In plant morphology, thorns, spines, and prickles, and in general spinose structures, are hard, rigid extensions or modifications of leaves, roots, stems or buds with sharp, stiff ends, and generally serve the same function: physically deterring animals from eating the plant material.

What does it mean when someone calls you a thorn?

: a person or thing that repeatedly annoys someone or something or causes problems He's been a thorn in my side for years.

What are thorns?

a sharp excrescence on a plant, especially a sharp-pointed aborted branch; spine; prickle. any of various thorny shrubs or trees, especially the hawthorns belonging to the genus Crataegus, of the rose family.