Pistia stratiotes, (water lettuce, water cabbage, shell flower) might well be termed a living fossil. The genus Pistia is monotypic, meaning there is only one species in the genus. Water lettuce is known from paleobotanical records from the late Cretaceous, Paleocene, Oligocene and Miocene eras from Europe, Western Siberia, Eastern Asia and North America. It was known to the ancient Egyptians and was first described in modern times by Linnaeus in 1753.
It is found globally but its spread is checked by severe cold. It can regrow leaves damaged by a moderate frost however. Allowing it to dry totally or incinerating it is the preferred method of killing it as it has been know to survive composting.
In the US, it is found in the Southern tier of states up to the level of about New Jersey on the East Coast. It is in dispute whether this plant is native to the US or not. If not a native, it is believed to have been introduced from ballast water dropped by early explorers on sailing ships.
This plant is the only free swimming member of the Aracaea. It doesn't shoot or form tubers, but can reproduce vegetatively by means of stolons, or from seeds. The flowers are tiny and often overlooked. In fact for many many years, it was assumed that the plant did not flower because the flowers are hard to see. The inflorescence produces a single female flower and between 3-9 male flowers.
Water lettuce is a threat to natural waterways and is on the list of noxious weeds in most Southern states including Florida, Texas, Louisiana and others. It is illegal to sell or possess in some states. It can be legally sold in colder states, but is prohibited from being shipped into states that outlaw it. Fines can be significant. The plants are densely mat forming, and will block out all sunlight from emersed plants growing under the mats. Biological diversity is decreased in infested areas because it alters the habitat by blocking animals from obtaining water to drink or fish in and blocking them from access to the plants that they depend on for food, shelter and nesting. The long trailing roots are termed 'water purifiers' by pond enthusiasts because they suck up all the nutrients needed for algae to grow and prosper; at the same time, these nutrients are also denied to other plants.
Programs for removing and destroying water lettuce from natural waterways have been in place for a long time in Florida, and currently the weed is considered to be under maintenance control.