Plants have evolved many defenses against herbivory, ranging from the subtle to the obvious. Physical defenses, which are mostly constituitive (always present) tend more to the obvious side; for instance, thorns and spines have evolved in many groups as a defense against herbivores.
Thigmonasty, or the ability to move in response to a stimulus, can be a defense against herbivory. For example, sensitive plant (Mimosa pudica) will suddenly droop if it is touched, making it much less appetising.
Coconut palms and many hardwood nut trees protect their seeds by enclosing them in layers of woody or fibrous tissue which are difficult to get through.
Chemical defenses may be constituitive or induced, meaning that they appear or intensify when herbivory is detected by the plant. Many chemicals are simply strong tasting, such as the oils that give hot peppers their heat or the bitter tasting tannins present in many plant families.
Other chemical defenses are similar to biological warfare, affecting the metabolism of the herbivore in some negative way. For instance, foxglove (Digitalis purpurea), a common garden plant, contains enough digitalis in its leaves to stop a human heart. Even ordinary plants that are commonly eaten can have poisonous parts; these include potato and tomato plants, kidney beans, and rhubarb.
Haptens are another chemical defense; these small molecules are not dangerous by themselves, but they can elicit a strong immune response in an herbivore. Poison ivy is an example of a plant that uses this type of defense.