The Columbia Encyclopedia, Sixth Edition. 2001-07.
thorn, in botany
sharp-pointed projection on some plants, usually protective in function. Botanically, thorns are distinguished as modified stems (as in the honey locust and hawthorn) from spines, which are modified leaves (as in the barberry), and from prickles, which are epidermal outgrowths of the bark (as in the rose and blackberry). Cacti have both thorns and spines.
