The role of scent is thought to have developed from odors produced during plant metabolism. In some plants, these odors caught the attention of potential pollinators, leading to a more successful spread of the genes of some plants over others.
Chemically, plant esters are formed when an acid and alcohol combine. According to Karen Delahaut of the University of Wisconsin Department of Horticulture, the alcohol portion of these essential oils is what gives each plant its fragrance. The exact makeup of these compounds varies among flowering plants. Flowers may produce only a few or many different essential oils.
The oils are heavier than air and often linger near the ground for this reason. They also evaporate readily in warm weather. The time of day a plant gives off an aroma is frequently tied to the time pollinators for that plant are active.
Usually, the physical construction of a particular flower is also created to deposit scent on visitors. The attractive aroma can travel along with the pollinator and lead others back to the area or the plant itself.
Attracting pollinators is the goal of aromatic plants, whether the scent produced is of rotting flesh or the sweetest perfume. Pollinators include birds, bees, butterflies and even some mammals. Plants depend upon these creatures because the visitors come into contact with grains of pollen and then carry those grains along with them as they visit other flowers.
As they brush against or shed pollen grains near the sticky surface of the stigma, the pollen adheres. The pollen grain may then potentially grow to reach one of the plant's ovules and achieve fertilization, creating a seed.
Many plants have co-evolved with certain creatures. Their pollinators develop specific features or body construction which allow them to best pollinate a particular plant. One example of this is the hawk moth (Xanthophan morgani praedicta) and the Malagasy orchid (Angraecum sesquipedale) of Madagascar. Plants may also use their scent to drive away insects that may cause damage to the plant.
Many plants capable of self-pollination have adapted to both produce their own pollen and be receptive to pollen at different times to maximize their opportunities to gain new genes.
Plants may decrease or stop producing their scents once they have achieved fertilization. Fertilization triggers the release of a hormone which works to stop the genetic process leading to scent. Pollination alone is not enough to inhibit scent.
Plants invest energy in scent production and by ending the process when the goal has been achieved, they conserve this energy for use in creating fruit or seed and for use in other vital processes.