Saguaro cacti (Carnegiea gigantea) respond to injury by secreting callus tissue. The secretion molds itself to the shape of the injury, whether a deep hole made by a woodpecker or a shallow tunnel made by a moth caterpillar. Callus tissue hardens into a corklike layer that remains in place through the life of the cactus, protecting it from invasion by bacteria and fungi. The "Saguaro boot" becomes the nesting cavity for woodpeckers, owls, martins, and other desert birds. The cavity remains relatively cool during the day and warm at night, offering relief from the desert's temperature extremes. Tucson, AZ.