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Why are plants so colourful?

Streptocarpe, © JBM (Gilles Murray)
Why are plants so colourful?

We are immersed in a world of colour. Colours influence our choices and make objects easier to spot and to recognize. In plants, colours are concentrated in strategic places that are essential to life.

“Floral” attraction

Flowers are beautiful to look at, but that is not their raison d’être. Plants surround their sexual organs with attractive, brightly coloured petals. In addition to providing protection from the sun, the pigments play an active role with regard to the flower’s visibility. Why do they want to attract attention? Flowers need to be fertilized in order to produce seeds. For this to happen, the grains of pollen — tiny capsules containing the sperm cells of flowering plants (angiosperms) and conifers (gymnosperms) — must travel towards other similar flowers. But finding a partner when your “feet” are firmly rooted in the ground is quite the challenge. To achieve its objective, a flower must capture animals’ attention and offer them a “food reward” in the form of nectar.

It attracts them with its colours and scent, with contrasting lines and dots serving as guides that insects learn to recognize. When the animals feed on the nectar — at the bottom of the flower’s corolla — they are covered in pollen, which they then unwittingly transport to other flowers. Narrow, tubular red or dark orange flowers attract hummingbirds; these are abundant in the tropics, where hummingbirds are important pollinators. Bees prefer white, blue or yellow flowers with wide corollas; their ultraviolet vision allows them to see things that are invisible to us.

Fruit to be dispersed

Before the seeds are mature, fruit is often green so it is better camouflaged. The change of colour that follows maturation then allows the fruit to be seen by fruit-eating animals, which will disperse the seeds.

Colourful leaves

Leaves may also become brightly coloured during blooming, to assist small flowers. This is the case of poinsettias and many bromeliads. Coevolution — that is, the simultaneous evolution of flowers and pollinators — partly explains the vast diversity of the plant world. If all these flowers were pollinated by the wind, our environment would be a lot less colourful! Leaves may also become brightly coloured during blooming, to assist small flowers. This is the case of poinsettias and many bromeliads.

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