Cactus was the name given to several varieties of desert plants found on Earth and other worlds, which were usually green in colouration and covered with pointed spines. The Zolfa-Thurans were similar in appearance to cacti. (TV: Meglos)
Wilfred Mott likened the Vinvocci to cacti due to their spiky green faces. Such a remark was apparently racist. (TV: The End of Time)
In Aztec Mexico, Ixta slashed Ian's wrist with a cactus needle the First Doctor had given him for a fight. Ian's energy was drained by this. (TV: "The Warriors of Death")