Urticaria dioica

Without silica, nettles do not develop the ability to sting. Barber and Shone (112b) described experiments in which the nettle Urticaria dioica was grown in a culture solution nominally free of silica. The leaves showed little stinging ability. Then a solution of silica was added to the culture medium and in 2 weeks the stinging hairs... 

Oliver, F, E.U. Amon, A. Breathnach, et al. 1991. Contact urticaria due to the common stinging nettle (Urtica dioica)— Histological, ultrastmctural and pharmacological studies. 

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