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glochid or glochidium: small barbed hair (modified spine) found only on members (like prickly-pears and chollas) of the cactus subfamily Opuntioideae.
spine: a modified, sharp, pointed leaf
Glochids or glochidia, which grow in tufts, are prominently visible (and felt) by gardeners weeding or otherwise working around prickly-pear cactus.