Prof Kathy Willis concludes her major new history series by asking how much plant biodiversity is worth, and examines new research into securing the future of our staple crops.

Understanding the distribution, diversity and potential of plants for food, lay at the heart of the 18th century botanical impresario Joseph Banks' vision to "improve Britain's estates of the world". To secure future resilience of crops in today's world there's a growing need to conserve the closest wild relatives of our staple crops.

Kathy Willis discovers, given climatic threats to some of our most substantial crops such as coffee - for which the industry currently depends on a single species, the economic value of wild relatives of today's domestic crops is considerable.

And as we hear, some important future crops are still to be found from previously overlooked plants.

With contributions from Richard Thompson, Business valuations partner at Price-Waterhouse Cooper; historian Jim Endersby; head of coffee research at Kew, Aaron Davis; Kew's head of yams Paul Wilkin.

Producer Adrian Washbourne

Music for the series was composed by Mark Russell.