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The mating grain

These beautiful spheres ensure the survival of a species. Peter Cotgreave looks at the art and science of plant reproduction

Many people avoid pollen because it triggers the painful and annoying symptoms of hay fever, but this photograph, taken through a powerful electron microscope, shows just how much beauty and complexity there is in each individual grain of pollen. It is not surprising that we find it difficult to appreciate something we can hardly see: you would need to line up 20 grains of this knotweed pollen in a row to make the thickness of a full stop.

Unlike animals, plants have to rely on something other than their own mobility to transport the male genetic material to the female. They rely on natural elements such as wind or water, or the accidental intervention of animals such as insects or hummingbirds. Pollen grains have rough or furry surfaces, partly so that they can stick to insects, such as the fluffy bumblebees that can be seen in summer covered in bright yellow specks of pollen.

Flowers have evolved as colourful advertising billboards, letting an insect know that there is a meal of nectar awaiting. While enjoying a free lunch, the insect unwittingly picks up some pollen and carries it to the female part of another flower, known as the style. If a pollen grain lands on the style of a suitable flower, it sends out a tiny shoot to carry the male genes into the egg, which eventually develops into a seed.

For every successful pollination, millions more grains of pollen never reach the female part of a suitable plant: they land in soil, get eaten by insects, or are blown up your nose. If one lands on the style of the wrong kind of plant, it cannot send out its shoot, because each plant species produces pollen with a different arrangement of structures sticking out from the surface - the knotweed pollen is quite complex - so the pollen grains and styles can recognise when they are incompatible. If this grain of knotweed pollen landed on the receptive female part of a rose or a daisy, its only chance of success would be to sit tight, on the off-chance that another insect might brush past and it could then possibly be carried to the style of another knotweed flower.

Peter Cotgreave is a biologist and director of Save British Sciencewww.savebritishscience.org

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