how does a cactus grow on a oak tree?

Answer 1

do you want to start one or have you seen one somewhere,i've seen ferns & orchids thriving on large trees,some have naturally set root there others have been coaxed to gro w on the tree for ornamental display,a depression or deep crack that will give a foothold as well as provide moisture & nutrition are the basic requirments,natural dispersal of seed in suitable climatic conditions result in the plant setting root,to transfer a plant you need to secure it in place till it has taken root.sorry got away from your question,have never seen a cactus .flourising on a tree

Answer 2

It sounds like a riddle.