his story begins with a cliff-hanger. On the Spanish side of the Pyrenees mountains, around 850 metres above sea level, two adjacent cliff faces hold the entire population of Borderea chouardii – one of the world’s rarest plants. It’s a small herb that grows into crevices in the rock. Its leaves are heart-shaped and its flowers green and unassuming. There are around 10,000 individuals here, all growing on a square kilometre of vertical rock.