Minecraft wiki entry for Development_versions, associated text: A new type of sapling.
Can be grown by bonemealing the mangrove leaves, grows from the exposed bottom of it.
Propagules can also grow spontaneously from leaves, but only if they're naturally generated (not player placed leaves).
Have 4 stages to fully grown, can be accelerated by bonemealing.
Can break off a fully grown one, plant it to growing a mangrove tree (currently not added, will generate a oak tree instead).
Can be planted underwater.
Can be placed in flower pots.
Bees can pollinate them.
Bees follow players holding them and can be bred with them.