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 rarity of this substance, the only Daleks known to have used it are the first Dalek Emperor (TV21: Genesis of Evil) and later Dalek Sec. (DWF: Birth of a Legend)