Touch is the sense of direct environmental contact. Plants are very much in touch with their habitat.
Roots can distinguish between wet and dry soil. They pattern their growth based upon the minute difference in moisture on one side of a root filament versus the other.
Plants have palpation perception, whether by the breeze or more substantial contact. The plant sense of touch extends to recognizing the saliva of herbivores that prey upon them. Touch is at its most developed in climbing plants, which possess an extraordinary tactile sense.
Unlike animals, plants are unable to run away. Instead, plants developed intricate systems to sense their environment and respond appropriately. Reactions can be triggered by rain drops falling, the wind blowing, an insect moving across a leaf, or even by clouds casting a shadow over a plant. Plants are very sensitive, and can redirect gene expression, defense, and their metabolism because of it. ~ Australian botanist Olivier Van Aken
Touch is ultimately an electrical sense. Established ion channels are energized by movement of fluid within and among cells.
The Venus flytrap literally has hair-trigger response to tactile stimulation. Charles Darwin showed that a Venus flytrap can be anaesthetized, just like animals. A flytrap gets back in action when the effect wears off. Ether, chloroform, or morphine may render a flytrap senseless.
Vines feel when they have latched onto something and initiate rapid growth when attached to reliable support. A bur cucumber can feel a string weighing 0.00025 mg, whereupon it sets itself to wrapping and growing. In contrast, it takes 8 times as much pressure for a human finger to sense a string.
Trees growing on a mountain ridge, exposed to high winds, adapt by limiting branch growth and growing short, thick trunks. In contrast, the same species of tree in an idyllic valley will be tall, thin, and have fulsome branches.
Heliconia tortuosa is a tropical plant that allows only 2 species of hummingbirds to pollinate its flowers. The plant knows who is guzzling its nectar by the shape of the bird bill put into its flower.
Plants generally don’t like to be touched. Simply touching or shaking a plant can lead to growth arrest.
A researcher studied cocklebur, a North American weed with small burrs that readily cling to passersby. This was done by measuring leaf length. The researcher found that the leaves measured never reached normal length. Instead, they turned yellow and died, just from being touched a few seconds each day.
Many plants are hardier about being touched than the cocklebur. Typically, those that are often touched keep their defenses up by producing defensive metabolites.