HINT:

The observation was discovered in 1827 by a botanist looking through a microscope at pollen cavities suspended in water.

Email your answer to: competitions@cosmosmagazine.com with your name and address by 30 November. Three correct entries will win a copy of either:

Plant, Map or Universe by Phaidon. See review page 98.

If you are not one of the lucky winners you can purchase Plant, Map or Universe with a 30% discount using discount code CSM30 at the checkout at phaidon. com/cosmos