Plant Structures for Defense Plants have special structures that function for defense, survival, and reproduction. Structures for defense protect plants from threats. They might die without these defenses. Thorns Are sharp outgrowths on the stems of plants Defend the plant from being eaten by some animals Cactus Poisons Some plants have chemical substances designed to discourage animals from eating them. Some taste bad; some sicken or kill the predator. Some have high poison levels only until the fruit matures, then it decreases so the fruit can be eaten and seeds dispersed. Ex. Poison ivy, poison oak, sumac Poison Ivy Other Defense The ability to close its leaves when touched is called (thigmotropism).