Plants are subject to attack and infection by a remarkable variety of symbiotic
species and have evolved a diverse array of mechanisms designed to frustrate the
potential colonists. These can be divided into preformed or passive defense mechanisms
and inducible or active systems. Passive plant defense comprises physical and chemical
5 barriers that prevent entry of pathogens, such as bacteria, or render tissues unpalatable
or toxic to the invader. The external surfaces of plants, in addition to being covered by
an epidermis and a waxy cuticle, often carry spiky hairs known as trichomes, which
either prevent feeding by insects or may even puncture and kill insect larvae. Other
trichomes are sticky and glandular and effectively trap and immobilize insects.