NEW YORK - The botanists forged through a thicket and crossed a rocky ravine bed. Their quarry, Polygonatum pubescens, was proving elusive. But they would not rest. They had promised a shipment of its seed to their partners in a bunker across the ocean.

Just beyond a derelict chain-link fence, a man with a beard spotted pea-sized green balls dangling from a miniature awning of shiny oval leaves.

“There’s your plant,” he called out to another bearded man.

On a wooded Staten Island hillside last Wednesday, a few hundred feet from a busy road, the botanists from the city’s native plant center scored another find for a project called the Millennium Seed Bank.........