With some 7 million dried plant specimens, the herbarium at Kew Gardens in London is one of the largest collections of its kind in the world. Building on the Linnaean system, through Darwin to DNA, scientists there have traced the relationship between plant life-forms and the timeline of their development over the ages – from algae to mosses to flowers. While the plant family tree is thought to be 95 per cent complete, this short documentary reveals that continuing to study plants gives us an important framework for asking questions about how our ecosystems actually work.