At this point, I’m thinking this series of posts should perhaps be “Botany Word of Whenever I Get Around to It.” Sigh …
Anyway, this is a fun one! Thigmotropism is a plant’s growth response to touch. If you have a vining plant with tendrils, you may have watched, fascinated, as the tendrils wrap around a support. Epidermal cells in the tendril (which, in some plants, can be ten times as sensitive to touch as human skin!) cause it to reach and latch on when it contacts a solid object.
The tendrils use differential growth to wrap themselves around objects such as another plant, trellis, or wall. The side of the tendril that is opposite to the side that is in contact with the object grows faster due to the production of the growth hormone auxin by the side that is closest to the object. This causes the side that is touching the object to compress at the same time the other side elongates. The tendril then curves towards the object in a positive response.
Typically, thigmotropism is a fairly slow response, but in some plants, it occurs quickly. This is called rapid contact coiling, which occurs due to turgor pressure. (Turgor pressure is the pressure exerted by water that pushes plant cell membranes against cell walls. It maintains the rigidity of the cell walls and helps support the plant.)
Roots also exhibit thigmotropism (in addition to gravitropism, which is a term for another day, perhaps!). Unlike tendrils, however, roots react in a negative way to encountering a solid object such as a stone in the soil – that is, they move away from it, rather than towards it. This makes sense, as roots want unencumbered access through the soil, to facilitate the uptake of nutrients and water.
And, on a side note, you may be thinking that sensitive plants (Mimosa pudica) also display thigmotropism, but their response to touch is actually non-directional; that is, they don’t move towards or away from a stimulus. Instead, they experience what is called a nastic movement.
Why is thigmotropism important? It allows some climbing plants the opportunity to reach for more sunlight … which means more efficient photosynthesis can occur.
Sweet peas are an example of a common plant that exhibits thigmotropism. (This cultivar is ‘America’.)