| Rhamnus | |
|---|---|
 | |
| Rhamnus cathartica | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Rosids |
| Order: | Rosales |
| Family: | Rhamnaceae |
| Tribe: | Rhamneae |
| Genus: | Rhamnus L. |
| Species | |
See text | |
Rhamnus is agenus of about 140 acceptedspecies ofshrubs or smalltrees, commonly known asbuckthorns, in the familyRhamnaceae. Its species range from 1 to 10 m (3 to 33 ft) tall (rarely to 15 m, 50 ft) and are native mainly ineast Asia andNorth America, but found throughout the temperate and subtropicalNorthern Hemisphere, and also more locally in the subtropicalSouthern Hemisphere in parts ofAfrica andSouth America. One species, the common buckthorn (Rhamnus cathartica), is able to flourish as aninvasive plant in parts ofCanada and theUnited States, where it has becomenaturalized.[1]
Bothdeciduous andevergreen species occur. Theleaves are simple, 3 to 15 cm (1 to 6 in) long, and arranged alternately, in opposite pairs, or almost paired (subopposite). One distinctive character of many buckthorns is the way theveination curves upward towards the tip of the leaf. The plant bears fruits which are black or red berry-likedrupes. 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.[2]
Rhamnus species areshrubs or small to medium-sizedtrees,[3] withdeciduous or rarelyevergreenfoliage.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 generallydioecious, with male and femaleflowers on separate plants.[4] Most species have yellowish green, small,unisexual or rarelypolygamous flowers; which are produced singly or in axillarycymes, cymoseracemes, or cymosepanicles containing a few flowers.Calyx tube campanulate to cup-shaped, with 4 or 5 ovate-triangularsepals, 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 5stamens which are surrounded by and equal in length the petals or are shorter. Theanthers are dorsifixed. The superiorovary 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 fleshyendosperm.[5]
As of October 2024[update],Plants of the World Online accepted the following species:[6]
Rhamnus has a nearlycosmopolitan distribution,[9] 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.[5]
North American species includeRhamnus alnifolia, alder-leaf buckthorn, occurring across the continent, andRhamnus crocea, hollyleaf buckthorn, in the west. Though not native to this region,Rhamnus cathartica can be found in North America.[10]
Buckthorns may be confused withdogwoods, which share the curved leafvenation; indeed, "dogwood" is a local name forR. 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.
Rhamnus cathartica, the common buckthorn, is considered aninvasive species in theUnited States[11] and by many local jurisdictions and state governments, includingMinnesota[12] andWisconsin.[13]
The common buckthorn is well-adapted to spreading inCanada and the U.S.[1] It is an efficient grower that does not need much sunlight and or fertile soil.[1] Its seeds are hardy, as well as being able to grow and spread easily in a variety of environmental conditions.[1] Also other animals prefer to leave buckthorns alone because their leaves are not appetizing and their fruits are toxic to some animals.[1] Overall they are known to have a negative effect on their surrounding environment.[1] For example, the European buckthorn is blamed for increased frog egg mortality from a chemical it releases.[14] Other species, such as theRhamnus alaternus also make chemicals that prevent other animals from consuming them.[2]
Rhamnus alaternus is considered an invasive species in New Zealand[15] and Australia.[16] In New Zealand, it is particularly considered a problem in coastal shrubland and forest, including islands in the Hauraki Gulf such as Rangitoto.[17] In New Zealand, it is prohibited from sale, propagation and distribution under theNational Pest Plant Accord.[18] In Australia, it is particularly a problem in coastal parts of south-eastern Australia.[16]
Some species areinvasive outside their natural ranges.R. cathartica wasintroduced into theUnited States as agarden shrub and has become an invasive species in many areas there. It is a primary host of thesoybean aphid (Aphis glycines), apest forsoybean farmers across the US. The aphids use the buckthorn as a host for the winter and then spread to nearby soybean fields in the spring.[19] Italian buckthorn (R. alaternus), an evergreen species from the Mediterranean region, has become a serious weed in some parts ofNew Zealand,[20] especially onHauraki Gulf islands.
Buckthorns are used as food plants by thelarvae of many Lepidoptera species.
The American species are known to be hosts for theoat fungusPuccinia coronata. In a 1930 study, both kerosene and salt were employed for eradication ofR. lanceolata and both proved to be less expensive than felling these bushes.[21]
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.[5] Many species have been used to makedyes.R. utilis provides china green, a dye used to give a bright green color to silk and wool.[22] The bark of some species ofRhamnus afford a beautiful yellow dye.[23] The fruit of the Avignon buckthorn (R. saxatilis), called thePersian berry, produces a yellow dye.
Some species may causedemyelinating polyneuropathies.[24]
The purging buckthorn (R. cathartica) is a widespreadEuropean native species used in the past as apurgative. It was in mid 17th-century England the only native purgative.[25] It was also known pre-Linnaeus asSpina Cervina.[26] The berries ofSpina Cervina are black and contain a greenish juice, along with four seeds apiece; this serves to distinguish them from those of theblack alder anddogberry, which contain only one or two apiece. Its syrup is said to be churlish.[27] Its toxicity makes this a very riskyherbal medicine, and it is no longer in use.[28]
Rhamnus prinoides is known asgesho inEthiopia, where it is used to make amead calledtej.
The speciesRhamnus alaternus shows some promise for medicinal use as well.[2]
