As biodiversity declines, locating and preserving the planet’s plant life has become more important. The Millennium Seed Bank in Wakehurst, West Sussex, has been doing just that for 25 years, collecting seeds, storing them and holding them in trust in countries all over the world in case they are needed. To commemorate the anniversary, Patrick Greenfield toured the site. Madeleine Finlay tells about the journey a seed takes from arrival to cold storage, and how some are already helping to return endangered plant species to the wild