Arkellis

The Arkellis flower was a flower native to Skaro. It had evolved to survive in the harsh irradiated environment of post-war Skaro, only taking root in metal. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)

The sap from this flower was used to infuse some Dalek casings. The first Dalek Emperor's casing was enhanced with Arkellis sap. (COMIC: Genesis of Evil) The Cult of Skaro were also enhanced with Arkellis sap; by that time, the flower had become extinct. (PROSE: Birth of a Legend)

Behind the scenes
The Arkellis flower was introduced in The Dalek Dictionary, which first established the idea that the Arkellis is rare and would only take root in metal.