|Photo by Marty Horowitz 12/18/2013
This beauty is a crested barrel (Arizona Barrel Cactus, Ferocactus wislizenii). You've probably seen a crested saguaro or two. (The site has loads of photos of crested saguaros. And there's even a Crested Saguaro Society!) Other cacti can form crests, too. Barrel crests form for the same reason as saguaros crests; namely, cell division gone awry. Like saguaros, barrel cacti can be big (okay, not as big) and long lived. The more times cells make copies, the more opportunities there are for mistakes to occur. Mistakes in cell division may be brought about by contaminants; by viruses; and by other environmental factors. But age and size are risk factors in living things.