In Oblivion, Nirnroots were a collectible - there was a finite number of them*, and they did not respawn.
I've been noticing, however, that when I pick Nirnroots in Skyrim, the roots of the plant remain behind. This got me wondering...
Do Nirnroots regrow in Skyrim?
What about the Crimson Nirnroots of Blackreach?
*discounting the respawning food-nirnroot in Frostcrag Spire