Arkellis

Arkellis was a flower native to Skaro. The sap from this flower was used to infuse some Dalek casings. The first Dalek Emperor 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
According to The Dalek Dictionary, the Arkellis is rare and would only take root in metal.