Rheum is a genus of about 60 herbaceous perennial plants in the family Polygonaceae. Species are native to eastern Europe, southern and eastern temperate Asia, with a few reaching into northern tropical Asia. Rheum is cultivated in Europe and North America. The genus includes the vegetable rhubarb. The species have large somewhat triangular shaped leaves with long, fleshy petioles. The flowers are small, greenish-white to rose-red, and grouped in large compound leafy inflorescences. A number of cultivars of rhubarb have been domesticated both as medicinal plants and for human consumption. While the leaves are toxic, the stalks are used in pies and other foods for their tart flavor.
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Rheum species are herbaceous perennials growing from fleshy roots. They have upright growing stems and mostly basal, deciduous leaves growing from short, thick rhizomes. They have persistent or deciduous ocrea. The inflorescences are terminal and panicle-like with pedicels. The hermaphrodite flowers consist of a whitish green to pinkish green, hairless and campanulate (bell-shaped) perianth, composed of six tepals. The outer three tepals are narrower than the inner three and all are sepal-like in appearance. The flowers have nine (sometimes six) stamina inserted on the torus at the base of the peranthium, they are free or subconnate at their base. The anthers are yellow or pinkish green, elliptic in shape. The ovary is simple and triangular shaped with three erect or deflexed styles. The stigmas are head-like. The fruits are a three-sided achene with winged sides, and the seeds are albuminous with a straight or curved embryo.
The genus Rheum was erected in 1753 by Carl Linnaeus, initially for three species: R. rhaponticum, R. rhabarbarum and R. ribes. Linnaeus did not explain the origin of the genus name. Rheum is usually derived from the Greek rheon, mentioned by Dioscorides as an alternative name for medicinal rhubarb; the word rheon is itself thought to be derived from the (old) Persian rewend. Dioscorides calls the plant rha, but mentions the Romans call it rha ponticum, and it was also called ria or rheon. It is theorised the Ancient Greek word rha was derived from an ancient Scythian name for the Volga River in Russia, Rā, near from where the plant was supposedly brought. (See Volga River § Nomenclature.)
Rheum is placed in the family Polygonaceae, subfamily Polygonoideae. Within the subfamily, it is in the tribe Rumiceae, along with the two genera Oxyria and Rumex. It is most closely related to Rumex.
The genus is represented by about 50–60 extant species. The many cultivars of culinary rhubarb more usually grown for eating are recognised as Rheum × hybridum in the Royal Horticultural Society's list of recognised plant names. The drug rheum is prepared from the rhizomes and roots of another species, R. officinale or medicinal rhubarb. This species is also native to Asia, as is the turkey rhubarb, R. palmatum. Another species, the Sikkim rhubarb, R. nobile, is limited to the Himalayas.
- Rheum acuminatum Hook.f. & Thomson
- Rheum alexandrae Batalin
- Rheum australe D.Don
- Rheum compactum L.
- Rheum cordatum Losinsk.
- Rheum coreanum Nakai
- Rheum darvasicum V.S.Titov ex Losinsk.
- Rheum delavayi Franch.
- Rheum fedtschenkoi Maxim. ex Regel
- Rheum forrestii Diels
- Rheum globulosum Gage
- Rheum hotaoense C.Y.Cheng & T.C.Kao
- Rheum × hybridum Murray
- Rheum inopinatum Prain
- Rheum khorasanicum Baradaran & Jafari
- Rheum kialense Franch.
- Rheum laciniatum Prain
- Rheum lhasaense A.J.Li & P.K.Hsiao
- Rheum likiangense Sam.
- Rheum lucidum Losinsk.
- Rheum macrocarpum Losinsk.
- Rheum maculatum C.Y.Cheng & T.C.Kao
- Rheum maximowiczii Losinsk.
- Rheum nanum Siev. ex Pall.
- Rheum neyshabourense Baradaran & Jafari
- Rheum nobile Hook.f. & Thomson
- Rheum officinale Baill.
- Rheum palaestinum Feinbrun
- Rheum palmatum L.
- Rheum persicum Losinsk.
- Rheum platylobum Rech.f.
- Rheum przewalskyi Losinsk.
- Rheum pumilum Maxim.
- Rheum racemiferum Maxim.
- Rheum rhabarbarum L.
- Rheum rhaponticum L.
- Rheum rhomboideum Losinsk.
- Rheum ribes L.
- Rheum spiciforme Royle
- Rheum subacaule Sam.
- Rheum sublanceolatum C.Y.Cheng & T.C.Kao
- Rheum × svetlanae Krassovsk.
- Rheum tanguticum (Maxim. ex Regel) Balf.
- Rheum tataricum L.f.
- Rheum tibeticum Maxim. ex Hook.f.
- Rheum turkestanicum Janisch.
- Rheum uninerve Maxim.
- Rheum webbianum Royle
- Rheum wittrockii C.E.Lundstr.
- Rheum yunnanense Sam.
Other species within this genus that have been recognized include:
Many rheum species have food and medicinal uses. Some of these uses originated in Asia more than 2,000 years ago. All parts of the plant contain slightly poisonous oxalic acid, but its concentration in the leaf stems or petioles used in food preparation is very low, and their tart flavor instead is caused by nontoxic malic acid. The plants also produce other compounds, including citric acid and anthraquinone glycosides, and the raw or cooked leaf blades are poisonous to humans and livestock if consumed in large enough amounts. Plants in cultivation are propagated by cutting up the crowns of larger plants and by seeds.
Some species are grown for their ornamental qualities, including R. acuminatum, R. alexandrae, R. australe, R. kialense, R. palmatum, R. rhabarbarum and R. ribes.
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- From Ancient Greek ῥῆον.
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- Rheum rhabarbarum in Flora of North America @. Efloras.org. Retrieved on 2015-05-02.
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