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Fragrances and Flavours - Floral scents and pollination

’A rose by any other name would smell as sweet” - Romeo and Juliet

EnvironmentGardening
By Patrick Campbell

Sunday 18 November 2018, 10:00AM


Have you ever paused to think why so many flowers smell so sweet? In bald terms, it is because they produce volatile chemicals, often in the form of essential oils, which evaporate into the air and produce these delicious scents.

It is generally assumed that these scents initially developed to deter grazing animals; now, eons later, and as a consequence of the evolutionary process, specific fragrances act positively to attract pollinating insects.

The biological purpose of these scents is to proclaim the fact that food in the form of nectar or pollen is available. Come and get it… They do. Insects can make out floral scents at a distance; the hummingbird hawk moth, for instance, can detect them up to a kilometre away.

Plants tend to have their perfume output at maximum levels only when the flowers are ready for pollination. Those that up their output during the day are pollinated primarily by bees and butterflies. Those that release their scent at dusk or nightfall are pollinated mainly by bats and moths.

For example, nicotiana, tabernaemontana and cestrum nocturnum (which blooms at night), release their perfume at the end of the day. Scents are also more pronounced after rain.

If the symbiotic principle has worked, then the nectar-seeking insect will take some pollen on its body when it leaves the flower, and will deposit some of these grains on the female stigma of the next one it visits, normally of the same species. In this way, cross-pollination takes place.

Sometimes the insect is duped into raiding a plant for nectar that does not exist. The sphinx moth, one of few insects that can reach down into the plumeria’s long flower throat, goes from blossom to blossom in the fruitless search. For despite the plumeria’s wondrous scent, it harbours no nectar. So the moth searches in vain, but inadvertently pollinates the flower.

Insects, and especially butterflies, are capable of distinguishing between complex scent mixes, though ironically they are not at all aware that they are performing a crucial self or cross-pollinating function.

Incidentally, self-pollination requires distribution of pollen only from one flower’s anther to its sticky female parts or stigma. The cross pollination process involves transference to another flower’s stigma of the same species.

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Specific flower fragrances attract specific pollinators; indeed, the scent cues are almost certainly much more important than the visual ones, a point expanded in the next section.

These cues are not always sweet-smelling. A group of plants known as carrion flowers derive their name from their unpleasant odour. Both the huge titan arum lily and rafflesia arnoldii, which boasts the world’s largest bloom (one metre across), attract scavenging beetles and blow-flies that normally feed on, or lay eggs in, rotting flesh.

One orchid, the bulbophyllum, is fly pollinated, and produces flowers which smell of bad meat. When in bloom, it is surrounded by clouds of tiny flies. All these scents are mundanely based on identifiable chemicals such as methyl jasmonate (jasmine) or phenylpropanoids (cinnamon and nutmeg).

Yet most floral fragrances are sweet-smelling. While the rose tops the list of delicately perfumed temperate flowers, along with the lily, violet and lavender, many tropical species such as jasmine, plumeria, gardenia, calatropis, champaca, ylang ylang and buddleia are wonderfully odiferous.

The jasmine flower, perhaps the most widely used for perfume in the tropics, must be harvested as a bud just before dawn when its fragrance is at its peak.

Interestingly, the quest for yet more colourful and more durable cultivars sometimes results in a loss of perfume. The old fashioned variety known as the damask rose – which is mentioned by Shakespeare – is still cultivated in Turkey and Bulgaria for its rose oil (rose otto), and is widely used in the perfume industry because of its intense scent. But in appearance it is relatively ordinary, with small pink blooms.

But you can’t have it both ways. My father used to bemoan the fact that some of his newest and most exotic hybrid tea roses, sumptuous in appearance, had lost most of their smell in the hybridist’s quest for longer lasting blooms and, above all, greater visual appeal.

Stand corrected, Mr Shakespeare. “A rose by any other name” does not necessarily “smell as sweet”.


Patrick has been writing for ten years about gardening in Phuket and allied topics. If you have horticultural or environmental concerns, please contact him at drpaccampbell@gmail.com. Many of his earlier creative and academic publications can be found at Wordpress: Green Galoshes.

 

 

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