The Arizona state tree, the Palo Verde tree, has green bark because the leaves are not big enough to provide adequate food for the tree. The bark helps in photosynthesis.
Saguaro cacti (the big ones with arms reaching up to the sky) usually grow in a bush or small tree in the beginning so they will be protected from the elements and have a better start. They then out live their "nurse trees" and stand tall and alone when they are more mature. Their roots stem out from their base 100 feet, but are only a couple inches below the surface of the ground.
There are many different types of chulla cacti...teddy bear chulla, chain fruit chulla, etc. However, all of them are painful if touched a have a tendency to "jump" out at a passerby because the slightest brush up against them will cause them to dislodge part of the plant.