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Could plants and trees someday release toxins that would eliminate us as a threat?

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"Plants respond to human stimulation; it has been scientifically proven." "Plants can communicate, adapt quickly, and attack specific threats." That is the basis of the science fiction that is poured into the film "The Happening" about global warming in which plants massively release toxins into the air, blocking the neurotransmitters responsible for human self-preservation. But how far is it from reality?

With the current events that we are experiencing with the current pandemic and its possible origins, it is inevitable to begin to wonder how much damage nature can cause by approaching us without any respect. The film The Happening raises an interesting question: what happens if the environment, stimulated by centuries of pollution and contempt, turns against us?

When the director of the film M. Night Shyamalan came up with the idea, he sent investigators to find out if this really could happen. He wanted to know from one to 10 whether this idea was entirely possible, probable, or impossible. Shyamalan said: "When they returned, the pile of information on how plants work, how a cotton plant can send a signal to the other side of the field to tell them that this insect is coming, and then they send toxins: and I spoke to the University of Massachusetts and some other institutes on toxin-free brain function and how they affect each other. It's fun to watch, in the process of research, all of these great scientific facts that came up about other great ones for writing, for making movies".

However, according to three botanical experts, The Happening is made up of real grains that have been greatly exaggerated for the benefit of history.

In The Happening, the foliage of the trees reacts to the mere presence of humans: first it is the city parks intoxicating the urban masses, then the fields of prairies that go along rural roads. The deadly pattern, which radiates outward within 36 hours, leads to the assumption that plants are communicating. "If you take a very broad definition of communication, which means any kind of signal made by one organism that can be detected by another, then yes, in some circumstances, plants can communicate," says Joe Armstrong, professor of botany at the University of Illinois.

Still, it is not a communication like a conversation. Plants perceive the presence of other vegetation through photo receptors and chemical means. When herbivores chew some types, for example, a plant's response is twofold: chemical, to deter the herbivore; and a volatile emission to the air that other plants can feel and then respond with protective chemicals. The same chemicals are not produced when the sheets are simply torn or damaged. Some plants have been shown to recognize nearby plants that compete for sunlight. Ripening fruits release hormones that can cause a response in surrounding plants, and parasitic plants can detect chemicals released into the air and soil by other plants.

There's research suggesting that plants may have a social life that we don't understand so far: According to a study recently published by scientists at McMaster University in Hamilton, Ontario, the Great Lakes marine rocket herb may recognize plants related to that. "This is important," says Susan A. Dudley, an evolutionary plant biologist who conducted the study, "species often live with relatives and, like animals, can increase their physical condition by benefiting relatives." It is interesting, but it hardly adds to a network of conniving and conversational trees.

The film claims that plants can attack specific threats. While some plants may target specific plants, in the film it seems to imply that the plants were intentionally direct behind humans, and that's not exactly how it works. "Some plants send out volatile hormones when they're attacked," says Dudley. "The signals are evoked by a combination of insect damage and saliva. Predators like wasps use this signal to find their prey." In other words, Armstrong says, in all likelihood wasps have developed a sensory apparatus to detect damaged plant hormones. So it is not the plant that is calling the wasp in its defense.

Shyamalan seems to suggest that plants could evolve to the point of attack because humans pose a threat to the planet. But Armstrong emphasizes that individuals do not evolve, populations do, by selecting genetic variants that promote survival. How quickly everything happens depends on how fast those plants reproduce. "Something like trees are going to be very slow because their generation time is low," he says. "Something like weeds can evolve quite quickly."

In the case of the killer vegetation, some plants would have had to present the killer genetic variant at random, and over time those plants would have been selected because they were more fit than their less toxic relatives. Only in this case, it is not entirely clear why killing humans would help these plants evolve, especially in places like parks.

Botanists agree that plants can emit volatile compounds. There are literally thousands of plants that have neurological impacts on humans when ingested, but researchers have not yet discovered any that can emit neurotoxins into the air. And there's certainly no evidence to suggest that plants emit them on a large scale, because developing chemical defenses requires extraordinary amounts of energy.

However, smaller plants with smaller populations are a different story. "When eating is a problem for the smallest plants, that's a common situation where plants are toxic," says Armstrong. But that's not the setting portrayed in The Happening, where characters spend most of their time running away from toxins in the fields of central Pennsylvania, not exactly the center of life for exotic plants.

All our scientists agree that plants can respond to certain stimuli, it is a phenomenon called tropism, but speech is not one of them. "You can talk to them a lot and raise the CO2 content in their neighborhood," Armstrong jokes, "but I don't think plants can sense our presence. There are plants that respond to touch, carnivorous plants like the Venus flytrap have trigger hairs, and if you play them in the correct sequence the plant will shut down. I don't think most other plants have a mechanism to respond to the presence or absence of people. "

Apparently the M. Night Shyamalan film will continue to remain science fiction. Or perhaps the Tenth Man Rule could be applied to analyze future probability. The tenth man rule is a common sense tactic stating that whenever nine people agree that something is true, a tenth person must defend the contrary thesis; even if you disagree with it, to be prepared for an unlikely event or thesis.

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