This is a saguaro, the unmistakeable icon of the Sonoran desert of southern Arizona and northern Mexico. I think they would be perfect for a beginner's life drawing course.
Cholla species come in a variety of forms, each one nastier than the next, at least if you walk into one. The staghorn species is a pleasant purplish colour, with interesting fruits that stay around for a long time. But beware the teddy bear - it's the nastiest of all.
Barrel cactuses have the curious habit of leaning towards the south - convenient if you happen to be lost in the desert when it is too cloudy to see the sun and too foggy to see any mountains. Which is fairly unlikely. Apparently the number one natural cause of death of barrel cactuses is falling over. The evolutionary advantage of this is a bit obscure.
And finally, one wannabe cactus, the ocotillo. This shrub mostly stands around looking like 10-foot-tall dead sticks, until there's enough rain for it to produce little leaves for a few weeks. It's thorns are a little less deadly than cactuses', but still not the sort of thing to bush-whack through. It was the only thing flowering in the desert in February, but it was doing so in a spectacular way.