Back in 2004 when I took a short tour that included Scotland, our tour guide told us a neat story about why the thistle is one of the symbols of Scotland. I was too busy looking out the window and admiring the scenery to pay much attention, but I think the gist of it involved night-time invaders getting poked in the foot and screaming with pain, thereby revealing their presence.
The moral of the story? Wear thick boots when invading countries that have lots of thistles. Also, pay more attention when the tour guide is talking.