The Plunger Flower is Mario's special item in Super Mario Nation. He uses it in Dark-Lit Cave for the first time, where it turns him into Plunger Mario, where he gets the ability to climb up steep cliffsides. He will only loose this when he hits an enemy.
In Super Mario Nation, only Mario can get the Plunger Flower, from first hitting a Power Block. In Skyhigh Hills, Mario is the only one that can beat a few missions and in Dark-Lit Cave, Mario must use the Power Block to get a Red Coin, up on the wall. Since the Power Block appears in every area, Mario can use the Plunger Flower just about everywhere.
In New Super Mario 3D, it grants Mario the ability to climb up steep hills/walls.