Collier's New Encyclopedia (1921)/Thorn (plant)
THORN, a sharp conical projection constituting the growing point of a branch which has proved abortive. That this is its origin is shown by the fact that sometimes trees, which are thorny in their wild state, have their spines converted into branches when long cultivated in a garden, as is the case with the apple and the pear. A thorn differs from a prickle, which is so superficial that it comes away when the bark is peeled off, while in similar circumstances a thorn, being deep seated, remains. Sometimes thorns bear leaves, as in the whitethorn.