I discovered an Cactus Apple or Engelmann's Prickly Pear (Opuntia engelmannii) growing on top of a dead Saguaro (Carnegiea gigantea).

Two common ways a Saguaro can be killed are by being struck by lightning or by suffering bacterial rot. Engelmann's Prickly Pear cacti can sprout in very strange places like this one, such as the debris-filled crotch of a live Saguaro's arm or on a dead tree stump. I don't know how long this particular Prickly Pear's growing spot will remain standing, but when it falls, the Prickly Pear can root again from the base of any stem pad that touches the ground.