Back in the heady (hazy?) days of the early 1970’s, a book was making the rounds on college campuses that suggested plants possessed a sort of sentience.
This book was The Secret Life of Plants.
The professor teaching my Introductory Botany class at the time loathed this book. He actually stated in class that any student he caught in possession of this book would receive a grade of “F”. (True story!)
Although this prof’s reaction was perhaps an extreme example, the story serves to illustrate the general attitude within the scientific community against any suggestion that plants possess a nervous system .
The notion that plants could feel pain, for instance, or move rapidly in response to stimuli was solely within the realm of fantasy and science fiction. (Please see here and here, for a couple of examples.)
There are, however, some plants that exhibit relatively rapid movements in response to external stimuli, the most famous of which is the Venus fly trap.
In this case, as well as in the Venus fly trap, it’s not so much that the plant is moving in response to mechanical stimulation, but that the touch is triggering a sort of spring-loaded mechanism. Think of an old-fashioned mouse trap – gently touch the triggering mechanism, and the trap snaps shut.
In these plants, however, it’s kind of a hydraulic spring-loading. That is, when some cells within a thickening at the base of the leaves called a pulvinus have a high turgor pressure, this causes the leaves to open. And if these cells lose turgor pressure, the leaves close.
But how does mechanical stimulation trigger the rapid loss of cell turgor pressure in these plants?
Plant cells, like animal cells, generate an electrical membrane potential across their cellular membranes.
Some of the energy in this membrane potential is used by cells to accumulate solutes such as sugars and mineral ions such as potassium. This accumulation of solutes draws water into the cells via osmosis.
This is how the pulvinus cells in the Venus fly trap and the touch-sensitive plants likely generate their turgor pressure to open the leaves.
Now the leaves are hydraulically “spring-loaded”, and ready….
Next time: How electrical signals “spring the trap”. Also, an introduction to the emerging field of Plant Neurobiology.
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