Glyceria fluitans (syns Festuca fluitans, Poa fluitans, Panicularia fluitans), known as floating sweet-grass^[1] and water mannagrass, is a species of perennial grass in the genus Glyceria native to Europe, the Mediterranean region and Western Asia and occurring in wet areas such as ditches, riverbanks and ponds.

Quick Facts Glyceria fluitans, Scientific classification ...

Glyceria fluitans
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Monocots
Clade:	Commelinids
Order:	Poales
Family:	Poaceae
Subfamily:	Pooideae
Genus:	Glyceria
Species:	G. fluitans
Binomial name
Glyceria fluitans (L.) R.Br.

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It has a creeping rootstock, a thick stem which rises to one metre. The leaves are long, narrow and pale green, rough on both sides, often folded at the keel which lies on the surface of the water.

The species epithet fluitans is Latin for "floating".^[2]

Before the 19th century, the grains were widely harvested in Central Europe and Sweden, and traded as far as England.^[3] It was cooked and eaten as gruel.^[3]

When a shoot is submerged in floodwater there is a strong reduction of diffusion of gases which limits oxygen and carbon dioxide availability.^[4] To handle the poor gas exchange while submerged, Glyceria fluitans forms a gas film around the leaves.^[5] The gas film allows to increase the gas exchange since the diffusion of gas within the film is rapid.^[6] Glyceria fluitans has two kind of leaves: floating leaves and aerial leaves. Floating leaves form a gas film only on the adaxial side, instead aerial leaves form it on both sides.^[5] The formation of a gas film is caused from the superhydrophobicity of the leaves, which is provoked by the special structure of the leaves. Indeed, they have a plicate shape with ridges and grooves, on the microscale they have convex papillose epidermal cells forming papillae and three-dimensional epicuticular waxes.^[5] A gas film vanishes typically after 2–6 days of submergence because the leaves become hydrophilic.^[7] Regarding the beneficial trait of the gas film development, it can be said that Glyceria fluitans through it enhances the gas exchange and therefore the photosynthesis under flood conditions, but it is seen as a short term strategie.^[5]

Glyceria fluitans is a component of purple moor grass and rush pastures, a type of Biodiversity Action Plan habitat in the UK. This habitat occurs on poorly drained neutral and acidic soils of the lowlands and upland fringe. It is found in the South West of England, especially in Devon.

At the Great Fen, researchers are running trials with Glyceria fluitans in order to assess its physical and financial viability as a crop in wet farming systems.^[8] Glyceria fluitans was chosen because it has been consumed by humans for a long time and because of its tolerance to a wide range of growing conditions.^[9] This tolerance originates from the fact that Glyceria fluitans is to some extent self-sterile and therefore outbreeding, which allows for adaptation.^[10]

The use and cultivation of Glyceria fluitans disappeared little by little during the 19th and 20th century due to its long gathering time and the disappearance of wetlands.^[11] For these reasons, known farming techniques are old and not very applicable to today's production systems anymore.

Glyceria fluitans grains were popular in Poland, Germany, Hungary and other European countries for their sweet taste and nutrient richness. In Poland, foods containing Glyceria fluitans were pointed out as specialities of the local cuisine. Originally, Glyceria fluitans was used to prepare dishes as sweet cakes, flatbreads, dumplings, with butter or boiled milk, broths and soups, with oriental spices, as well as in sausages as binding additive.^[11]

Due to its healthy properties, between the 17th and 18th century, its consume was advised by pharmacopoeias to people with debilitating diseases as well as malnourished young people.^{[13][14][15][16]}

• Glyceria fluitans (1)
  Illustration in:
  Jakob Sturm: "Deutschlands Flora in Abbildungen",
  Stuttgart (1796)