Fig.
9.2-5. Transverse section of leaf of stinging
nettle (Urtica). If you do not know about stinging nettle
first hand, good. The large structure projecting up out of the upper epidermis
is a stinging cell, a secretory cell that produces and stores a toxic compound.
As long as the cell is undamaged, the secretory product is kept in a harmless
location. But if the point of the cell is broken (and its wall is made brittle
with silica), the toxin is released. Because the cell is so pointed, it will
puncture your skin and be inside your body by the time it breaks and releases
its secretion.