Sensitive Plant: Am I Bothering You?

A Sri Lankan byway. Alongside the curb is a small roadside weed, very easy to miss. It has little lacy leaves and a small purple pom-pom of a flower. Pretty but not remarkable.

But it’s one of the most incredible plants I’ve ever encountered.

It moves.

Sensitive plant, it’s called. Mimosa pudica. And it’s sensitive, all right. Touch it with a fingertip, and the leaves close up, the tiny leaflets clutching themselves together nervously. Poke it again, and the whole leaf swings down, moving away from your annoying persistence.

As a general rule, plants don’t seem to pay much attention to people. You can touch them, pick their fruit, even cut them down, and they don’t squeak, wince, or shed a tear.

But this little plant is aware of you. 

This plant is native to South America, but like so many others that manage to survive curbside, it’s a hardy traveller that has invaded many continents. It’s classed as a minor agricultural weed, but is not hugely invasive. It mostly inveigles itself into lawns and pokes up through cracks in the sidewalk.

How on earth does it do that moving thing? Plants move, for sure, but generally too slow for the human eye to see. But this one moves fast. Check out the Wikipedia article on sensitive plants for a real-time video of how promptly the plant closes up shop. It’s as graceful as a dancer. http://en.wikipedia.org/wiki/Mimosa_pudica

It’s not really understood exactly how the process works. Apparently the plant responds to touch by lessening the water pressure in the leaves–instantly pumping water molecules through the cells. This lessening of pressure lets the leaves furl and the stem drop away from danger. Why does the plant go to all this trouble? Perhaps it’s an adaptation to keep bugs from eating the leaves. Having my food move as I took a bite would certainly discourage me from eating it.

I really get a kick out of this plant. Even if it doesn’t seem to be equally thrilled by me. The frustrating thing about plants is that no matter how much love you lavish on them, they’re never going to jump up and lick your face. But this species at least acknowledges your presence–even if it seems to give an exasperated sigh.