All plants produce a diverse group of chemicals whose main function is to protect the plant against herbivores and diseases; these are the plant's chemical defenses. Many of these compounds seem to have no role in such core plant functions as growth and reproduction, and they are synthesized in unique pathways in the plant. As a result, they are often called secondary compounds or secondary metabolites. Others, including many enzymes, also have functions in growth, reproduction, and acquiring light or nutrients. Even though humans have exploited these plant products for thousands of years, it was not until the 1950s that scientists reasoned that these chemicals might be produced as defenses. While many plants have physical defenses such as thorns or spines, and some are just too tough to chew, those traits block feeding by large animals only, and do nothing against diseases. Chemical defenses are potentially...