From the Super Mario Wiki
A Lava Lotus is a plant in Super Mario Bros. 3 that lives underwater. They are capable of creating and storing Fireballs under their transparent leaves and attack by opening up to release them. The fireballs can go through platforms and make navigating underwater difficult. The number they shoot out is random and can be up to five at a time. The fireballs travel rather slowly and in a straight line from their point of release. The Japanese name of the Lava Lotus, Suichūka, refers to a type of artificial flower that blossoms underwater, and even the American manual refers to them as artificial.
Lava Lotuses were given a land counterpart, the Volcano Lotus, in Super Mario World, which did not feature Lava Lotuses. They had a very similar attack pattern, however they were terrestrial as opposed to aquatic. These Volcano Lotuses have also made an appearance in Super Princess Peach, leaving Lava Lotuses with only one game appearance (not including Super Mario Bros. 3's remakes).
 Appearances in Other Media
A single Lava Lotus appeared briefly in The Adventures of Super Mario Bros. 3 episode "Oh, Brother!". This Lava Lotus unleashed its fireballs on Frog Luigi, who managed to swim past them with ease, mainly due to the Frog Suit that Princess Toadstool had advised him to wear.
 Notes and references