Acorn Plains

Acorn Plains is the first world of New Super Mario Bros. U. It is a plains world with acorn trees, shrubs, jagged mountains reminiscent of some of the backgrounds in Super Mario World and the world's most defining feature, the Acorn Tree. This special tree is one of main sources of the game's new power-up, the Super Acorn. It can be found in the game's intro, Acorn Plains' map, and said world's first level. This world has at least seven levels, two of which are the Fortress and Castle levels, a red and green Toad House, a Piranha Plant Enemy Course, and a spot where a magenta Baby Yoshi walks near. The Castle boss of this world is Lemmy Koopa.

Trivia

 * In early builds of New Super Mario Bros. U, the Acorn Plains map looks substantially different. The map was significantly less detailed, the Acorn Trees did not show the Super Acorns, the map had two red Toad Houses, and the green Toad House was in a different spot, there are two Enemy Courses instead of one, and the spot where the magenta Baby Yoshi was is gone.
 * There is a pipe at the beginning of the world. It could possibly be a way to get to later finished worlds easier, as Layer Cake Desert and Frosted Glacier has the same pipe at the beginning of the world.