Arkellis

Arkellis was a rare flower native to Skaro. Now extinct, it would only take root in metal. (DWA: The Dalek Book)

The sap from this flower was used to infuse metalert Dalek casings. Due to the rareness of this substance, the only Daleks known to have used it are the Dalek Emperor and Dalek Sec. (DC: Genesis of Evil, DWF: Birth of a Legend)