Spines, prickles and thorns are common terms bandied about when referring to sharp objects protruding from plants. There is a distinction to be made between these three terms. Spines denote a structure that is evolutionarily derived from a leaf or part of a leaf (cactuses have spines). Thorns circumscribe modified shoot systems (honey locust trees have thorns). Prickles make up the rest of these plant defensive structures as outgrowths of the skin (epidermis) and underlying tissues (roses are a great example).
Today, prickles are on the agenda, with Aralia chinensis, the Chinese angelica tree (291-2011*A). The stems have amazingly sharp prickles. Bottom left is the tip of the now dormant shoot system. Bottom right is a shot I took this summer of additional prickles growing out of the compound leaves. In the upper photograph, the broad U-shaped area with a series of circular spots is a leaf scar (where the leaf attached to the stem before it abscised in the fall). The circular spots are the scars of the vascular bundles that carried water into and sugars out of the leaf. Above the leaf scar, is a lateral bud covered by another patch of prickles. This is one seriously well protected plant – and great to enjoy in the winter in the Leventritt collection of vines and shrubs.
-Ned Friedman, Director of the Arnold Arboretum