One thing we did not see when we went to Nevada was any cactus. I am not sure why. It is generally thought that cactus, as a distinct, New World family – Cactaceae – originated in Mexico and radiated out from there. The family has colonized area pretty far into South America but not as far into North America.
It is not that North America is too cold. The Andes have lots of cactus and the plant shown in the picture above lives at an elevation of over 8,000 feet – actually this particular plant lives in my greenhouse, but it's ancestors came from an area in Bolivia at 8,800 feet.