Plants have both male and female parts in their flowers – the male part is the stamen, which produces pollen. The female part is the stigma and style (together called the pistil); ovary and ovule; seeds and fruit develop in the female part of the flower. For a seed and fruit to develop pollen has to be transferred from the stamen to the stigma, this process is called pollination.
Self-pollination occurs in a minority of species, the stamen and stigma contact within the same flower, pollination occurs and seeds and fruit are produced. However most plants are cross pollinators; pollen from one plant, must be transferred to the stigma of another genetically different plant of the same species, in order to produce seed and fruit.
Since plants are unable to make any deliberate movements, they must involve another agent or organism to move and transfer pollen from stamen to stigma.
Many species of grasses, conifers, deciduous trees and a proportion of flowering plants use wind as a method of pollination. Aquatic plants use water to move pollen.
The vast majority of plants use other organisms for pollination, these organisms are called pollinators – most are insects, but birds and bats can also be pollinators. This type of pollination occurs when pollen first sticks to the pollinator, who then transports and rubs the pollen off onto the stigma. Pollination is inadvertent rather than deliberate, so plants have to possess a strategy to make the pollinators work for them.
To attract a pollinator, plants have to signal their location and readiness, provide an incentive for the pollinator to visit, ensure that the pollinator contacts and retains the pollen, and encourage pollinators who visit their own species and exclude those who do not.
The main incentive for pollinators to visit flowers is to obtain food – sweet, protein-rich pollen may be the food source. A plant may also produce a food called nectar; it consists mainly of sugar.
Two major cues plants use to communicate and advertise to pollinators are through the scent and colour of flowers.
The creation of fragrances in flowers is a complex biochemical process by which genetically coded enzymes convert a wide variety of compounds into volatile biochemicals, which evaporate into the air and produce fragrance.
Most fragrances advertise the availability of pollen and nectar. Some fragrances attract under false pretences, such as the Australian Broad-lipped orchid, which attracts pollinator male wasps by mimicking the scent of female wasps or the corpse flower, which attracts pollinator carrion flies and beetles, by emitting an odour of decaying flesh.
The process of pollination is vital to all forms of life on Earth but it is also vulnerable to human disturbance through pollution, habitat degradation and the loss of pollinators.
Scientists have discovered that the fragrances of some flowers activate genetic activity and blood chemistry that repress the symptoms of stress. Another good reason to smell the roses.
- Written by Nadia O'Carroll