How do plants fight back when they are being eaten?

A plant that is being eaten cannot run, so it fights with chemistry, armour and allies. Plant defense against herbivory is a range of adaptations that improve survival and reproduction by reducing the impact of herbivores.

The most common weapons are chemical. Many plants make secondary metabolites that act as repellents or toxins or make the plant harder to digest. Over 3,000 alkaloids are known, including nicotine, caffeine, morphine and quinine, all made by plants. The plant often keeps the poison in a harmless form until it is needed. Horseradish stores its pungent compound in a harmless form, separate from an enzyme called myrosinase, and the two meet only when the flesh is crushed. That is the sting in your sinuses: allyl isothiocyanate, the pungent ingredient in horseradish sauces and a compound toxic to insects.

Plants also have armour, such as spines and thorns. Many plants also take silicon up from the soil and deposit it as silica, which wears down teeth and insect mandibles.

And there are allies. When a caterpillar chews a plant, it triggers a hormone called jasmonic acid, which leads the plant to release chemical signals that attract parasitic wasps and predatory mites to hunt the herbivores. Some Acacia trees even grow swollen, hollow spines that house protective ants and feed them nectar. In other words, plants hire bodyguards.