Here’s a nifty, fancy, sciencey word for you all: Thigmonasty. This is when plants respond to touch, like the Venus flytrap or the so-called “sensitive plant” (Mimosa pudica). Cactus flowers in the Opuntia group, meaning the prickly pears and chollas, sometimes also exhibit thigmonasty in their stamens, which bend and curve inwards when touched by an insect or a human finger. This rapid touch response is possibly an adaptation to dab pollen onto the visiting insect that is scrambling around in the flower, so that when it flies away it will transfer that pollen to the next flower and help the plants cross-breed. I took a series of 5 photos showing this interesting floral activity in the span of about 2 seconds. Enlarge them to see a better view of a dynamic coastal cholla flower (Cylindropuntia prolifera) in action.

It might be a bit hard to see unless one scrolls through the entire series in sequence like a video, but the stamens at the top of  the flower in the last photo are bent significantly inwards from the first photo. That is where I touched them. If I had touched more, those would have bent inwards too. This motion happened in only about 1.5 to 2 seconds.

In researching this post I discovered that there are two separate types of touch-activated plant motion: Thigmonasty and thigmotropy. The difference is that thigmonasty has the same motion regardless of the direction of the tactile stimulus – i.e. the Venus flytrap leaf always folds inwards and closes regardless of whether the insect was approaching from the left, right, above, or below. Thigmotropy is sensitive to the direction of the stimulus – i.e. a grapevine tendril moves and coils in the direction of the twig that is touching it, and wraps around the twig to help support the plant. Since the vine tendril needs to be sensitive to the direction of stimulus to be effective while the Venus flytrap leaf doesn’t, this is why a distinction was made. A final note is that the root word “thigma” is Greek for “touch”.
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Note: This post was originally made on Facebook on May 4, 2015. It’s worth having online for others to see, so I’m converting it to a blog post as well.