What does "plated" mean here?

Context:Second, almost every chemical tested is toxic to isolated, naked, literally defenseless cells in petri dishes. An elevated concentration of table salt, for example, causes plated cells to wither and die; many are likewise sensitive to small changes in pH. This situation is very different from an intact, living organism: Animals have evolved elaborate defenses against the millions of chemicals present in the environment that can harm cells. The first line of defense is as simple as their skin, and the cells that line the gastrointestinal tract constitute a similar barrier. Bt proteins cannot penetrate those cells, so other cells and organs in intact animals are not exposed to Bt proteins. This fact — which Séralini conveniently ignores — has been known for decades.More:http://www.forbes.com/sites/henrymiller/2012/02/22/the-science-of-things-that-arent-so/