London

WHEN Eileen and Derrick Severs disappeared suddenly from their home in central England, the police suspected murder. But they had no bodies—and no case against the chief suspect, the couple's son Roger. After a massive and fruitless search, police faced the daunting prospect of dredging Rutland Water, the largest man-made lake in Western Europe. First though, they called in the botanist.

A botanist? You'd better believe it. Detectives can get crucial leads from plants and their fragments. Sawdust in a safe-cracker's turn-ups, roots growing through a grave and pollen trapped in a stash of cannabis—all can link a criminal with the scene of the crime, show when a body was buried or link two batches of drugs to the same source. Pollen might even have laid to rest the lingering doubts over who killed O. J. Simpson's ex-wife.

Outside New Zealand, where forensic botany was pioneered, police rarely call ...