Plant defense against herbivory or host-plant resistance (HPR) describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. These chemical defenses can act as repellents or toxins to herbivores, or reduce plant digestibility. Plant defenses can be classified generally as constitutive or induced. Constitutive defenses are always present in the plant, while induced defenses are produced or mobilized to the site where a plant is injured. Induced defenses include secondary metabolic products, as well as morphological and physiological changes. An advantage of inducible, as opposed to constitutive defenses, is that they are only produced when needed, and is therefore potentially less costly, especially when herbivory is variable. Induced defenses include secondary metabolic products, as well as morphological and physiological changes. An advantage of inducible, as opposed to constitutive defenses, is that they are only produced when needed, and is therefore potentially less costly, especially when herbivory is variable. The evolution of chemical defences in plants is linked to the emergence of chemical substances that are not involved in the essential photosynthetic and metabolic activities. These substances, secondary metabolites, are organic compounds that are not directly involved in the normal growth, development or reproduction of organisms, and often produced as by-products during the synthesis of primary metabolic products. Secondary metabolites are often characterized as either qualitative or quantitative. Qualitative metabolites are defined as toxins that interfere with an herbivore’s metabolism, often by blocking specific biochemical reactions. Quantitative metabolites are digestibility reducers that make plant cell walls indigestible to animals.