First things first, we were extremely lucky in the storm. A few branches came down, but otherwise the Museum and the garden made it through unharmed. Some of our museum staff and many of our visitors weren’t so lucky. Please donate or volunteer if you can.
When it’s your job to convince kids that plants are cool, Mimosa pudica is your secret weapon. This unassuming little guy responds dramatically to the slightest touch. It folds up at night, and you can blow it out like a birthday candle.
How does it work? The simple answer is that water rushes into the cells on the top of the stem and water flows out of the cells on the underside of the stem. The extra water pressure on top pushes the stem downward, and the wilted cells on the bottom aren’t strong enough to resist.*
That’s pretty complicated for a third grader to wrap his/her mind around, so when I teach a Gardening with Greta program about these dancing plants on Monday November 12th, I think I’ll use a water balloon as a prop, squeezing the bottom to show how pressure on top would force the stem downward, then letting it equalize to show how the plant rights itself again.
The mystery is why Mimosa pudica acts the way it does. Reaction to insects? Herbivores? I’ll collect hypotheses from the kids next week.
These plants are totally showstoppers, but they’re not perfect as classroom plants. The tough seed coat makes germination a bit unreliable, so some kids will get sprouts before others. Mine often get whiteflies, which can be controlled by having a fan blow on them all the time, but then the leaflets stay folded up, which defeats the purpose. In my experience, plants that are touched constantly won’t survive more than a few weeks. I’m going to try this year to stagger plantings every few weeks so I always have reinforcements.
* The more complicated answer is that when the plant is stimulated, electrical signals zap tiny gates in the membranes of pulvini cells (cells at the base of a plant stem), opening some gates so that potassium ions flow in, closing others. Potassium ions are normally surrounded by water, and when they squeeze through the tiny gate in the cell membrane, they leave those water molecules outside of the cell. Water wants to be equal across a membrane, so water rushes into the cell through osmosis, making that cell swell with the added pressure. The reverse process happens on the underside of the stem, and all the water inside those cells drains out. When the top side of the stem has swollen and the underside has wilted, the stem swings downward.
At least that’s as far as I understand it. Biochemists, correct me if I’m wrong!