Are Plants Conscious?

 by Rukhsar Naguman

Charles Darwin first proposed the idea of plant consciousness through the concept of the “root brain” in which he suggested that the tips of the roots acted as brains; receiving and processing information from its environment and sending signals to different parts of the plant to respond or react accordingly. This concept eventually led to plant neurobiology. Although plants do not have neurons, they do, in some sense, have a nervous system like us where they similarly use neurotransmitters (chemical signals) and electrical signals to respond to stimuli (a change in the environment). Plants have senses that are analogous to humans such as sight, touch, smell, and hearing.

First, how can plants have the sense of sight without eyes? To see, we need photoreceptors to detect light. In humans, 4 types of photoreceptors reside in the retina (located at the back of the eye) helping us respond to a spectrum of 400 - 700 nanometres (nm). Whereas in plants, there are about 13 types of photoreceptors located in all their cells enabling them to respond to a spectrum of 300 - 750 nm which is higher in comparison to humans. This detection of light enables the plant to differentiate between seasons and to grow towards light (phototropism). In terms of smell, plants can detect volatile organic compounds (a class of odour) allowing them to attract birds and bees, deter pests, and react to disease in nearby plants. Scientists have found that the smell of freshly cut grass which is a mix of organic compounds can send aromatic distress signals to nearby plants indicating danger.

Plants also possess the sense of “hearing”. Though they do not have ears, they respond to sound in their local environment. According to research from Tel Aviv University, the Oenothera drummondii flowers produced sweeter nectar within 3 minutes of a playback sound of a flying bee or to synthetic sound signals at similar frequencies. These flowers showed no response to random, higher-frequency noise suggesting that the nectar-producing reaction from the flowers was not triggered by random sounds. Similarly, in another experiment, plants released organic compounds (which deter insects) when they sensed a caterpillar eating their leaves. When an audio recording of a caterpillar was played by the plant, it once again released the organic compounds showing signs of “hearing”.

Finally, the sense of touch in plants is vital for acting as a defense mechanism or catching prey. In the Mimosa Pudica plant, its leaves curl inwards when touched. In an experiment, the Mimosa Pudica plant displayed learning and memory behaviours. When dropped for the first time, it curled its leaves, perceiving the fall was dangerous. However, after the 2nd or 3rd drop, it figured that the fall was non-dangerous and thus did not curl its leaves. What is even more bizarre is that a month later, when the same plant was dropped again, it did not curl its leaves indicating signs of memory. The Venus Flytrap uses the sense of touch to catch prey. It has 6 trigger hairs that are sensitive to touch. However, for the two lobes of the leaves to snap shut to trap the prey, two trigger hairs need to be touched in a span of 25 seconds. This indicates that Venus Flytrap has “short-term memory” to register the two triggers in the given period.

In conclusion, despite lacking organs such as ears, plants display senses analogous to humans showing signs of awareness and response to their changing environment. They do not possess a brain meaning they do not necessarily “think” like humans but by the definition of consciousness – the state of being aware of and responsive to one’s surroundings – plants, to some extent, are conscious.

Useful links to learn more about plant neurobiology:

https://www.youtube.com/watch?v=gBGt5OeAQFk

https://www.youtube.com/watch?v=pvBlSFVmoaw

https://www.youtube.com/watch?v=MPql1VHbYl4