Cacti have a long and quite successful history of growing out of the ground.
Sinking roots into dirt just seems to work.
Now and then, though, a free-spirited cactus takes a very different approach.
One example: a prickly pear cactus growing out of the trunk of a mesquite tree in Catalina State Park north of Tucson.
Look Ma, no dirt!
Many desert dwellers have seen cacti growing in unlikely places such as a gutter. But in such cases, the gutter usually has accumulated at least a thin layer of soil that sustains the plant.
How does this tree-dwelling prickly pear get by?
"The cactus almost certainly does not graft to the host plant, but its roots simply cling to the surfaces and gather rainfall," said Mark Dimmitt, director of natural history at the Arizona-Sonora Desert Museum.
"I've seen several prickly pears growing out of the crotches of saguaros and large trees," Dimmitt said.
"Cactus roots can absorb large amounts of water in a short time, and the cacti can then survive for many months with no rain."
Dimmitt said the Catalina Park example, pictured with this story, is one of the largest he's seen.
"Perhaps the tree has a hollow that collects water," he speculated.
The prickly pear is visible along a stretch of the short Canyon Loop Trail in the park. Etiquette calls for enjoying the sight - but not touching or harming the cactus.
To reach the park, drive north out of Tucson on Oracle Road, which becomes Arizona 77. Continue to the Catalina State Park entrance at milepost 81. Admission is $7 per vehicle. The loop trail begins at a trailhead at the end of the main park road.
Contact reporter Doug Kreutz at firstname.lastname@example.org or at 573-4192.