Deciding on the prime time to pick a bean pod is s tricky business. Leave it too late and the pod will become tough and stringy - and the reason for that is because as it grows the pod begins to prepare to shed its seeds. Members of the pea family - the leguminosae - carry their seeds in pods that naturally become brittle when they dry and ripen, when tensions developed along the suture between the two pod halves and in the pod wall eventually become so great that the pod splits open violently, hurling out the seeds. Plant breeders have worked hard to breed this trait out of legume crops, but species like runner bean still produce long strands of woody, lignified cells in their pod walls as they ripen. In this fluorescence micrograph, showing a cross section of the upper suture of a developing pod, the bright yellow arcs of cells at the top are the 'strings' that you need to strip out of the pod before you eat and cook it if you've left it too long before harvest. The yellow cells just creeping into the picture at bottom left belong to the parchment layer that develops in the pod wall. Together, these thock-walled cells develop the tensions in the pod as it dries that will eventually split it open along its longitudinal sutures and release the seeds.