, tissues dedicated to the transmission of electrical signals, which animals use to conduct nerve impulses. This would seem to present quite a problem: how to transmit signals without having tissues designed for that purpose? Plants have found a very functional solution: for short trips, these signals pass from one cell to another by means of simple openings in their cellular walls, called plasmodesmata (from the Greek plasma, “structure,” and desma, “connection”); for longer trips (for example, from the roots to the leaves), they use the main vascular system.
What? Plants don’t have a heart, but they have a vascular system? Yes: like animals, plants are equipped with a hydraulic system which serves mainly to move materials from one point to another within the organism, and which works like a true vascular system, much like our own, except for the fact that it lacks a central pump (that is, it lacks a heart, in accordance with the need to avoid unique organs, which we have already discussed). Thus plants have a circulatory apparatus that permits the transport of liquids from the bottom to the top and vice versa: a sort of system of arteries and veins, called xylem when the flow goes from bottom to top and phloem when the liquids flow from top to bottom. Xylem (from the Greek xulon, “wood”) is the conductive tissue principally adapted to the transport of water and mineral salts (but also other substances) from the roots to the crown of the plant, while phloem (from the Greek phloios, “cortex”) is the tissue that conducts in the other direction, transporting sugars produced by photosynthesis from the leaves to the fruit and roots.
The purpose of this circulation is readily apparent when you consider that the water absorbed by the roots is lost by the leaves in great quantities through transpiration, and so must be continually restored; meanwhile the sugars produced through photosynthesis—the plant’s main source of energy—must be continuously moved from the site of production (the leaves) to other parts of the organism.
By means of this complex vascular system, electrical messages circulate smoothly and fairly quickly, as in a tube filled with a conductive solution. Signals that would take a great deal of time to arrive at their destination if transmitted by chemicals can travel in a short time between the roots and the leaves, bringing urgent messages such as those concerning the water status of the soil. Is there only a little water, or a lot? The leaves, with sufficient notice, will adjust to the situation.
Before we come to a concrete example, let’s look at the functioning of the stomata (from the Greek stoma, “mouth,” “opening”), special structures on the surfaces of the leaves (usually on their undersides). These small openings put the inside of the plant in communication with the outside, much like the pores of our skin. Regulating each stoma are two “guard cells,” which control its opening and closing based on the current water and light conditions of the plant organism.
The stomata’s task is much more complex than it might seem. In fact, balancing the plant’s different requirements is far from simple: on the one hand, because carbon dioxide (CO2, necessary to carry out photosynthesis) enters through the stomata, the plant would seem to have every interest in keeping them open—at least during the daylight hours; but on the other hand, when the stomata are open the plant loses a great deal of water through transpiration.
Every plant has to respond to a real dilemma: keep the stomata open, and through photosynthesis produce the sugars necessary for survival, even if that means losing a great deal of water; or close them, conserving the water it needs but forgoing photosynthesis. It’s such a difficult problem that in order to understand how the plant can make the right decision, concepts such as “collective dynamic” or “emergent distributed computing” have been invoked, though they seem a bit out of place applied to plants.
However the plant does it, what’s certain is that it reaches a compromise between the exigencies of producing sugar and not losing water, both of which are essential to its survival. Let’s look at an example: the summer sun, with its powerful rays, is precious for photosynthesis, as it is for our solar panels. Unlike the latter, however, which produce more energy the more sunlight they’re exposed to, a plant must take into account not only light but its reserves of water. This is why during the midday hours—the hottest—it closes its stomata, depriving them of a great opportunity for photosynthesis. By doing so the plant protects itself from the risk of getting too dehydrated.
Imagine a tree (for example an oak or a very tall sequoia), whose roots suddenly notice that there’s not enough water available in the soil. Communicating that fact to the leaves is now imperative: if the stomata remained open, continuing to transpire water, the plant could die in a very short time. A very grave danger! So this message is essential for the tree’s survival and must travel fast.
To speed it along, as a first option, the plant utilizes an electrical signal, which in a short time reaches the leaves, prompting the stomata to close. At the same time as the electrical signal, chemical/hormonal signals also set out, moving through the vascular system, and taking more time to reach the leaves. These signals move in the same way as chemicals and hormones in our vascular system, but in plants, they are transported by a nutritive solution, rather than in the bloodstream. In a very tall tree, the trip may take several days! But the arrival of the chemical/hormonal signals guarantees that the leaves will get more complete information.
The hydraulic (vascular) system is also very useful for transporting messages of another sort. Picture a plant organism as a closed system. Have you ever cut into or broken off a branch, a leaf, or the stem of a flower, and noticed a liquid coming out of the wound? The sudden loss of tissue causes a small hydraulic failure in the plant, which communicates a simple but fundamental message to the organism: Attention! There’s a leak somewhere! Thus alerted, the plant immediately proceeds to locate the loss and form a scar at the site of the wound.
Thus, as we’ve seen, the three systems of internal signaling are complementary. They can function over long and short ranges and carry multiple types of information, and each contributes to keeping the plant alive and in equilibrium. From this perspective, too, plants are not so different from us.
Yet despite the similarities, a plant’s internal communication pathways have a completely different architecture from those of an animal. Whereas animals are equipped with a central brain toward which all the signals are directed, plants—by virtue of their modular and iterative construction—utilize multiple “data processing centers” which permit them a very different sort of signal handling.
We human beings can’t direct a message from our foot to our hand or mouth: all signals, with few exceptions, must first be processed by the brain. Plants, however, can communicate not only from their roots to their crown and vice versa, but also from one root or leaf to another. Their intelligence is distributed! Having no central processing center means that in a plant, information needn’t always take the same pathway; instead, it can be transmitted quickly and efficiently right where it’s needed.
In our discussion of plants’ senses, we saw that they can communicate with each other by means of a real language, composed of thousands of chemical molecules which are released into the air or the water and contain various types of information. Emitting these molecules is plants’ preferred means of communication, just as releasing articulated sounds is preferred by human beings. But we also communicate by means of gestures, facial expressions, bearing, and body language: a system of communication which, though differing from species to species, exists among many animals, especially the higher animals.
And plants? They can communicate with each other, too, by touching (generally with their roots, but sometimes also with their aerial parts) or by positioning themselves in particular ways relative to their neighbors. This is what happens with competing plants during “escape from shade,” when they assume different positions relative to each other, vying to win the race to capture light.
Another example of “gestural” communication is “crown shyness,” so named by the French botanist Francis Hallé (b. 1938). This phenomenon, in which some trees tend to avoid touching each other’s crown even when growing very close to each other, is not seen in all species, however. Trees usually aren’t shy at all about intermingling their crowns. But some species of the families Fagaceae, Pinaceae, and Mirtaceae—to mention a few of the most common—are quite reserved and don’t appreciate such interweaving. Just go into a pine woods and look up. The trees manage never to let their crowns touch, but leave a bit of empty space between their own leaves and their neighbor’s, thus avoiding contact that we might assume would be unwelcome. Though why and how it happens isn’t clear, this phenomenon implies a type of signaling by which the crowns communicate their presence mutually and agree to a sort of territorial partitioning (in this case, of air and of light) so as not to disturb each other.
Plants interact at many levels and in their interactions exhibit different personalities. Are some species more or less competitive than others, more or less aggressive, collaborative, shy? Sure. But that isn’t all. Plants’ similarities with the animal world, while not numerous at the anatomical level, are plentiful in the behavioral sphere. This shouldn’t be surprising: all living beings have the same fundamental aims, and presumably the means of attaining them are similar in some way, too. Yet despite real affinities between animal and plant behavior, one domain would of necessity seem to be excluded: that of the family. Indeed, plants don’t have families. There’s nothing like the kind of connection that occurs between related individuals of the same animal species. Or is there?
In the plant world, we don’t expect to find the concept of kinship or clan; we tend to associate these notions with very evolved species, such as humans and some other higher animals, but certainly not with plants. And yet . . . plants definitely can recognize their relatives and in general are much friendlier to them than they are to strangers. To understand why plants developed this ability, we should ask ourselves what this trait is useful for. It’s an appropriate question because in nature no capacity develops without a reason, including the recognition of familial bonds. Being able to identify the individuals with whom one has strong genetic similarities is important for all species and results in outstanding evolutionary, behavioral, and ecological opportunities. For example: organisms with this capability manage their territory better, defending themselves against enemies without wasting energy fighting against kin; they can avoid reproducing with close relations; and above all they can benefit indirectly from the success of individuals who closely resemble them genetically.
To fully understand these advantages, we need to keep in mind that in nature the main purpose of life is to protect one’s genetic inheritance, that is, oneself and one’s close kin: parents, brothers and sisters, children. Competing with one of those is such a waste of energy! Much better to cooperate and join forces to overcome adversity, passing our genes to the next generations. From this perspective, the capacity to recognize one’s kin is a great advantage; but are we sure that plants behave differently toward other plants based on their degree of genetic kinship?
In the animal kingdom, this process of recognition makes use of the senses: sight, hearing, smell, and in some cases even taste. In plants, it occurs through the exchange of chemical signals released by the roots and probably the leaves (though with regard to leaves, research results aren’t yet conclusive).
Plants are stationary, as we’ve said: a point that bears repeating because this is their main difference from animals. Unable to move from their place of birth, plants clearly evolved as territorial organisms, and their capacity to defend their territory must necessarily be greater than that of any animal. Plants are fierce fighters—and it’s not hard to see why. An animal that is in an unfavorable position with respect to another can always go and live somewhere else. A plant doesn’t have that option and must resign itself to sharing the resources of its environment with the other beings coexisting in the same area, sometimes even a few centimeters away. But this doesn’t mean simply accepting their presence; on the contrary, it means a continuous struggle for one’s own space, which must be defended against all intruders. A plant protects its territory by investing much of its energy in its underground part. By producing a great many roots, it occupies the soil like a military force and claims possession of it against its neighbors. But not always: if neighboring plants are part of the same clan, and thus relatives, there’s no need to compete, and the roots can be kept to a minimum for the benefit of the aerial part.
In 2007, a simple but important study shed light on this type of familial behavior. The experiment consisted of growing thirty seeds from the same plant in one pot, and in a second pot, identical to the first, growing thirty seeds from different plants. Observing the behavior of the young specimens growing in the two pots led to the discovery of several evolutionary mechanisms formerly thought to be present only in animals. The thirty plants from different mothers behaved as expected, developing a greater number of roots in an attempt to dominate the territory and assure themselves sufficient food and water, to the disadvantage of the other plants. But the thirty plants from the same mother, though they too found themselves coexisting in a restricted space, produced many fewer roots, advantaging the plants’ aerial growth. In their case, what was observed was noncompetitive activity linked to their genetic proximity. This was a fundamental discovery: it replaced the traditional view that plants would adopt a stereotypical and repetitive mechanism (neighboring plant = necessity for defense and competition for territory) with a much more complex estimation that takes into account different factors, including genetic kinship. The plant, it turns out, checks out a potential rival before attacking or defending, and if it discovers a genetic affinity, instead of competing it chooses to cooperate.
This excerpt has been reprinted with permission from Brilliant Green: The Surprising History and Science of Plant Intelligence, by Stefano Mancuso and Alessandra Viola, and published by Island Press, 2015.