See the review of mechanical defenses by Lucas et al., 2000, which remains relevant and well regarded in the subject . Many plants have external structural defenses that discourage herbivory. Structural defenses can be described as morphological or physical traits that give the plant a fitness advantage by deterring herbivores from feeding. Depending on the herbivores physical characteristics (i.e. size and defensive armor), plant structural defenses on stems and leaves can deter, injure, or kill the grazer. Some defensive compounds are produced internally but are released onto the plants surface; for example, resins, lignins, silica, and wax cover the epidermis of terrestrial plants and alter the texture of the plant tissue. The leaves of holly plants, for instance, are very smooth and slippery making feeding difficult. Some plants produce gummosis or sap that traps insects.