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.
Fragrance is difficult to define due to the limited number of descriptive terms that can be used.
Some of the common essential oils found in flowers include rose, violet, and jasmine while eucalyptus, lemon, and pine contribute to the scents of foliage. Some flowers such as orchids produce 100 essential oils while others may only produce ten. Many of the scented plants bloom exclusively at night. These plants typically have white, or light-colored flowers and are pollinated by moths. Some of these nocturnal flowers actually open during the day but don t release their fragrance until after the sun sets.