Chapter 2: The language of signals and fragrances
The philosopher and linguist Günther Witzany developed a theory on “biocommunication”. As I was reading several scientific articles on plant communication, his work in particular captured my attention. He examines the subtle mechanisms by which plants, cells, and even genes exchange information. He understands life as an ongoing process of communication that requires a system of signals. These visible, audible, and olfactory signals include chemical messengers as well as electrical and mechanical stimuli. Communication is always about the meaning and interpretation of such signals.
I meet Günther Witzany at the venerable café Tomaselli, in the centre of Salzburg’s old town. On the first floor of the café, where it is quiet, he waits for me sipping a rose-hip tea.
I get straight to the point and ask what communication means. “Communication is a complex process,” says Günther Witzany. “Linguistics has taught us that communication is never merely an exchange of signs or signals. Lets look at the tomato plant for example. A diverse cloud of fragrances constantly surrounds neighbouring tomato plants; however, when caterpillars attack one plant, she ejects the fragrance methyl jasmonate to warn her neighbours. When methyl jasmonate becomes part of this mixture of fragrances, the neighbouring tomato plants must first recognize and differentiate it from the other fragrances, then interpret it as a signal of grave danger. Only after accomplishing these tasks can the plant produce substances to make her leaves unpalatable for the caterpillars.”
Günther Witzany continues to explain that an individual’s highly differentiated interaction with her partners and her environment is a prerequisite for communication. The parties must share a common understanding of the basic vocabulary of signals and the rules to decipher that vocabulary.
He sips his tea thoughtfully and continues: “Any communication has to follow three levels of rules. First comes syntax, which determines the correct sequence of characters and detailed rules on how the characters are properly combined. Syntax is obviously important for our verbal language. We understand ‘water’, but ‘teraw’ carries no meaning for us because the order of characters is combined incorrectly. The second level deals with pragmatic rules. At this level, an individual’s immediate vicinity and life situation play a crucial role. The meaning of a signal depends on the specific circumstances of the signal’s use.”
“The third and last level,” Günther Witzany continues, “is about semantics – what does a particular character mean to the receiver of the character? The letter sequence in w-a-t-e-r is determined by the syntax. Thanks to our semantics, we know what it means – the liquid in the glass in front of me. According to the semantic rules for tomato plants, methyl jasmonate means an imminent invasion of caterpillars. But in other situations and at other times, methyl jasmonate may have other meanings.”
He adds another example how the meaning of a character depends on its surroundings and might lead to misunderstandings, even within the same group. The Austrian bee researcher Karl von Frisch once added a bee colony from Italy to an existing colony in the Wolfgangsee area of Austria, causing great commotion and conflict among the bees. The bee researcher soon knew the reason: The distance and direction to a good food source, as communicated in the dance language by the Wolfgangsee bees, was 300 metres away. For the Italian bees, however, that same information meant the source was 500 metres away. When the Italian bees found no food source and flew back, they responded aggressively towards the Austrian bees for getting the “wrong” message. But the information was correct for the native Austrian bees. According to Günther Witzany, “The expression, namely the bee dance, was syntactically the same, but the context in which the colonies had learned the importance of these dance signals was different.”
I tell Günther Witzany of my visit to Wilhelm Boland, the Jena professor. He said a plant is essentially a kind of machine that responds and reacts like a programmed system, with pre-encoded answers. Günther Witzany objects vehemently: “A machine performs preprogrammed steps that give the impression of communication, but beyond that programming, a machine does not work. It cannot respond to the unexpected or to information for which it has not been preprogrammed, but plants can. By communicating with one another, plants are able to deal with new, non-predictable situations. With machines, the first level of communication, namely the syntax, is relevant; however, the other two levels always play a role. How does the plant grasp the correct character and interpret the meaning correctly? Communication requires partners ‘listen’ to each other, ‘respond’ to one another, and coordinate their activities. The partners must constantly choose between options and make decisions.”
Communication, he says after a pause, is a form of social interaction. Without community, communication is not possible. An individual cannot develop skills to use fragrance signals, characters, codes, rules, or languages in isolation. These skills can only be acquired at the community level among individuals acting independently, but as part of a community. While machines can exist independent of their surroundings, plants cannot. They are social beings, part of a dynamic web of relationships.
At the very end of our conversation, Günther Witzany points out the amazing fact that plants have been and continue to be incredibly successful in evolving. “They are the youngest living creatures on this earth. The first higher land plants formed about 400 million years ago. Today, approximately 99 per cent of the world’s total biomass consists of plants, of which more than 80 per cent are trees. They are by far the most adaptable creatures on the planet. Without a highly refined communication system, that would not have been possible.”
Example: Acacias fend off giraffes
In the savannahs of Kenya, the giraffe is the biggest enemy of acacias. Giraffes are capable of reaching the high treetops and eating acacia leaves, despite the thorns. But the tree defends herself. The moment a giraffe starts to eat leaves, the physical action of biting causes the leaf to release a gaseous messenger – ethylene – that warns neighbouring trees of the giraffes. Within minutes, the acacia transports tannins to the leaves. The tannins make the leaves bitter and upset the stomach of the giraffe. But giraffes have learned, over the course of their evolutionary history, to adapt also. They approach acacia trees against the wind, so that the trees cannot warn others that they are approaching. Also, giraffes eat for no more than ten minutes before moving to the next tree.
Coevolution between acacia, giraffe, and ants, or why it is important to be eaten
Acacias use another defensive strategy against giraffes and other herbivores. They produce small droplets of sugar-rich nectar (so-called extrafloral nectar) between their stems and thorns to attract ants. The acacias accommodate these ants by allowing certain thorns to swell, offering them a habitat. The ants are good allies and attack all predators, whether they be insects or mammals.
In an experiment led by Todd M. Palmer of the University of Florida in Gainesville, his research team installed a high fence to protect 1700 acacias from giraffes and other large herbivores. The trees were expected to grow better in the absence of giraffes – but the opposite happened. The acacia did not grow better and, in fact, became sicker. When the giraffes could not eat the acacia, the tree made fewer thorns and produced less nectar for the ants. As a result, the ants came no more which caused the trees to be strongly afflicted by other pests, specifically the bark beetles. The beetles caused slower and poorer growth among the acadias.
The extinction of one animal can have a domino effect on many other creatures. This experiment clearly illustrates the principle of coevolution.
This is an excerpt from the book: “Plant Whispers | A journey through new realms of science.” by Florianne Köchlin, English translation by Thomas Rippel