There are walking tree stumps called Stumpers here. Their sides are spiky, but you can walk on top of them. Climb up the ladder and cross on top of the Stumpers to a P-Switch. This flips a bridge and wakes up a Piranha Creeper. Cross the bridge and you'll find a plant with a key. Follow the Piranha Creeper down a ladder, then cross the bridge to reach the door. Unlock it to find the first gem. On that level, you'll find a lone plant that moves. Pick it for the second gem. Go up the ladder and the ramp near the Piranha Creeper to hit another P-Switch to make a bridge fall into place. Use the tall Stumpers to reach the third gem. Drop down from the bridge and walk to the Star.
Special Challenge: Defeat all the Piranha Creepers.