12.0.7 12.12: Unable to escape, plants must resist predation in other ways.

The interactions of plants and animals are not simple. On the one hand, plants depend on animals to pollinate their flowers and disperse their seeds. On the other hand, many kinds of animals eat plants, and plants are vulnerable because they can’t run away.

Just because plants can’t run away from plant-eating animals, however, they are not defenseless. A host of defensive devices that have evolved in plants give them some protection against being eaten. These defenses fall into two categories: anatomical structures, such as thorns, and chemical compounds, including hallucinogens (FIGURE 12-26).

Spines, prickles, and thorns are a common way to discourage herbivores, and some of these structures are impressively large. The acacias, a group of plants that grow as large bushes or small trees, are notoriously spiny. In these plants, the deterrent effects of spines are sometimes enhanced by the presence of ants that live in the swollen bases of the spines and fight off other animals—from insects to mammals, including humans—that would feed on their host acacia. Spines on palm fronds carry bacteria that infect the wounds that the spines create, providing a long-lasting reminder that it’s not a good idea to try to eat a palm frond. In most plants, the surfaces of leaves and stems have microscopic hairs that protect the plant against insect herbivores by exuding a sticky fluid trap, but stinging plants such as nettles use larger hairs to inject a potent fluid into any animal that brushes against a leaf.

Many chemical defenses make plants bitter or otherwise unpalatable, but some plants play hardball by producing chemicals that affect the nervous or reproductive systems of their predators. Locoweeds, which occur all over western North America, contain alkaloids that can cause permanent damage to the nervous system. When cattle and horses eat locoweed, they tire quickly and stop eating. Other defensive chemicals are hallucinogens. Tetrahydrocannabinol, or THC, in the leaves and buds of marijuana plants disorients mammals and causes them to stop eating.

Many of the chemicals that plants produce to protect themselves have physiological effects in humans, and for thousands of years humans have used plants for medicinal purposes. The aboriginal peoples of North America and Eurasia, for example, knew that extracts of willow or aspen bark could relieve pain. Salicin is the compound in the bark that relieves pain, and we still use acetylsalicylic acid (aspirin) for pain relief. Opium poppies produce a more powerful pain reliever; foxglove flowers contain a compound that slows the heart rate; and a compound from the ipecacuanha plant can induce vomiting.

Plants with medicinal uses are sought after (a process called “bioprospecting”), and representatives of pharmaceutical companies trudge through rain forests and deserts to find them. The rewards from such a discovery can be great. For example, artemisin from Chinese sweet wormwood is effective in treating malarial infections that are resistant to conventional pharmaceutical compounds.

There’s another role that chemicals can play in a plant’s defense systems. Although plants can’t move, they do send messages. They release volatile chemicals when they are attacked by insects, and these airborne chemicals can warn nearby plants of the impending threat or can call protective insects to their aid. When plants are nibbled by insects, they release the chemical methyl jasmonate (MJ), which is carried with the breeze to nearby plants. When these plants detect the presence of MJ, they ramp up production of their own defensive chemicals so that they are prepared if the insects move in to feast on them. Insects can also detect MJ, and insects that prey on other insects are attracted to plants emitting the chemical, because it signals the presence of prey. When corn and cotton plants are attacked by caterpillars, release of MJ summons parasitic wasps that deposit their eggs inside the caterpillars. When the eggs hatch, the wasp larvae eat the caterpillars from the inside out.

About 900 species of plants turn the tables—and eat insects. They use the protein in their insect meals to supplement the nitrogen compounds they get from the soil. Insectivorous plants are most common in boggy areas, because boggy soil often has low nitrogen concentrations (FIGURE 12-27). Pitcher plants are an example—their name comes from conspicuous pitcher-shaped structures that capture insects. It’s easy for an insect to enter the open top of a pitcher, but hard to get out, because the inner walls are lined with hair-like structures that point downward, forcing the insect into a pool of liquid. Some pitcher plants merely absorb the nitrogen that is released when the trapped insects decay, but other species secrete digestive enzymes into the pool. These enzymes, which are basically the same as the enzymes in your stomach, digest the insects and make the nitrogen available more rapidly.