Rhamnus (plant)

thumb|right|[[Rhamnus pumila, dwarf buckthorn]]

Rhamnus is a genus of about 140 accepted species of shrubs or small trees, commonly known as buckthorns, in the family Rhamnaceae. Its species range from tall (rarely to ) and are native mainly in east Asia and North America, but found throughout the temperate and subtropical Northern Hemisphere, and also more locally in the subtropical Southern Hemisphere in parts of Africa and South America.

Both deciduous and evergreen species occur. The leaves are simple, long, and arranged alternately, in opposite pairs, or almost paired (subopposite). One distinctive character of many buckthorns is the way the veination curves upward towards the tip of the leaf. The plant bears fruits which are black or red berry-like drupes. The name is due to the woody spine on the end of each twig in many species. One species is known to have potential to be used medicinally. with deciduous or rarely evergreen foliage. Branches are unarmed or end in a woody spine. The leaf blades are undivided and pinnately veined. Leaf margins are serrate or rarely entire. Rhamnus species are generally dioecious, with male and female flowers on separate plants. Most species have yellowish green, small, unisexual or rarely polygamous flowers; which are produced singly or in axillary cymes, cymose racemes, or cymose panicles containing a few flowers. Calyx tube campanulate to cup-shaped, with 4 or 5 ovate-triangular sepals, which are adaxially ± distinctly keeled. Petals 4 or 5 but a few species may lack petals. The petals are shorter than the sepals. Flowers have 4 or 5 stamens which are surrounded by and equal in length the petals or are shorter. The anthers are dorsifixed. The superior ovary is free, rounded, with 2-4 chambers. Fruits are a 2-4 stoned, berrylike drupe, which is obovoid-globose or globose shaped. Seeds are obovoid or oblong-obovoid shaped, unfurrowed or abaxially or laterally margined with a long, narrow, furrow. The seeds have fleshy endosperm.

Species

, Plants of the World Online accepted the following species:

Rhamnus alaternus L. – Italian buckthorn
Rhamnus alnifolia L'Hér. – alderleaf buckthorn, alder-leaved buckthorn
Rhamnus alpina L.
Rhamnus arguta Maxim. – sharp-tooth buckthorn
Rhamnus arnottiana Gardner ex Thwaites
Rhamnus aurea Heppeler
Rhamnus baldschuanica Grubov
Rhamnus × bermejoi P.Fraga & Rosselló
Rhamnus biglandulosa Sessé & Moc.
Rhamnus bodinieri H.Lév.
Rhamnus brachypoda C.Y.Wu
Rhamnus bungeana J.J.Vassil.
Rhamnus calderoniae R.Fern.
Rhamnus cathartica L. – common buckthorn, purging buckthorn (orth. var. R. catharticus)
Rhamnus collettii Bhandari & Bhansali
Rhamnus cordata Medw.
Rhamnus coriophylla Hand.-Mazz.
Rhamnus cornifolia Boiss. & Hohen.
Rhamnus costata Maxim.
Rhamnus crenulata Aiton
Rhamnus crocea Nutt. – redberry buckthorn (subsp. crocea), hollyleaf buckthorn (subsp. pilosa)
Rhamnus crocea subsp. ilicifolia (Kellogg) C.B.Wolf – synonym Rhamnus ilicifolia
Rhamnus dalianensis S.Y.Li & Z.H.Ning
Rhamnus daliensis G.S.Fan & L.L.Deng
Rhamnus darii Govaerts
Rhamnus davurica Pall. – Dahurian buckthorn
Rhamnus depressa Grubov
Rhamnus diffusa Clos
Rhamnus disperma Ehrenb. ex Boiss.
Rhamnus dolichophylla Gontsch.
Rhamnus dumetorum C.K.Schneid.
Rhamnus erythroxyloides Hoffmanns.
Rhamnus erythroxylum Pall.
Rhamnus esquirolii H.Lév.
Rhamnus fallax Boiss.
Rhamnus flavescens Y.L.Chen & P.K.Chou
Rhamnus formosana Matsum.
Rhamnus fulvotincta F.P.Metcalf
Rhamnus × gayeri Kárpáti ex Soó
Rhamnus gilgiana Heppeler
Rhamnus glandulosa Aiton
Rhamnus glaucophylla Sommier
Rhamnus globosa Bunge – Lokao buckthorn
Rhamnus grandiflora C.Y.Wu
Rhamnus grubovii I.M.Turner
Rhamnus hainanensis Merr. & Chun
Rhamnus hemsleyana C.K.Schneid.
Rhamnus heterophylla Oliv.
Rhamnus hirtella Boiss.
Rhamnus hupehensis C.K.Schneid.
Rhamnus × hybrida L'Hér.
Rhamnus imeretina J.R.Booth ex G.Kirchn.
Rhamnus infectoria L.
Rhamnus integrifolia DC. –
Rhamnus × intermedia Steud. & Hochst.
Rhamnus ishidae Miyabe & Kudô
Rhamnus iteinophylla C.K.Schneid.
Rhamnus japonica Maxim. – Japanese buckthorn
Rhamnus kanagusukii Makino
Rhamnus kayacikii Yalt. & P.H.Davis
Rhamnus kurdica Boiss. & Hohen.
Rhamnus kwangsiensis Y.L.Chen & P.K.Chou
Rhamnus lamprophylla C.K.Schneid.
Rhamnus lanceolata Pursh – lanceleaf buckthorn
Rhamnus laoshanensis D.K.Zang
Rhamnus ledermannii Lauterb.
Rhamnus × lemaniana Briq.
Rhamnus leptacantha C.K.Schneid.
Rhamnus leptophylla C.K.Schneid.
Rhamnus libanotica Boiss. – Lebanese buckthorn
Rhamnus liboensis Y.F.Deng
Rhamnus liukiuensis (E.H.Wilson) Koidz.
Rhamnus lojaconoi Raimondo
Rhamnus ludovici-salvatoris Chodat – endemic to Mallorca
Rhamnus lycioides L. – black hawthorn, European buckthorn, or Mediterranean buckthorn
Rhamnus maximovicziana J.J.Vassil.
Rhamnus × mehreganii Alijanpoor & Khodayari
Rhamnus × mercieri Briq.
Rhamnus microcarpa Boiss.
Rhamnus mildbraedii Engl.
Rhamnus minnanensis K.M.Li
Rhamnus mollis Merr.
Rhamnus mongolica Y.Z.Zhao & L.Q.Zhao
Rhamnus × mulleyana Fritsch
Rhamnus myrtifolia Willk.
Rhamnus nakaharae (Hayata) Hayata
Rhamnus napalensis (Wall.) M.A.Lawson
Rhamnus nigrescens Lauterb.
Rhamnus nigricans Hand.-Mazz.
Rhamnus ninglangensis Y.L.Chen
Rhamnus nitida P.H.Davis
Rhamnus oleoides L.
Rhamnus orbiculata Bornm. – Orjen buckthorn
Rhamnus papuana Lauterb.
Rhamnus parvifolia Bunge
Rhamnus pentapomica R.Parker
Rhamnus persica Boiss. – Persian buckthorn (grows in Iran)
Rhamnus persicifolia Moris
Rhamnus petiolaris Boiss. & Balansa – (endemic to Sri Lanka)
Rhamnus philippinensis C.B.Rob.
Rhamnus pichleri C.K.Schneid. & Bornm.
Rhamnus pilushanensis Y.C.Liu & C.M.Wang
Rhamnus × pissjaukovae O.A.Popova
Rhamnus prinoides L'Hér. – shiny-leaf buckthorn
Rhamnus procumbens Edgew.
Rhamnus prunifolia Sm.
Rhamnus pulogensis Merr.
Rhamnus pumila Turra
Rhamnus punctata Boiss.
Rhamnus purpurea Edgew.
Rhamnus pyrella O.Schwarz
Rhamnus qianweiensis Z.Y.Zhu
Rhamnus rahiminejadii Alijanpoor & Assadi
Rhamnus rhodopea Velen.
Rhamnus rosei M.C.Johnst. & L.A.Johnst.
Rhamnus rosthornii E.Pritz.
Rhamnus rugulosa Hemsl.
Rhamnus salixiophylla S.S.Ying
Rhamnus sargentiana C.K.Schneid.
Rhamnus saxatilis Jacq. – rock Buckthorn, Avignon buckthorn, Avignon berry, dyer's buckthorn (syn. R. infectoria, R. infectorius)
Rhamnus schlechteri Lauterb.
Rhamnus seravschanica (Kom.) Kamelin
Rhamnus serpyllacea Greuter & Burdet
Rhamnus serrata Humb. & Bonpl. ex Schult. – sawleaf buckthorn
Rhamnus sibthorpiana Schult.
Rhamnus smithii Greene – Smith's buckthorn
Rhamnus songorica Gontsch.
Rhamnus spathulifolia Fisch. & C.A.Mey.
Rhamnus staddo A.Rich. – (syn. R. rhodesicus)
Rhamnus standleyana C.B.Wolf
Rhamnus subapetala Merr.
Rhamnus sumatrensis Ridl.
Rhamnus sumbawana Lauterb.
Rhamnus tangutica J.J.Vassil.
Rhamnus taquetii (H.Lév. & Vaniot) H.Lév. – Jejudo buckthorn
Rhamnus thymifolia Bornm.
Rhamnus tonkinensis Pit.
Rhamnus tortuosa Sommier & Levier
Rhamnus triquetra (Wall.) Brandis
Rhamnus tzekweiensis Y.L.Chen & P.K.Chou
Rhamnus utilis Decne. – Chinese buckthorn
Rhamnus velutina Boiss.
Rhamnus virgata Roxb.
Rhamnus wightii Wight & Arn. – Wight's buckthorn
Rhamnus wilsonii C.K.Schneid.
Rhamnus × woloszczakii Kárpáti
Rhamnus wumingensis Y.L.Chen & P.K.Chou
Rhamnus xizangensis Y.L.Chen & P.K.Chou
Rhamnus yoshinoi Makino

Distribution

Rhamnus has a nearly cosmopolitan distribution, with about 140 species which are native from temperate to tropical regions, the majority of species are from east Asia and North America, with a few species in Europe and Africa.

Buckthorns may be confused with dogwoods, which share the curved leaf venation; indeed, "dogwood" is a local name for R. prinoides in southern Africa. The two plants are easy to distinguish by slowly pulling a leaf apart; dogwoods will exude thin, white latex strings, while buckthorns will not.

Invasive species

Rhamnus cathartica, the common buckthorn, is considered an invasive species in the United States and by many local jurisdictions and state governments, including Minnesota and Wisconsin.

The common buckthorn is well-adapted to spreading in Canada and the U.S. It is an efficient grower that does not need much sunlight and or fertile soil. Other species, such as the Rhamnus alaternus also make chemicals that prevent other animals from consuming them.

Rhamnus alaternus is considered an invasive species in New Zealand and Australia. In New Zealand, it is particularly considered a problem in coastal shrubland and forest, including islands in the Hauraki Gulf such as Rangitoto. In New Zealand, it is prohibited from sale, propagation and distribution under the National Pest Plant Accord. In Australia, it is particularly a problem in coastal parts of south-eastern Australia. Italian buckthorn (R. alaternus), an evergreen species from the Mediterranean region, has become a serious weed in some parts of New Zealand, especially on Hauraki Gulf islands.

Buckthorns are used as food plants by the larvae of many Lepidoptera species.

The American species are known to be hosts for the oat fungus Puccinia coronata. In a 1930 study, both kerosene and salt were employed for eradication of R. lanceolata and both proved to be less expensive than felling these bushes.

Uses

right|thumb|[[Rhamnus cathartica]]

The fruit of most species contain a yellow dye and the seeds are rich in protein. Oils from the seeds are used for making lubricating oil, printing ink, and soap. The bark of some species of Rhamnus afford a beautiful yellow dye. The fruit of the Avignon buckthorn (R. saxatilis), called the Persian berry, produces a yellow dye.

Some species may cause demyelinating polyneuropathies.

The purging buckthorn (R. cathartica) is a widespread European native species used in the past as a purgative. It was in mid 17th-century England the only native purgative. It was also known pre-Linnaeus as Spina Cervina. The berries of Spina Cervina are black and contain a greenish juice, along with four seeds apiece; this serves to distinguish them from those of the black alder and dogberry, which contain only one or two apiece. Its syrup is said to be churlish. Its toxicity makes this a very risky herbal medicine, and it is no longer in use.

Rhamnus prinoides is known as gesho in Ethiopia, where it is used to make a mead called tej.

The species Rhamnus alaternus shows some promise for medicinal use as well.