Although they seem sessile organisms that lack all sensitivity, the truth is that the behavior of plants is far from being that of simple objects, as many people perceive them. Plants, without a nervous system like that of animals, have a complex biochemical system for recognizing and responding to stimuli.
A holly or oak plant that is repeatedly attacked by herbivores will produce tougher, spinier leaves. The leaves of a venus flytrap will close like a spring, at the slightest contact with an insect. And a bean stalk can solve a dark labyrinth only guided by the light that comes from its entrance, reflected on its walls.
Gnosiophysiology of plants
There are multiple examples of stimulus-response interactions in plants. These events are included in the study of the gnosophysiology of plants , sometimes called in an anthropomorphized way as ‘plant cognition’ or ‘plant neurobiology’; These terms are not entirely correct, since plants lack a nervous system and, therefore, cognitive systems.
Apart from the simple reactions already mentioned, it has been suggested that plants have capacities that, without having the same evolutionary basis as in animals, can be considered analogous with respect to animal behavior. One of the most fascinating behaviors reported in plants is the ability to anticipate and remember events .
The memory of plants
What we call “memory” in a plant has nothing to do with human brain processes. We are faced with processes analogous to those of animals, and care must be taken not to fall into the anthropomorphization of plants.
Let us remember that they are organisms without a nervous system, and therefore, without cognitive processes. All memory-related processes in plants are pure biochemistry ; an extraordinarily complex biochemistry, the product of thousands of years of evolution; but biochemistry after all.
Not only has it been proven that plants have the ability to have previous experiences, they also present a plasticity in the response based on said experiences ; what in animals we would call conditioning.
Charles Darwin already observed that the response of the first leaves of a plant to light was influenced by its previous exposure. Since then, many other forms of, say, “plant conditioning” have been observed.
The growth of clover branches depends not only on the presence of neighboring clovers, but also on the neighbors of the previous year; plants that are stimulated by an initial attack respond more quickly, aggressively and effectively to new attacks, also producing a higher concentration of secondary metabolites for defense.
The anticipation of plants
The anticipation behavior in plants began to be studied less than three decades ago. Being such a young field of study, there are still many unknowns to be resolved, although we do have some clear and well-established aspects.
Plants, for example, use the photoperiod —the length of daylight—as an indicator to anticipate winter chill. They also orient the leaves of the lower branches in the position in which they will receive less shade with respect to those of the upper branches, before they grow and cast said shade.
For this type of anticipation they use physical signals, such as the reception of far red light , reflected by the upper leaves from incipient moments, which indicates to the lower ones where the shadow will be projected when they grow.
Other examples of anticipation, even more complex, can be found in the dodder , a parasitic plant that actively selects the most nutritious hosts and rejects those that are less so, before feeding —we do not yet know how it does so—.
In the 1980s, in South Africa, a fascinating case was reported. Hundreds of kudus , a type of antelope, were killed by acacia leaves , their usual food source. To defend themselves, acacia leaves produce a type of molecule called tannin , which is astringent and has the ability to precipitate proteins. The tannins are responsible for the fact that when eating a banana that is too green, the tongue becomes “spongy”. Normally, the levels of tannins are not so high as to harm the antelopes, but during the dry season, and after a strong predation event by herbivores, the plants produced such high concentrations of tannins that they were capable of inactivating the antelopes. enzymes from the liver of the kudus, killing them in a few weeks.
The most surprising thing is that all the plants in the population had produced the overdose, and not only those that had suffered predation. In fact, the closer a tree was to another that suffered the predatory effect, the more tannins it accumulated in its leaves, thus anticipating the attack of the kudus. The overdose of tannins could be produced in less than half an hour. It was discovered that the key lay in ethylene, a volatile molecule that the attacked acacias emitted into the environment, and that induced the formation of defenses in the receiving acacias.
The plants were communicating .
plant communication
As with the rest of gnoseophysiological processes, communication in plants is drastically different from that of humans or other animals. Again, it is a phenomenon of a strictly biochemical nature.
The acacia and kudu event in South Africa is one of the most visual examples. However, the biologists IT Baldwin and JC Schultz already observed this phenomenon of chemical communication between plants in 1983. Since then, several cases of communication between plants have been verified. And not only between them.
Plants can also communicate with herbivores, with mutualistic organisms and their parasites, as well as with organisms that are predators or parasites of herbivorous animals or their own parasites.
Sometimes, some plants that are attacked by herbivores emit volatile substances that indicate to predators where their potential prey is , or guide the parasites of those animals to new possible hosts. Examples of this would be yarrow , which attracts ladybugs that prey on aphids, or tansy , which attracts tachinid flies, whose juveniles are parasitic on bedbugs, beetles, and butterfly caterpillars.
It is convenient to remember that all these plant behaviors, although they are very real, do not imply intelligence, consciousness, or any analytical capacity in plants. They are adaptive responses, acquired in the evolutionary process, of an exclusively biochemical nature. Plants are organisms without a central nervous system, without a brain, and therefore without consciousness .
Although they do not need it to carry out the most fascinating behaviors.
References:
Affifi, R. 2018. Deweyan Psychology in Plant Intelligence Research: Transforming Stimulus and Response. En F. Baluska et al. (Eds.), Memory and Learning in Plants (pp. 17-33). Springer International Publishing. DOI: 10.1007/978-3-319-75596-0_2
Baldwin, I. T. et al. 1983. Rapid changes in tree leaf chemistry induced by damage: evidence for communication between plants. Science (New York, N.Y.), 221(4607), 277-279. DOI: 10.1126/science.221.4607.277
Calvo Garzon, F. 2007. The Quest for Cognition in Plant Neurobiology. Plant Signaling & Behavior, 2(4), 208-211. DOI: 10.4161/psb.2.4.4470
Karban, R. 2008. Plant behaviour and communication. Ecology Letters, 11(7), 727-739. DOI: 10.1111/j.1461-0248.2008.01183.x
Phillip, Y. 1990. Acacia trees kill antelope in the Transvaal. Scientific American, 263(6), 28.
