How do plant toxins work? They do whatever will keep herbi- vores from eating the plants. Why are there so many different toxins? Herbivores would quickly find a way around any one defense, so the arms race creates many different ones. The list of different toxins and their diverse actions is impressive. Some plants make precursors of cyanide, which is released either by enzymes in the plant or by the intestinal bacteria of the consumer. The bitter almond is noteworthy in this regard, but apple and apricot seeds use the same strategy, as do cassava roots, which are used for food in many cultures.
All adaptations, however, have costs, and plants’ defensive chem- icals have theirs. Toxin manufacture requires materials and energy, and the toxins may be dangerous to the plant that produces them. In general, a plant can have high toxin levels or rapid growth, but not both. To put it from the herbivore’s point of view, rapidly growing plant tissues are usually better food than stable or slowly growing structures. This is why leaves are more vulnerable than bark and why the first leaves of spring are especially vulnerable to caterpillars and other pests.