The theory is that cactus spines evolved from modified leaves or modified bud scales which are also modified leaves except spines do not contain any of the cells or tissues characteristic of leaves and leaves lack all of the features characteristic of spines. Spines are made up of a fiber core surrounded by sclereid-like epidermis cells. See Cactus Spines from the University of Texas. In short, the evolution of spines is still a mystery.
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