Stiff hairs or
trichomes 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. 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
Urtica species, particularly some subspecies of
Urtica dioica, have been attributed to a number of substances, including
histamine,
acetylcholine,
serotonin, and
formic acid. 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
vertebrates, histamine is a
neurotransmitter. 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. In tests on rats, the long-lasting pain caused by stings was attributed to
oxalic and
tartaric acid, 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==