Several plants have evolved various mechanisms both morphological and chemical to protect themselves against herbivory.
Morphological defence mechanisms:
Cactus leaves (Opuntia) are modified into sharp spines (thorns) to deter herbivores from feeding on them.
Sharp thorns along with leaves are present in Acacia to deter herbivores.
In some plants, the margins of their leaves are spiny or have sharp edges that prevent herbivores from feeding on them.
Chemical defence mechanisms:
All parts of Calotropis weeds contain toxic cardiac glycosides, which can prove to be fatal if ingested by herbivores.
Chemical substances such as nicotine, caffeine, quinine, and opium are produced in plants as a part of self-defence.