A stinging plant or a plant with stinging hairs is a plant with hairs (

trichome Trichomes (); ) are fine outgrowths or appendages on plants, algae, lichens, and certain protists. They are of diverse structure and function. Examples are hairs, glandular hairs, scales, and papillae. A covering of any kind of hair on a p ...

s) on its leaves or stems that are capable of injecting substances that cause pain or irritation. Other plants, such as

opuntia ''Opuntia'', commonly called prickly pear or pear cactus, is a genus of flowering plants in the cactus family Cactaceae. Prickly pears are also known as ''tuna'' (fruit), ''sabra'', '' nopal'' (paddle, plural ''nopales'') from the Nahuatl word ...

s, have hairs or spines that cause mechanical irritation, but do not inject chemicals. Stinging hairs occur particularly in the families

Urticaceae The Urticaceae are a family, the nettle family, of flowering plants. The family name comes from the genus ''Urtica''. The Urticaceae include a number of well-known and useful plants, including nettles in the genus ''Urtica'', ramie (''Boehmeri ...

Loasaceae Loasaceae is a family of 15–20 genera and about 200–260 species of flowering plants in the order Cornales, native to the Americas and Africa. Members of the family include annual, biennial and perennial herbaceous plants, and a few shrubs ...

, Boraginaceae (subfamily

Hydrophylloideae Hydrophylloideae is a subfamily of the plant family Boraginaceae. Their taxonomic position is somewhat uncertain. Traditionally, and under the Cronquist system, they were given family rank under the name Hydrophyllaceae, and treated as part of th ...

) and

Euphorbiaceae Euphorbiaceae, the spurge family, is a large family of flowering plants. In English, they are also commonly called euphorbias, which is also the name of a genus in the family. Most spurges, such as '' Euphorbia paralias'', are herbs, but some, ...

. Such hairs have been shown to deter grazing mammals, but are no more effective against insect attack than non-stinging hairs. Many plants with stinging hairs have the word "nettle" in their English name, but may not be related to "true nettles" (the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

Urtica ''Urtica'' is a genus of flowering plants in the family Urticaceae. Many species have stinging hairs and may be called nettles or stinging nettles, although the latter name applies particularly to '' Urtica dioica''. ''Urtica'' species are food ...

''). Though several unrelated families of plants have stinging hairs, their structure is generally similar. A solid base supports a single elongated cell with a brittle tip. When the tip is broken, the exposed sharp point penetrates the skin and pressure injects toxins. The precise chemicals involved in causing pain and irritation are not yet fully understood.

Structure and function of stinging hairs

Stiff hairs or

s without the ability to inject irritating compounds occur on the leaves and stems of many plants. They appear to deter feeding insects to some degree by impeding movement and restricting access to the surface of the stem or leaf. Some plants have glandular hairs, either as well as non-glandular hairs or instead of them. Glandular hairs have regions of tissue that produce secretions of secondary metabolites. These chemical substances can repel or poison feeding insects. Brennhaar-40x

Stinging hairs may be defined as those with ability to inject a chemical substance through the skin of an animal causing irritation or pain. Since some glandular hairs can cause irritation merely by contact, the difference between "stinging hairs" and "irritating hairs" is not always clear. For example, the hairs of ''

Mucuna ''Mucuna'' is a genus of around 100 accepted species of climbing lianas (vines) and shrubs of the family Fabaceae: tribe Phaseoleae, typically found in tropical forests. The leaves are trifoliolate, alternate, or spiraled, and the flowers are pe ...

'' species are described in both ways. Some species of ''Mucuna'' have sharply tipped hairs, in which the upper part easily breaks off, whereas other species have hairs that are blunter. In those subspecies of ''

Urtica dioica ''Urtica dioica'', often known as common nettle, burn nettle, stinging nettle (although not all plants of this species sting) or nettle leaf, or just a nettle or stinger, is a herbaceous perennial flowering plant in the family Urticaceae. Ori ...

'' that have stinging hairs (stinging nettles), these also have a point that easily breaks off, allowing the irritants in the cell below to enter through the skin. Being stung in this way has been shown to deter grazing mammals, such as rabbits, and even large herbivores such as cows. Many plant species respond to physical damage by producing a higher density of trichomes of all kinds. The general structure of a stinging hair is very similar in all the families of plants that possess them (except ''

Tragia ''Tragia'' is a genus of flowering plants in the spurge family, Euphorbiaceae. It is widespread across North and South America, Africa, the Arabian Peninsula, the Indian Subcontinent, northern Australia, and to various islands in the Caribbean ...

'' and ''

Dalechampia ''Dalechampia'' is a genus of plant of the family Euphorbiaceae and of the monogeneric subtribe Dalechampiinae. It is widespread across lowland tropical areas (generally below 2,000 m ASL) primarily in the Americas with smaller numbers of spe ...

''). A multicellular base supports a single long thin cell, typically 1–8 mm long, with a brittle tip that easily breaks to form a sharp point that can penetrate skin. Stinging hairs of ''Urtica'' species have been studied in some detail. Each hair contains a fine tube, stiffened with calcium carbonate (calcified) at its base and with

silica Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...

(silicified) at its tip. In '' Urtica thunbergiana'', individual hairs contain around 4 nanolitres () of fluid. The silicified tip breaks off on contact, and the resulting fine point pierces the skin. Pressure forces the fluid out of the hair. Different toxins may be involved. The stinging hairs of ''

'' spp, notably '' Tragia volubilis'', a South American member of the

, are capable of injecting a crystal of

calcium oxalate Calcium oxalate (in archaic terminology, oxalate of lime) is a calcium salt of oxalic acid with the chemical formula . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydrate ...

. The stinging sensation is initially caused by the mechanical entry of the stiff hair into the skin, but is then intensified by the effect of the oxalate. The effects of the stinging hairs of ''

'' species, particularly some subspecies of ''

'', have been attributed to a number of substances, including

histamine Histamine is an organic nitrogenous compound involved in local immune responses, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Since histamine was discover ...

acetylcholine Acetylcholine (ACh) is an organic chemical that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

serotonin Serotonin () or 5-hydroxytryptamine (5-HT) is a monoamine neurotransmitter. Its biological function is complex and multifaceted, modulating mood, cognition, reward, learning, memory, and numerous physiological processes such as vomiting and va ...

, and

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid, and has the chemical formula HCOOH and structure . It is an important intermediate in chemical synthesis and occurs naturally, most notably in some ants. Est ...

. Histamine is a component of the stinging hairs of other ''Urtica'' species (e.g. '' U. urens'' and '' U. parviflora'') and of '' Cnidoscolus urens'' and ''

Laportea ''Laportea'' is a genus of plants in the family Urticaceae. They are herbaceous, either annual or perennial. Like many plants of the Urticaceae, they have stinging hairs. There are stinging and non-stinging hairs on the same plant. The genus was ...

'' species. In

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...

s, histamine is a

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neu ...

. When it is released naturally, inflammation of the skin results, causing pain and itching. Injection of histamine by stinging hairs has been considered to have the same effect. This traditional interpretation was challenged in 2006 by research on ''Urtica thunbergiana'', the main species of ''Urtica'' present in

Taiwan Taiwan, officially the Republic of China (ROC), is a country in East Asia, at the junction of the East and South China Seas in the northwestern Pacific Ocean, with the People's Republic of China (PRC) to the northwest, Japan to the no ...

. In tests on rats, the long-lasting pain caused by stings was attributed to oxalic and

tartaric acid Tartaric acid is a white, crystalline organic acid that occurs naturally in many fruits, most notably in grapes, but also in bananas, tamarinds, and citrus. Its salt, potassium bitartrate, commonly known as cream of tartar, develops naturally ...

, although a synergistic effect of the other components of the stinging hairs was not ruled out. Fu et al. concluded that "stinging hairs, although studied for a long time, are still mysterious, particularly concerning the mechanism of the skin reaction after being stung."

Plants with stinging hairs

''Urtica''

Many plants with stinging hairs belong to the genus ''

''. Between twenty-four and thirty-nine species of flowering plants of the genus ''Urtica'' in the family

fall into this category, with a

cosmopolitan Cosmopolitan may refer to: Food and drink * Cosmopolitan (cocktail), also known as a "Cosmo" History * Rootless cosmopolitan, a Soviet derogatory epithet during Joseph Stalin's anti-Semitic campaign of 1949–1953 Hotels and resorts * Cosmopoli ...

though mainly temperate distribution. They are mostly

herbaceous Herbaceous plants are vascular plants that have no persistent woody stems above ground. This broad category of plants includes many perennials, and nearly all annuals and biennials. Definitions of "herb" and "herbaceous" The fourth edition o ...

perennial plant A perennial plant or simply perennial is a plant that lives more than two years. The term ('' per-'' + '' -ennial'', "through the years") is often used to differentiate a plant from shorter-lived annuals and biennials. The term is also wide ...

s, but some are

annual Annual may refer to: *Annual publication, periodical publications appearing regularly once per year ** Yearbook ** Literary annual *Annual plant *Annual report *Annual giving *Annual, Morocco, a settlement in northeastern Morocco *Annuals (band), ...

and a few are

shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from tree ...

by. The most prominent member of the genus ''Urtica'' is the

stinging nettle ''Urtica dioica'', often known as common nettle, burn nettle, stinging nettle (although not all plants of this species sting) or nettle leaf, or just a nettle or stinger, is a herbaceous perennial flowering plant in the family Urticaceae. ...

, ''Urtica dioica'', native to

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...

, and

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...

Other Urticaceae

The family Urticaceae also contains some other plants with stinging hairs that are not members of the genus ''Urtica''. These include: Gardenology

* ''

Dendrocnide ''Dendrocnide'' is a genus of approximately 40 species of plants in the nettle family ''Urticaceae''. They have a wide distribution across North East India, Southeast Asia, Australia and the Pacific Islands. In Australia they are commonly known a ...

'' spp. ** ''

Dendrocnide excelsa ''Dendrocnide excelsa'', commonly known as the giant stinging tree or fibrewood, is a rainforest tree in the family ''Urticaceae'' which is endemism , endemic to eastern Australia. It occurs from Tathra, New South Wales to Imbil in southeastern ...

'' (giant stinging tree) ** ''

Dendrocnide moroides ''Dendrocnide moroides'', commonly known in Australia as the stinging tree, stinging bush, Queensland Stinger or gympie-gympie, is a plant in the nettle family Urticaceae found in rainforest areas of Malaysia and Australia. It is notorious for it ...

'' (gympie-gympie) ** '' Dendrocnide peltata'' (stinging tree, ''jelaton'') * '' Girardinia diversifolia'' (synonym: ''G. leschenaultiana'') (Nilgiri nettle, Himalayan giant nettle), source of allo fibre * ''

Laportea canadensis ''Laportea canadensis'', commonly called Canada nettle or wood-nettle, is an annual or perennial herbaceous plant of the nettle family Urticaceae, native to eastern and central North America. It is found growing in open woods with moist rich soi ...

'' (wood nettle) * '' Urera'' spp. ** '' Urera baccifera'' (nettle tree) ** '' Urera tenax''

Other families

There are also plants with stinging hairs that are unrelated to the Urticaceae: Cnidoscolus urens var

* Boraginaceae: Hydrophylloideae ** '' Phacelia malvifolia'' (stinging phacelia) ** ''

Wigandia ''Wigandia'' is a genus of flowering plants within the waterleaf subfamily, Hydrophylloideae. They are found mainly in Central America and South America, though one or two species are found as far north as the United States. Some are grown a ...

'' spp. *

** ''

Cnidoscolus ''Cnidoscolus'' is a plant genus of the family Euphorbiaceae first described as a genus in 1827. The group is widespread across much of North and South America, including the West Indies. The name is derived from the Greek words κνίδη (''k ...

'' spp. *** ''

Cnidoscolus stimulosus ''Cnidoscolus stimulosus'', the bull nettle, spurge nettle, tread-softly or finger rot, is a perennial herb covered with stinging hairs, native to southeastern North America. A member of the family Euphorbiaceae (spurge family), it is not a true ...

'' (bull nettle or spurge nettle) *** '' Cnidoscolus urens'' (stinging spurge or mala mujer) ** '' Croton ciliato-glandulosus'' (ciega-vista) ** ''

'' spp. ** ''

'' spp. (noseburn) *

Fabaceae The Fabaceae or Leguminosae,International Code of Nomenc ...

** ''

'' spp. *** '' Mucuna pruriens'' (cowhage) *

** '' Loasa'' spp. ** ''

Caiophora ''Caiophora'' is a genus of flowering plants belonging to the family Loasaceae Loasaceae is a family of 15–20 genera and about 200–260 species of flowering plants in the order Cornales, native to the Americas and Africa. Members of the fa ...

'' spp. ** ''

Cevallia ''Cevallia'' is a genus of flowering plants belonging to the family Loasaceae Loasaceae is a family of 15–20 genera and about 200–260 species of flowering plants in the order Cornales, native to the Americas and Africa. Members of the fam ...

'' spp. *

Solanaceae The Solanaceae , or nightshades, are a family of flowering plants that ranges from annual and perennial herbs to vines, lianas, epiphytes, shrubs, and trees, and includes a number of agricultural crops, medicinal plants, spices, weeds, and or ...

** ''

Solanum carolinense ''Solanum carolinense'', the Carolina horsenettle, is not a true nettle, but a member of the Solanaceae, or nightshade family. It is a perennial herbaceous plant, native to the southeastern United States, though its range has expanded throughou ...

'' (Carolina horsenettle)

Toxicity

Though plants with stinging hairs can cause pain and acute

urticaria Hives, also known as urticaria, is a kind of skin rash with red, raised, itchy bumps. Hives may burn or sting. The patches of rash may appear on different body parts, with variable duration from minutes to days, and does not leave any long-last ...

, only a few are seriously harmful. The genus ''

'' (stinging trees) has been said to cause the most pain, particularly the Australian ''

'' (gympie-gympie), although other sources describe the pain of stinging trees as only differing from that of nettles in terms of persistence rather than severity. There are reports of dogs and horses being killed, and once of a human death. The researcher Marina Hurley reports being hospitalized after being stung by a dead leaf. Deaths are probably due to heart failure caused by pain and shock. ''

Urtica ferox ''Urtica ferox'', commonly known as tree nettle and in Māori: ongaonga, taraonga, taraongaonga, оr okaoka, is a species of nettle endemic to New Zealand. Unlike the other species in the genus ''Urtica'' found in New Zealand, all of which are he ...

'' (tree nettle or ongaonga) is

endemic Endemism is the state of a species being found in a single defined geographic location, such as an island, state, nation, country or other defined zone; organisms that are indigenous to a place are not endemic to it if they are also found else ...

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...

. One recorded human death is known: a lightly clad young man died five hours after walking through a dense patch. After cooking, some plants with stinging hairs, such as ''

'' (stinging nettle), are eaten as vegetables.

References

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