In the picture of the Tree Cholla at you may have noticed the slender, gray stems at the base of the cactus, arching to the left. You can see a close-up showing that plant's smooth, gray skin and short side-shoots below:
This is a typical resident of dry, gravelly ridges in the Chihuahuan Desert from New Mexico and western Texas south into Mexico. Called Leatherstem or sometimes Witch's Fingers, I think it's the same species I saw so much of in Big Bend National Park, JATROPHA DIOICA, during my backpacking days there. In Spanish the plant is sometimes called Sangre de Drago, or "Dragon's Blood," because when you puncture its skin a thick, clear fluid emerges, which soon turns red upon exposure to air.
This is another plant that's leafless most of the year, but after a good rain quickly sprouts leaves and looks very different from the photo. You can see a leafy plant at http://botany.cs.tamu.edu/FLORA/swts/euph001.jpg.
This species' genus, Jatropha, is worth knowing, for several remarkable species belong to it. In some parts of arid, upland Mexico I've run into "bottle plant" Jatrophas with enormously swollen bases. Jatrophas are often used as bonsai specimens. Currently the greatest interest in a Jatropha species focuses on Jatropha curcas, which is being grown extensively in India and elsewhere for the oil in its seeds, which can be used as "bio-diesel" fuel for engines.