Why do some saguaros form crests?

Photo by Pam Bridgmon 10/31/2013

Pam sent in this photo of a wonderful crested Saguaro (Carnegiea gigantea) located in her neighborhood. We don't yet have a definitive answer why some saguaros form crests, but I'm partial to the cancer theory. As with all living things, the longer you live, the more your cells have divided and continue to divide. The more often your cells divide, the more opportunity for errors to occur. As with all living things, the bigger you are, the more cells you have. The more cells you have, the more you need to replace. Saguaros like the one in the photo are both big and old. Note, too, that the crest (or cancer) is occurring at one of the growing tips.
Some cancers are caused by exposure to certain toxins (e.g., in humans, lung cancer from cigarette smoke). I think that the many crested barrel cacti in my neighborhood are examples of this type of cancer. Yes, it does make me wonder about the soil in my neighborhood!
Interesting summary of George Johnson's recent book "The Cancer Chronicles: Unlocking Medicine's Deepest Mystery."