Kelp

"Kelp is the first plant to utilise a new kind of visual feature: Animation. Before, the only things that [were] animated were either particles or the lava/water and some other solid blocks. But we never animated a plant that used transparency. This was a new challenge for the style of Minecraft - as it has never been done before, we had no frame of reference to adapt the animation style to - so we had to find it."

- Jasper Boerstra

Kelp is an upcoming non-solid plant block that generates in all oceans. It has an item form that can be smelted to dried kelp.

Obtaining
Kelp can be mined with any tool or without a tool. Breaking part of a kelp stalk will destroy the entire stalk. Each block has an undetermined chance to drop a kelp item.

Natural generation
Kelp naturally generates in ocean biomes, near sea grass.

Farming
Kelp, when planted, will grow at an undetermined rate. It will only grow, however, if there is a water block above it.

Usage
Kelp can be placed underwater by hand, or anywhere by the use of commands such as. Placing it by hand will give it a random age value between 0 and 19.

Trivia

 * This, along with sea grass, is the only plant that can grow underwater.