This photo shows saguaro ribs. Saguaros are adapted to the intense desert sun. On the north side of cacti, the ribs are wider apart. On the south and west, they are closer together to provide a more shade for the fleshy surface.
Even the cacti can get sun burned. Whenever we buy a cactus, we examine the ribs to see which side should face west, otherwise they may die if not positioned properly.
We have travel photos posted at Viva la Voyage from our wonderful trip to Atacama, Chile.