Water Lettuce

Appearance

"Pistia stratiotes" is a perennial monocotyledon with thick, soft leaves that form a rosette. It floats on the surface of the water, its roots hanging submersed beneath floating leaves. The leaves can measure 2 – 15 cm long and are light green, with parallel venations and wavy margins. The surface of the leaves is covered in short, white hairs which form basket-like structures that can trap air bubbles and increase the plant's buoyancy. The spongy parenchyma with large intercellular spaces in the leaves also aids the plant in floating. The flowers are dioecious, lack petals, and are hidden in the middle of the plant amongst the leaves. "Pistia stratiotes" has a spadix inflorescence, containing one pistillate flower with one ovary and 2 - 8 staminate flowers with two stamens. The pistillate and carpellate flowers are separated by folds in the spathe, where the male flowers are located above the female flowers. Oval, green berries with ovoid seeds form after successful fertilization. The plant undergoes asexual reproduction by propagating through stolons, yet evidence of sexual reproduction has also been observed in the ponds of Southern Brazil.

"Pistia stratiotes" are found in slow-moving rivers, lakes, and ponds. The species displays optimal growth in the temperature range of  22 - 30 °C, but can endure extreme temperatures up to 35 °C. As a result, "Pistia" "stratiotes" do not grow in colder temperatures, beyond the tropics of Cancer and Capricorn. The species also require slightly acidic water in the pH range of 6.5 - 7.2 for optimal growth.

Habitat

The high sorption property of water lettuce makes it a great candidate for biodegradable oil sorbents in marine oil spills. Particularly, the leaves of "Pistia" "stratiotes" can efficiently absorb significant amounts of hydrocarbons due to its large surface area and hydrophobicity.

As a hyper-accumulator, "Pistia" "stratiotes" has been studied as a potential candidate for wastewater treatment plans. The roots and leaves of the plant have been found to absorb excess nutrients and heavy metals, such as zinc, chromium, and cadmium in contaminated waters.

"Pistia" "stratiotes" can be grown in water gardens to reduce harmful algal blooms and eutrophic conditions. The plant is able to control the growth of algae by restricting light penetration in the water column and competing for nutrients, with significant uptake of phosphorus and ammonia nitrogen.

Evolution

"Pistia"-like plants appear in the fossil record during the Late Cretaceous epoch in rock strata from the western interior of North America. They were first described as †"Pistia corrugata" by Leo Lesquereux in 1876 based on specimens from the Almond Formation of Wyoming. However, based on more complete specimens from the Campanian Dinosaur Park Formation of southern Alberta, Canada, and other areas, they were redescribed as a separate genus, †"Cobbania", primarily due to differences in leaf morphology. Younger fossils attributed to "Pistia" "stratiotes" have described from Eocene strata in the southeastern United States, and 350 fossil seeds of †"Pistia sibirica" have been described from middle Miocene strata of the Fasterholt area near Silkeborg in Central Jutland, Denmark. Fossils of this species have also been described from the Oligocene and Miocene of Western Siberia and from the Miocene of Germany.

A specimen of "Pistia" "stratiotes" from the Florida peninsula dating from at least 3,550 years Before Present, as well as a report of early Holocene "Pistia" pollen from a lake in Florida, have cast doubt on the claim that "Pistia" "stratiotes" is not native to the southeastern United States.

Uses

There are various medical uses of "Pistia" "stratiotes" throughout regions in Asia and Africa. In Nigeria, the dried leaves are prepared into a powder form and are applied to wounds and sores for disinfection. A similar use is present in Indian traditional medicine, where the powdered leaf is applied to syphilitic eruptions and skin infections. In Nigeria and Gambia, the leaf is infused in water to create an eyewash to treat allergic conjunctivitis. The eyewash is known to have a cooling and analgesic effect. Therefore, the plant is commonly called 'eye-pity' in Africa. In addition, the leaves of "Pistia" "stratiotes" can be burned into ash, and in Indian and Nigerian traditional medicine, the ash is used in treating ringworm infections of the scalp.