Similarly, herbs such as rosemary and thyme produce a wonderful fragrance when the leaves are crushed. Some plants release their odor when touched, while others release theirs into the breeze. Scent in plants is thought to have evolved from waste products produced by the plants through normal metabolic processes. Over time, these scents attracted pollinators such as bees, butterflies, and moths to the plant. Although less important than flower color, scent plays an important role in plant reproduction by attracting these, and other pollinating insects.
Plants also use scents to repel or kill insects or to communicate with other plants.
This latter example involves the use of pheromones, chemical substances that are often imperceptible to the human nose. From a chemical standpoint, the fragrance of plants results from essential oils that are produced when an acid and an alcohol are combined. It is the alcohol component that gives the flower or foliage its characteristic smell. These molecules, technically called esters, tend to be heavier than the surrounding air and consequently are found close to the ground. The answer may have to do with their exposure to scents because of their relative positions to the ground.