Saguaro boot

A saguaro boot is the hard shell of callus tissue, heavily impregnated with lignin, that a saguaro cactus creates to protect the wound created by a bird's nesting house. The bird pecks through the cactus skin, then excavates downward to hollow out a space for its nest. When the saguaro dies, its soft flesh rots, but its woody infrastructure lasts much longer. So does the hollowed-out callus whose roughly boot-like shape gives it the name of "saguaro boot."

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