A question I am often asked is: Are orchids scented? Many people tend to think that orchids are not fragrant. But in fact, it is believed that as many as 75% of all orchids are scented. That is, they emit detectable chemical compounds and some of them are extremely fragrant.

Scent is a mechanism that some orchids use to attract their pollinators. Unlike animals, most plants (except for some primitive ones) are unable to move sexual cells from one individual to another. Instead, they rely on agents such as wind, insects, birds or other animals as pollinators.

The pollinators are often attracted to the plants by floral colours, shape and fragrances. Of these attractants, fragrances are some of the most reliable and specific. The only attractant that is useful at night is fragrance.

Insects are the most important pollinators of orchids. Because of the nature of their compound eyes, insects have great difficulty in seeing the colours of orchid flowers from any great distance, yet insects often have preference for flowers of specific colours. It is believed that when an insect approaches a flower attracted by a specific fragrance, it ultimately gets close enough to be guided visually to a successful landing.

Human nose can only detect some of the complex odoriferous compounds released by orchids and other flowers; what smells good to the pollinators may not necessarily appear so to us. Individuals interpret scents differently, and very often will describe a given fragrance variously.

In terms of function, fragrant compounds can be classified into three categories. First, these compounds are a food source for the pollinators; they are usually associated with nectar, a blend of highly nutritious water-soluble sugars which is eagerly sought by birds, bees, butterflies and moths. Secondly, some fragrant compounds are sexual attractants. These compounds are highly complex and often pollinator-specific. The fragrance is often interrelated with shape, colour, and movement of accessory features of the flowers. Lastly, some of these compounds act as a general attractant.

All floral parts from sepals and petals, calluses and basal spurs, to petioles are known to be able to produce fragrances. Fragrances are produced in specialized glands called “osmophores.” Osmophores can be distributed all over a flower as a general attractant or they can be confined to certain regions of the flower to which pollinators are attracted, causing them to inevitably remove or deposit pollinia in the process.

Fragrance production consumes energy. In order to use the least energy to achieve the maximum effect, the timing of scent production often coincides with the time of visitation of the pollinators. For example, bee-pollinated flowers are fragrant early in the day when bees are most active. The timing of fragrance production is so specific that most bee-pollinated orchids are not fragrant after the morning and are less scented on cloudy days.

Because orchid-pollinator interaction is so specific, orchids that are pollinated by a specific group of pollinators often possess some common characteristics. For example, bee-pollinated orchids are (to most humans) pleasantly scented and have bright colours of orchid purple, violet, blue, yellow and green. Other characteristics of the flowers also help to guide the pollinators to a perfect landing. For example, orchids that are pollinated by bees often have a prominent lip with coloured nectar lines and osmophores at the base of the lip. Birds rely more on their superior vision to locate the flowers than insects. It is therefore not surprising that bird-pollinated orchids are often brightly coloured, but not fragrant.

Orchids pollinated by butterflies are generally similar to bee-pollinated orchids, except that butterflies can perceive some shades of red. Visual cues are probably more important than fragrance, but many flowers pollinated by butterflies are also fragrant.

Moth-pollinated orchids on the other hand, produce scents at night when their pollinators are most active. They also have plenty of nectar and are generally white or light green in colour. Examples can be found in the genera Angraecum and Brassavola. Most of these orchids have a prominent lip to allow moths to locate and land when they follow the scent to the flowers.

Here I would like to introduce you to some of the fragrant orchids that are displayed at our National Orchid Garden located in the Singapore Botanic Gardens.