Plants can not run away from their enemies nor get rid of troublesome pests as humans or other animals do, so what have they evolved to protect themselves? Whatever this protection is it must be successful, for the diversity and richness of green plants is extraordinary, and their dominance in most ecosystems of the world is unquestioned. Plant successes are closely intertwined with the evolution and production of highly diverse compounds known as secondary metabolites, compounds that are not essential for growth and reproduction, but rather, through interaction with their environment, enhance plant prospects of survival. These metabolites are therefore plant agents for chemical warfare, allowing plants to ward off microorganisms, insects, and other animals acting as predators and pathogens. Such compounds may also be valuable to humans for the same purposes, and therefore may be used as medicines.

What Characterizes Medicinal Plants

There are twenty...