Three naturally-occurring three-block-tall cacti.
Instead of dropping as cube-shaped, Minecraft: Pocket Edition cactus appears flat, and will also follow a player as they move around it.
This cactus planted in water on sand while in Creative gives off light for some reason.
Cactus next to a cactus farm
A cactus when placed
Cactus Green; obtained by smelting cacti
An example of a cactus farm
A lot of cacti.
Cactus in the Flower Pot.