Nether Wart

Nether Wart is a plant grown in The Nether. It can be found within Nether Ruins on Soul Sand. Nether Wart in inventory can be placed onto Soul Sand by right-clicking (as if it were a seed), where it will grow.

Nether Wart could be used to make potions.

Growth & Growing Conditions
When left alone, Nether Wart planted in Soul Sand will grow through three distinct stages. Nether Wart does not react to bonemeal. It will pop out of the ground if the area is sufficiently dark. In the Beta 1.9 Pre-release, though the player can place Nether Wart on Soul Sand in the Overworld, it will not grow.

Harvesting
There are three visual stages of development. During the first two stages, it will yield one Nether Wart. On the final, largest stage, breaking Nether Wart will produce 2-5.