''Pistia'' is a genus of

aquatic plant Aquatic plants are plants that have adapted to living in aquatic environments (saltwater or freshwater). They are also referred to as hydrophytes or macrophytes to distinguish them from algae and other microphytes. A macrophyte is a plant that ...

s in the arum family, Araceae. It is the sole genus in the tribe ''Pistieae'' which reflects its systematic isolation within the family. The single species it comprises, ''Pistia stratiotes'', is often called water cabbage, water lettuce, Nile cabbage, or shellflower. Its native distribution is uncertain but is probably

pantropical A pantropical ("all tropics") distribution is one which covers Tropics, tropical regions of both hemispheres. Examples of species include caecilians, modern sirenians and the plant genera ''Acacia'' and ''Bacopa''. ''Neotropical'' is a zoogeogra ...

; it was first discovered from the

Nile The Nile, , Bohairic , lg, Kiira , Nobiin language, Nobiin: Áman Dawū is a major north-flowing river in northeastern Africa. It flows into the Mediterranean Sea. The Nile is the longest river in Africa and has historically been considered ...

near

Lake Victoria Lake Victoria is one of the African Great Lakes. With a surface area of approximately , Lake Victoria is Africa's largest lake by area, the world's largest tropical lake, and the world's second-largest fresh water lake by surface area after ...

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways and is considered an

invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...

as well as a mosquito breeding habitat. The genus name is derived from the Greek word πιστός (''pistos''), meaning "water," and refers to the aquatic nature of the plants. The specific epithet is also derived from a Greek word, στρατιώτης, meaning "soldier," which references the sword-shaped leaves of some plants in the ''

Stratiotes ''Stratiotes'' is a genus of submerged aquatic plant commonly known as water soldiers, described as a genus by Linnaeus in 1753. Several specific names have been coined within the genus, but at present only one is recognized: '' Stratiotes aloi ...

'' genus.

Description

''Pistia stratiotes'' is a

perennial A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

monocotyledon Monocotyledons (), commonly referred to as monocots, (Lilianae ''sensu'' Chase & Reveal) are grass and grass-like flowering plants (angiosperms), the seeds of which typically contain only one embryonic leaf, or cotyledon. They constitute one of ...

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

flower A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants (plants of the division Angiospermae). The biological function of a flower is to facilitate reproduction, usually by providing a mechani ...

s are

dioecious Dioecy (; ; adj. dioecious , ) is a characteristic of a species, meaning that it has distinct individual organisms (unisexual) that produce male or female gametes, either directly (in animals) or indirectly (in seed plants). Dioecious reproductio ...

, 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 Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the fu ...

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.

Invasion

Water lettuce is among the world's most productive freshwater aquatic plants and is considered an

. The species can be introduced to new areas by water dispersal, fragmentation, and hitchhiking on marine transportation or fishing equipment. The invasion of ''Pistia stratiotes'' in the ecosystem can lead to environmental and socio-economic ramifications to the community it serves. In waters with high nutrient content, particularly those that have been contaminated with human loading of sewage or fertilizers, water lettuce can exhibit weedy overgrowth. It may also become invasive in hydrologically altered systems such as flood control canals and reservoirs. The severe overgrowth of water lettuce can block gas exchange in the surface water, creating hypoxic conditions and eliminating or disrupting various native marine organisms. Blocking access to sunlight, large mats of water lettuce can shade native submerged plants and alter communities relying on these native plants as a source of food. The growth of these mats can also get tangled in boat propellers and create challenges for boaters or recreational fishermen. ''Pistia stratiotes'' acts as a vector for

malaria Malaria is a mosquito-borne infectious disease that affects humans and other animals. Malaria causes symptoms that typically include fever, tiredness, vomiting, and headaches. In severe cases, it can cause jaundice, seizures, coma, or death. S ...

and filariasis. Mosquitoes of the genus '' Mansonia'' can lay their eggs under the leaves of aquatic plants, such as ''Pistia stratiotes.'' Twenty-four hours later, the emerging larvae attach to the plant's roots using its siphon tube for respiration. Within a week, larvae can develop into adult mosquitos, making ''Pistia stratiotes'' a breeding ground for infectious diseases. The moth '' Samea multiplicalis'' also uses ''Pistia stratiotes'' as its primary host plant. Eggs are laid among leaves and stems of the host plant and larvae hatch and feed intensively as they develop.

Control

# Chemical Control: Herbicides have been effective in controlling ''Pistia stratiotes:'' diquat, glyphosate, terbutryn, 2,4-D, among many others. Yet, the use of herbicides must be critically assessed to prevent negative environmental impacts and possible toxic effects on marine life and human health. # Physical Control: ''Pistia stratiotes'' can be controlled with mechanical harvesters that remove the water lettuce from the infested waters and transport it to disposal onshore. Larger infestations can be removed with the aid of hydraulic excavators and tractors. To prevent the re-growth of ''Pistia stratiotes'' colonies, a long-term maintenance program should be implemented. # Biological Control: Two species of

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...

s are also being used as a biological control. Adults and larvae of the

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

weevil Weevils are beetles belonging to the Taxonomic rank, superfamily Curculionoidea, known for their elongated snouts. They are usually small, less than in length, and Herbivore, herbivorous. Approximately 97,000 species of weevils are known. They b ...

'' Neohydronomous affinis'' feed on ''Pistia'' leaves, as do the larvae of the moth ''

Spodoptera pectinicornis ''Spodoptera pectinicornis'' (waterlettuce moth) is a moth of the family Noctuidae native to Asia, where it can be found from the northeastern parts of the Himalaya to Sundaland and in New Guinea. It has been introduced as a biocontrol agent of ...

'' from

Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is bo ...

. Both are proving to be useful tools in the management of ''Pistia'' ''stratiotes'' through the experimental recovery of benthic communities with hypoxic conditions.

Native range

The center of origin of ''Pistia stratiotes'' remains uncertain. However, the plant is thought to be native to South America or Africa. Described in

Egyptian hieroglyphics Egyptian hieroglyphs (, ) were the formal writing system used in Ancient Egypt, used for writing the Egyptian language. Hieroglyphs combined logographic, syllabic and alphabetic elements, with some 1,000 distinct characters.There were about 1,00 ...

and reported by Greek botanists,

Dioscorides Pedanius Dioscorides ( grc-gre, Πεδάνιος Διοσκουρίδης, ; 40–90 AD), “the father of pharmacognosy”, was a Greek physician, pharmacologist, botanist, and author of ''De materia medica'' (, On Medical Material) —a 5-vol ...

and

Theophrastus Theophrastus (; grc-gre, Θεόφραστος ; c. 371c. 287 BC), a Greek philosopher and the successor to Aristotle in the Peripatetic school. He was a native of Eresos in Lesbos.Gavin Hardy and Laurence Totelin, ''Ancient Botany'', Routledge ...

, in the

River, the plant suggests an African origin. In addition, the co-evolution of ''Pistia stratiotes'' with various insects native to Brazil and Argentina, such as th
water lettuce weevil
proposes a South American origin. It is a common aquatic plant in the southeastern

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

, particularly in

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and to ...

. A more recent argument for nativity in Florida notes that it was recorded by botanist, William Bartram, in the St. Johns River basin in 1765, and fossil specimens dating back to the late

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

(~12,000 BP) and early

Holocene The Holocene ( ) is the current geological epoch. It began approximately 11,650 cal years Before Present (), after the Last Glacial Period, which concluded with the Holocene glacial retreat. The Holocene and the preceding Pleistocene togethe ...

(~3,500 BP) period are reported from Florida. Yet, the rationale of southeastern, North American nativity remains controversial as disagreeing botanists claim the Spanish may have indirectly introduced into the plant into Florida's basins from their ship ballast.

Fossil record

''Pistia''-like plants appear in the

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

record during the

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

epoch in rock

strata In geology and related fields, a stratum ( : strata) is a layer of rock or sediment characterized by certain lithologic properties or attributes that distinguish it from adjacent layers from which it is separated by visible surfaces known as ei ...

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 Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...

(late Campanian age). However, based on more complete specimens from the Campanian Dinosaur Park Formation of southern

Alberta Alberta ( ) is one of the thirteen provinces and territories of Canada. It is part of Western Canada and is one of the three prairie provinces. Alberta is bordered by British Columbia to the west, Saskatchewan to the east, the Northwest Ter ...

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, and other areas, they were redescribed as a separate

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

, †''Cobbania'', primarily due to differences in leaf morphology.Stockey, R.A., Rothwell, G.R. and Johnson, K.R. 2007. ''Cobbania corrugata'' gen. et comb. nov. (Araceae): A floating aquatic monocot from the Upper Cretaceous of western North America. American Journal of Botany, vol. 94, no. 4, p. 609-624.Stockey, R.A., Rothwell, G.R. and Johnson, K.R. 2016. Evaluating relationships among floating aquatic monocots: A new species of ''Cobbania'' (Araceae) from the Upper Maastrichtian of South Dakota. International Journal of Plant Sciences, vol. 177, no. 8, p. 706-725. Younger fossils attributed to ''Pistia'' ''stratiotes'' have described from

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

strata in the southeastern

, and 350

seeds A seed is an embryonic plant enclosed in a protective outer covering, along with a food reserve. The formation of the seed is a part of the process of reproduction in seed plants, the spermatophytes, including the gymnosperm and angiosperm pl ...

of †''Pistia sibirica'' have been described from middle Miocene strata of the Fasterholt area near Silkeborg in

Central Jutland The Central Denmark Region ( da, Region Midtjylland), or more directly translated as the Central Jutland Region and sometimes simply Mid Jutland, is an administrative region of Denmark established on 1 January 2007 as part of the 2007 Danish mu ...

Denmark ) , song = ( en, "King Christian stood by the lofty mast") , song_type = National and royal anthem , image_map = EU-Denmark.svg , map_caption = , subdivision_type = Sovereign state , subdivision_name = Danish Realm, Kingdom of Denmark ...

. Fossils of this species have also been described from the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...

and Miocene of

Western Siberia Western Siberia or West Siberia (russian: Западная Сибирь, Zapadnaya Sibir'; kk, Батыс Сібір) is a part of the larger region of Siberia that is mostly located in the Russian Federation. It lies between the Ural region an ...

and from the Miocene of

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

. A specimen of ''Pistia'' ''stratiotes'' from the

peninsula dating from at least 3,550 years

Before Present Before Present (BP) years, or "years before present", is a time scale used mainly in archaeology, geology and other scientific disciplines to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s. Becaus ...

, as well as a report of early Holocene ''Pistia''

pollen Pollen is a powdery substance produced by seed plants. It consists of pollen grains (highly reduced microgametophytes), which produce male gametes (sperm cells). Pollen grains have a hard coat made of sporopollenin that protects the gametophyt ...

from a lake in Florida, have cast doubt on the claim that ''Pistia'' ''stratiotes'' is not native to the southeastern United States.

Uses

Consumption

While considered edible, ''Pistia'' ''stratiotes'' is not palatable as it is rich in

calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate ...

crystals that are bitter in taste. Nevertheless, there are records of the plant being utilized as famine food in India during the Great Famine of 1867-1878. The Hausa people of Nigeria utilized the ash of the plant as a substitute for salt due to its high concentration of potassium chloride, a mineral salt. This salt substitute, also called zakankau'',' was of high importance, especially when imported salt was unavailable. Caution is advised when consuming ''Pistia stratiotes'', as the plant is a

hyperaccumulator A hyperaccumulator is a plant capable of growing in soil or water with very high concentrations of metals, absorbing these metals through their roots, and concentrating extremely high levels of metals in their tissues. The metals are concentrated a ...

, and can absorb and accumulate toxic heavy metals present in its environment. The presence of high concentrations of

crystals can induce various health concerns such as, inhibited mineral absorption and kidney stones. In Singapore and Southern China, ''Pistia'' ''stratiotes'' is commonly grown or collected as animal feed for ducks and pigs. Water lettuce is also considered an alternative for poultry feed in Indonesia due to its high content of crude protein.

Medical treatment

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.

Medicinal properties

* Anti-Inflammatory Properties: Extractions of the leaves of ''P. stratiotes'' reduces mast infiltration and degranulation in allergic reactions and presents anti-inflammatory properties. The ethanolic extracts have also been positively correlated with a reduction in inflammatory disorders such as, arthritis and fevers. *Antifungal Properties: With the popular use of ''Pistia'' ''stratiotes'' as a traditional treatment for ringworms, researchers have tested ''P. stratiotes'' methanolic extracts on dermatophyte fungi. The results of the studies depicted significant fungicidal activity on

T. rubrum ''Trichophyton rubrum'' is a dermatophytic fungus in the phylum Ascomycota. It is an exclusively clonal, anthropophilic saprotroph that colonizes the upper layers of dead skin, and is the most common cause of athlete's foot, fungal infection of ...

, T. mentagrophytes, and E. floccosum.

Environmental remediation

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 Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environme ...

plants. The roots and leaves of the plant have been found to absorb excess nutrients and heavy metals, such as

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hardne ...

, and

cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

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 Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

and

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...

nitrogen.

References

"Biogeography of the Pistia clade (Araceae): Based on chloroplast and mitochondrial DNA sequences and Bayesian divergence time inference"

External links

*
''Pistia stratiotes'' information
from th
Hawaiian Ecosystems at Risk project (HEAR)Species Profile- Water Lettuce (''Pistia stratiotes'')
National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Water Lettuce. {{DEFAULTSORT:pistia / water lettuce Aroideae Monotypic Araceae genera Pantropical flora Aquatic plants Phytoremediation plants Invasive plant species in Sri Lanka