Plants’ escalations of the arms race are numerous and varied. Some plants make little defensive toxin until they are mechanically
damaged, after which toxin rapidly accumulates in or near the injured part. Damage to a tomato or potato leaf induces production of toxins (proteinase inhibitors) not only at the site of the wound but throughout the plant. A plant has no nervous system, but it does have electrical signaling and a hormone system that can keep all its parts informed about what takes place in a small region. Some aspen trees have even more impressive communication. When a leaf is damaged, a volatile compound (methyl jasmonate) evaporating from the wound can turn on the proteinase response in nearby leaves, even those on other trees. The usual result of such defenses is that insects are discouraged after feeding even briefly. Some particularly adept insects, however, begin their meal by cutting the main supply vein to a leaf so the plant cannot deliver more toxins. And so the arms race goes on.