Every plant to its own Science

Secrets of pollination

When we think of flowers, sweet-smelling jasmine garlands and rose bushes come to mind. The largest single flower on earth, however, has a nasty smell. It smells of a corpse.

The corpse flower (see pic to the right), Rafflesia arnoldi, is the national flower of Indonesia and can grow to a diameter of one metre and weigh up to 11 kilograms. The Indonesian name is in fact “padma raksasa” (monster flower)! Why do these flowers stink?

Flowers are the parts of the plant involved in reproduction. Pollen from the male part of the flower has to get transferred to the stigma, the female part of the flower, during the process of “pollination”. After pollination, fruits form and later seeds, which grow into the next generation. In short, to have babies, plants need to get pollinated.

Pollination can happen through different agents, which act as a connecting link between the pollen and the stigma. Insects, animals, birds all play their part. Plants design their flowers so that particular agents, the “pollinators”, find them attractive.

Rafflesia’s pollinator of choice is the fly. Yes, the annoying creatures you chase all the time. Flies like smelly things. The Rafflesia flowers remain in the bud stage for months and then bloom for just a few days. Within that time, flies have to find at least two flowers and pollinate them. Nastier the smell, more the number of flies, more pollination! And that’s why the flower stinks for miles in the rainforests of Indonesia.

Made for each other

Bees pollinate the flowers of a whole range of plants that humans cultivate for food — fruits like apples, spices like cardamom, onion, coffee, coconut — its a long list. Bee pollinated flowers bloom during the day, when bees fly to look for food. They contain nectar, a sugar rich liquid that bees love. Bees also feed on pollen. Nectar gives the bees energy while the pollen provides them with proteins.

The flowers are brightly coloured, with a mild, pleasant odour. A combination of the colour and the scent attracts the bees, which land on the flower.

Some flowers have lines on their petals, which are like a route map showing the bee where the nectar is. As the bee walks into the flower toward the nectar, pollen gets stuck on its hairy body. Bees even have little pouches on their hind legs to carry pollen, called the “pollen basket”! When the bee visits the next flower, pollen gets rubbed off on the sticky stigma and the flower is pollinated.

Some of you may have noticed that some flowers remain closed during the day and bloom during the night. These are pollinated by creatures of the night, like moths and bats.

Moths are attracted to purple, pink or white flowers, which have a strong, sweet smell.

Tobacco is pollinated by moths. Bat pollinated flowers have a strong, musty smell and are bowl shaped; pollen gets stuck onto the bat’s face as it drinks the nectar.

Some birds live off pollen and nectar, too. Sunbirds, with their characteristic curved beak, bright colours and chirpy calls are an example. The bird pollinates tubular flowers like those of Loranthus.

The flower remains in the bud stage until the bird applies a little pressure, almost squeezes the bud. This makes the bud open, and the sunbird puts its bill inside to sip nectar before it moves to another flower, pollinating in the process.

Some plants keep it simple — their flowers are pollinated by wind. They produce plenty of light pollen, which get blown into the wind; the stigma are feathery or net like, to catch the drifting pollen.

Sugarcane, grass and all the main cereals we grow (wheat, maize and rice) are pollinated by wind.

Give and take, but not always

A lot of the associations between flowers and their pollinators have evolved over millions of years, such that one cannot exist without the other. This reciprocal relationship is called “mutualism”.  

An extreme example of pollinator mutualism is the bee orchid. Each flower of this species looks like the female of the bee that pollinates them! They even emit an odour similar to the female bee, which male bees can sense from afar. When the male bees try to mate with the flowers, they get pollinated. For one species of bee orchid, Ophrys apifera, the bee species has unfortunately gone extinct. Still, we can get an idea of how the bee would have looked — because the orchid is the exact image of the female bee. “The only memory of the bee is a painting by a dying flower” (Read the comic that quotes this >here)

There are others who exist by exploiting this relationship — they steal nectar and pollen without pollinating the flowers in return. Some insects and birds are “pollen thieves”, which make a hole at the base of the flower and drink the nectar without stealing pollen. (See an exciting animation about these “pollen thieves” >here)

Sandhya is a science journalist who writes about weird and wonderful creatures. You can write to her at sandysek@gmail.com or read more on her website ‘The Melting Pot’ at www.sandhyasekar.com

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Printable version | Oct 20, 2021 8:19:35 AM | https://www.thehindu.com/in-school/sh-science/secrets-of-pollination/article6366768.ece

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