By Kit Flynn,
from the Durham Master Gardeners’ Newsletter
We feel plants: we feel the softness of lamb’s ear, we treasure the smoothness of a rose petal, and we learn to avoid the rose’s thorns. The question is: do plants feel? Does a tree know when the arborist had shorn it of one of its limbs? Chamowitz not only believes that plants can feel, but he asserts that they know the difference between hot and cold. A vine will quicken its growth pace when it has found an object to scale while the Venus flytrap knows when an insect has landed on its jaws.
When we talk about plants’ feeling, we are talking about the sense of touch, not emotional feelings. “[P]lants perceive tactile sensation, and some of them actually ‘feel’ better than we do” [576]. Our nervous system permits us to feel different sensations: “The initial stimulus starts a rapid electrochemical reaction known as depolarization, which is propagated along the length of the nerve. This electric wave hits the adjacent neuron, and the wave continues along the new neuron, and so on, until it reaches the brain”.14,
The sense of touch in plants works differently. That plants do feel is demonstrated by the Venus flytrap, as the hairs on the inside of each lobe will trigger the closing of the trap—if two hairs are touched within 20 seconds of each other: the touch of one hair is not enough to spring the trap [645]. “The hairs are extremely sensitive, but they are also very selective.” Darwin observed that water droplets falling on the hairs were not enough to close the trap, nor was a wind gust.
Mimosa pudica is an interesting tree. The leaves on this South American native are unique in that “its leaves are hypersensitive to touch, and if you run your finger down one of them, all the leaflets rapidly fold inward and droop,” only to reopen minutes later [685].15
When the pulvinus cells—the motor cells that move the leaves—fill up with water the leaflets open; when water leaves the cells, the leaflets droop. When an electrical action signal reaches the pulvinus cell, both the potassium and water leave, causing the leaflet to droop. After several minutes the cell will pump both the potassium and accompanying water back into the cell.
We feel in ways that plants are unable to because we have a brain. For example, plants feel but Chamowitz emphasizes that they cannot feel pain; they also have no ability to feel subjectively. “Our perception of touch and pain is subjective, varying from person to person” [782]. A light touch can be pleasant for one person and an agonizing tickle for another. Plants, lacking brains, “are free from these subjective restraints,” but they are able to respond to various stimuli to their advantage. For example: wound one tomato leaf and the other leaves know of a potential danger through electrical signals sent out by the damaged leaf.
In sum, plants do feel, although lacking a brain they do not feel emotions or pain. “Mechanical stimulation of a plant cell, like mechanical stimulation of a nerve, initiates a cellular change in ionic conditions that results in an electric signal. And just like in animals, this signal can propagate from cell to cell, and it involves the coordinated function of ion channels including potassium, calcium, calmodulin, and other plant components” [813].
14 Chamowitz, David. What a Plant Knows: a Field Guide to the Senses (Scientific American/Farrar, Straus and Giroux, chapter 3. Future references, in brackets [ ], refer to the Kindle location.
15Watch a demonstration of the leaves drooping at: