Significant as these morphological adaptation are, chemical defenses that occur so widely in plants are even more crucial.
Best known and perphaps most important in the defenses of plants against herbivores are secondary chemical compounds.
These are distingguished from primary compounds, which are regular components of the major metabolic pathway, such as respiration.
Many plants, and apparently many algae as well, contain very structurally diverse secondary compounds that are either toxic to most herbovores or disturb their metabolism greatly, preventing, for example, the normal development of larval insects.
Consequently, most herbivores tend to avoid the plants that posses these compounds.