Read more: “Rooted in experience: The sensory world of plants“
Plants live in a very tactile world. Branches sway in the wind, insects crawl across leaves, and vines search out supports to hang on to. Plants are even sensitive to hot and cold, allowing them to respond to the weather by doing things like changing their growth rates and modulating their use of water. Simply touching or shaking a plant is often enough to reduce its growth, which is why vegetation in windswept locations tends to be stunted.
All plants can sense mechanical forces to some degree, but tactile sensitivity is most obvious in the carnivorous Venus flytrap. When a fly, beetle or even a small frog crawls across its specially adapted leaves, these spring together with surprising force, sandwiching the unsuspecting prey and blocking its escape. The Venus flytrap (pictured) knows when to shut because it feels its prey touching large hairs on the two lobes of the trap. But it won’t just snap shut with any stimulation – at least two hair touches must occur within about 20 seconds of each other. This helps to ensure that the prey is the ideal size and will not be able to wiggle out of the trap once it closes.
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The mechanism by which the Venus flytrap feels its prey is uncannily similar to the way you feel a fly crawling on your arm. Touch receptors in your skin sense the insect and activate an electrical current that passes along nerves until it reaches your brain, which registers the fly’s presence and instigates a response. Likewise, when a fly rubs up against the Venus flytrap’s hairs, it induces a current that radiates throughout the leaves. This activates ion channels in the cell membrane and the trap springs shut, all in less than one-tenth of a second.
Although most plants do not react this fast, they feel a mechanical stimulus in the same way. What’s really fascinating is that even at the level of individual cells, plants and animals use similar proteins to feel things. These mechanoreceptors are embedded in the cell membranes and, when stimulated by mechanical pressure or distortion, they allow charged ions to cross the membrane. This creates a difference in electrical charge between the inside and the outside of the cell, which generates a current. Unlike us, plants lack a brain to translate these signals into sensations with emotional connotations. Nevertheless, their sensitivity to touch allows them to respond to their changing environments in specific and appropriate ways.
