Pistia

''Pistia'' is a genus of aquatic plants 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; it was first scientifically described from plants found on the Nile near Lake Victoria in Africa. It is now present, either naturally or through human introduction, in nearly all tropical and subtropical fresh waterways and is considered an invasive species as well as a mosquito breeding habitat. The specific epithet is derived from a Greek word, στρατιώτης, meaning "soldier", which references the sword-shaped leaves of some plants in the '' Stratiotes'' genus.

Description

''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.

Invasion

Water lettuce is among the world's most productive freshwater aquatic plants and is considered an invasive species. 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'' feature in the life cycles of certain insect vectors for malaria 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 potential breeding ground for vectors of infectious disease. 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 insects are also being used as a biological control. Adults and larvae of the South American weevil '' Neohydronomous affinis'' feed on ''Pistia'' leaves, as do the larvae of the moth '' Spodoptera pectinicornis'' from Thailand. Both are proving to be useful tools in the management of ''Pistia'' ''stratiotes'' through the experimental recovery of benthic communities with hypoxic conditions.
The species is set to be banned in the EU from August of 2024 to prevent spreading.

Range

The center of origin of ''Pistia stratiotes'' has long been a source of debate. Nativity to northern Africa is indicated by Egyptian hieroglyphics and reports of plants meeting the description of ''Pistia'' by Greek botanists, Dioscorides and Theophrastus in the Nile River. In addition, the co-evolution of ''Pistia stratiotes'' with various insects native to Brazil and Argentina, such as the water lettuce weevil, indicates a long-term native tenure in South America. Fossil specimens dating back to the late Pleistocene (~12,000 BP) and early Holocene (~3,500 BP) period are reported from Florida, indicating a native presence in the southeastern United States. Recent genetic evidence also suggests that ''Pistia'' is not actually a monotypic genus, as had been long assumed. Instead, ''Pistia'' appears to be composed of at least three genetically distinct, but morphologically and ecologically similar, species at a global scale.

Temperate occurrences

Though ''Pistia stratiotes'' is intolerant of cold temperatures, it has been recorded growing at least temporarily in temperate areas of North America and Europe. In the United States north of the Gulf of Mexico it has been found growing in Colorado, Connecticut, Delaware, Illinois, Kansas, Maryland,
Michigan, Minnesota, Missouri, New York, North Carolina, Ohio, Rhode Island, South Carolina,
and Wisconsin. One of these occurrences, in Idaho, survives in an area of a river fed by a hot spring. The rest are thought either to be completely eradicated by cold weather or possibly to survive by seed production.

Fossil record

''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 (late Campanian age). 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

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