When herbs are waving in the wind or you touch them with your hand, they give off a strong scent people like. Actually, however, the release of their scent is a way of protecting themselves against external aggression.
Caterpillars feed on pine needles, and cabbage worms feed on cabbage leaves. However, pine needles and cabbage leaves do not give themselves to insects and worms so easily. When attacked by a caterpillar, they release chemicals from their wounds. Upon smelling their scent, wasps fly in as swift as an arrow to prey on the caterpillar. A toxic substance in potato sprouts and allicin, the spicy scent of garlic, are also chemical substances to protect themselves.
Like that, plants defend themselves with chemical secretions when attacked, though they cannot take action in defense of themselves as humans or animals do. They release chemicals from their roots or leaves as a means of survival. These chemicals are called allelochemicals.
Every plant has its own allelochemical. Even seemingly fragile plants have strong life force for their survival.