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 plant ...

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 the prickly pear cactus, is a genus of flowering plants in the cactus family Cactaceae, many known for their flavorful fruit and showy flowers. Cacti are native to the Americas, and are well adapted to arid clima ...

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

families Family (from ) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictability, structure, and safety as ...

Urticaceae, Loasaceae, Boraginaceae (subfamily

Hydrophylloideae Hydrophylloideae is a subfamily of the plant family Boraginaceae. Its taxonomic position is somewhat uncertain. Traditionally, and under the Cronquist system, the taxon was given family rank under the name Hydrophyllaceae, and treated as part of ...

) 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 Euphorbia, the type genus of the family. Most spurges, such as ''Euphorbia paralias'', ar ...

. 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 (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

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 (the latter name applying particularly to ''Urtica dioica, U. dioica''). The generic name ''Urtica' ...

''). 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 110 accepted species of vines and shrubs of the Family (biology), family Fabaceae: tribe Phaseoleae, typically found in tropical and subtropical forests in the Americas, sub-Saharan Africa, southern, southeastern, ...

'' 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. Or ...

'' 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'' and '' Dalechampia''). 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 Calcium carbonate is a chemical compound with the chemical formula . It is a common substance found in Rock (geology), rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skel ...

(calcified) at its base and with

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

(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 '' Tragia'' 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 or . It forms hydrates , where ''n'' varies from 1 to 3. Anhydrous and all hydrated forms are colorless or white. The monohydr ...

. 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 communication, as well as regulating physiological functions in the gut and acting as a neurotransmitter for the brain, spinal cord, and uterus. Discovered in 19 ...

acetylcholine Acetylcholine (ACh) is an organic compound 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 (), also known as 5-hydroxytryptamine (5-HT), is a monoamine neurotransmitter with a wide range of functions in both the central nervous system (CNS) and also peripheral tissues. It is involved in mood, cognition, reward, learning, ...

, and

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

. 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'' species. In

vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s, histamine is a

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

. 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. The main geography of Taiwan, island of Taiwan, also known as ''Formosa'', lies between the East China Sea, East and South China Seas in the northwestern Pacific Ocea ...

. 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 tamarinds, bananas, avocados, and citrus. Its salt (chemistry), salt, potassium bitartrate, commonly known as cream of ta ...

, 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 Urticaceae fall into this category, with a

cosmopolitan Cosmopolitan may refer to: Internationalism * World citizen, one who eschews traditional geopolitical divisions derived from national citizenship * Cosmopolitanism, the idea that all of humanity belongs to a single moral community * Cosmopolitan ...

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 of ...

perennial plant In horticulture, the term perennial (''wikt:per-#Prefix, per-'' + ''wikt:-ennial#Suffix, -ennial'', "through the year") is used to differentiate a plant from shorter-lived annual plant, annuals and biennial plant, biennials. It has thus been d ...

s, but some are annual and a few are

shrub A shrub or 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 trees by their multiple ...

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. Or ...

, ''Urtica dioica'', native to

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

, and

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

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

'' spp. *

** '' Cnidoscolus'' spp. *** '' Cnidoscolus stimulosus'' (bull nettle or spurge nettle) *** '' Cnidoscolus urens'' (stinging spurge or mala mujer) ** '' Croton ciliato-glandulosus'' (ciega-vista) ** '' Dalechampia'' spp. ** '' Tragia'' spp. (noseburn) *

Fabaceae Fabaceae () or Leguminosae,International Code of Nomen ...

** ''

'' spp. *** '' Mucuna pruriens'' (cowhage) * Loasaceae ** '' Loasa'' spp. ** '' Caiophora'' spp. ** '' Cevallia'' spp.

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 or flesh-colored, 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 typically ...

, only a few are seriously harmful. The genus '' Dendrocnide'' (stinging trees) has been said to cause the most pain, particularly the Australian '' Dendrocnide moroides'' (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'' (tree nettle or ongaonga) is

endemic Endemism is the state of a species being found only 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 foun ...

New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...

. 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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