The genus Bidens Linnaeus

Remark The words or terms in red (actually dark orange) in the text are defined in a glossary.


sep_06_04.gthmb can't be loaded. Bidens is a genus of weedy herbs with over 200 species. The genus belongs to the Heliantheae tribe of the Asteraceae family. It was described by Linnaeus (1701-1778) in his Species Plantarum, 1753. The genus is distributed throughout the world. About 50 species, including introduced ones can be found in the USA. A dozen of species, flowering at the end of summer and in the fall can be found in Québec, some quite common.

The species of Bidens that is best known to botanists, horticulturists and herbalists is Bidens pilosa. It is used as an herbal medicine in many parts of the world. It is commonly found growing along roadsides, railroad tracks, vacant lots, and other places where the ground has been disturbed, including the shores of rivers of ponds.

Name

In Latin, bis means twice and dens means tooth so that bidens means two-toothed; hence the genus name refers to the bristles of the achenes of the plants, even if these seeds have, at times, three or four bristles.

Many species found in North America have the common names Bur-marigold, Sticktights, Beggar-ticks, Beggartick, Railway Beggar's Tick, Devil's Sticktight, Spanish Needles, Tickseed-sunflowers, the first vernacular name referring to the color of the flowers, the other ones referring to the achenes commonly functioning as stick-tights in animal fur and clothing, in feathers, by means of their barbs. This distinctive dispersal adaptation explains the widespread, distribution and colonization of the genus.

Synonyms

Some Bidens species have also been known with an other genus name:

Identification

The Bidens genus is distinguished by the combination of: Some species have divided leaves with toothed segments and long ray flowers; others have undivided, lance-shaped leaves and short ray flowers or none at all. sep_15_05.jpg can't be loaded.

Description

Bidens species comprise annual and perennial herbs and vary greatly in size and degree of branching. Leaves may be simple or ternately to pinnately compound; they are opposite (or alternate for the uppermost), obscurely to coarsely serrate, sessile or petiolate. The flower heads are discoid or radiate. The ray florets, when present, are rather few, sterile or rarely pistillate and have yellow ligules.

The involucral bracts are biseriate and dimorphic, the outer row being usually foliaceous and spreading, while the inner is row short, erect, membranous, and striate. The disk florets are perfect, usually numerous and with a usually yellow corolla but for some species a white corolla. The fruits are flattened achenes, narrowed at the base and widening upward, with 2 to 4 retrorsely barbed awns which persist atop the achene, the fruits functioning as stick-tights in animal fur and clothing by means of the barbs.

Notes

Several species of Bidens were used as sources of the traditional Chinese medicinal herb; they were known as guizhencao, where gui means demon or ghost, zhen means spike, needle and cao means weed, plant, the name making reference to the characteristic barbs on the fruits. The barbed fruits of these herbs are used as medicine and must be handled carefully or they can cause injury. Descriptions of injuries from Bidens spikes (mainly infection from cuts received) have appeared in the medical literature.

In Europe some Bidens species were used in the 16th and 17th century for their astringent, diaphoretic, and diuretic properties. Bidens frondosa and Bidens bipinnata were prescribed in the U.S. as emmenagogue and in the treatment of laryngeal and chest complaints.

Some Bidens species are used as food plants by the larvae of some Lepidoptera species.

Bidens pilosa, an annual native to tropical America, is one of the world's worst weeds.

The Bidens of Québec

In Québec one should find the following species: Quite often, it is difficult to identify with certainty each species, and one must be very careful before attributing a name to a given specimen; Bidens are well known for taxonomic difficulty, the species having plastic phenotypes.

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