With a little care, the stinging nettle can make for very interesting viewing under
a microscope. The stinging hairs that cover the leaves and stems are like tiny hypodermic
needles, and are made of silica. These hollow needles are pre-loaded with a cocktail
of chemicals including a neurotransmitter Acetylcholine, an irritant Histamine, and
a neurotoxin Moroidin that is responsible for the pain of the sting. A curious feature
of the needle is that it doesn’t actually end in a sharp point but a little ball
about 2um (0.002mm) in diameter. This ball forms a seal at the end of the needle
and stops the poison leaking out. Brushing against the needles causes the tips to
snap off , leaving razor sharp hollow points that can easily penetrate human skin.
Flexing at the thickened base of the needle forces the poison out through its tip,
injecting it into the hapless passerby.