We'd been pouring water into the tray holding the planting pots because it's designed to let the soil soak up water as it needs it, but for some reason, we decided we were going to wet Plucky's pot with a spray bottle last night--and lo and behold, he was tickled!
It was one of the coolest things I've ever witnessed, and I think Silkworm would say the same. It took three or four sprays before we noticed a reaction, but suddenly, in the midst of the mist, all of Plucky's ferny leaves folded into each other, and even his stem bent over! We waited for him to perk back up, but since it was so late at night, we ended up falling asleep before he unfolded himself. When we went to check on him this morning, he was still all folded up, so we asked Google if sensitive plants fold up at night, too, and thank goodness they do because we were a bit worried--we were afraid we'd killed him! We wanted to film the ticklish reaction, but we waited until this afternoon to poke at him again so we weren't bothering him too much. We're enchanted. I'm so proud of our little plant.
We've also discovered, when researching a bit more about sensitive plants on the Internet, that some sensitive plant seeds can take up to 90 days to germinate! Thus, we've changed our plans accordingly and we're going to keep taking care of the seeds still hiding under the soil along with Plucky. Maybe we'll have some luck later on.