Chapter 5: Poisons and Other Plant Defenses

Elaborate defense systems allow plants to live on and thrive, in spite of being mostly anchored in place. This chapter describes major classes of defensive chemical compounds that plants have evolved to deter animals, fight against fungal attacks, and keep other plants from growing too close. The number of different defensive compounds in plants is enormous and their chemical structures are highly diverse. Some families of compounds that were discussed earlier, like terpenes and phenolics, are encountered here in their roles as plant defenses. The sequence of chapter sections follows increasingly more potent chemical deterrents. Strong scents in leaves can provide a first line of defense. Viscous resins, sour acids, milky latex, and bitter or irritating saps discourage animals from feeding on certain plants. Sharp oxalate crystals provide defense in some leaves. Certain plants produce toxic cyanide when their leaves or seeds are bitten into. The structures of steroids are introduced with saponins and cardiac glycosides. Alkaloids conclude the chapter—they are a family of defensive plant bases that contains many well-known toxins, several with important uses in medicine. A separate section addresses how some insects have learned to cope with chemical plant defenses and use them for their own defense.