Definition: Spine

Spines are the ends of branches or leafs, that have been modified into rounded, hard structures with sharp ends. They are often also called thorns, which are reduced, sharp pointed stems.

Spines are used by plants to protect themselves from herbivores. Some plants with spines and animals that feed on them, have co-evolved in response to each other, with some plants having very long spines and the animals that feed on those species having long tongues to reach past the spines to feed on the leaves.

The spines of different cactus are leafs that have been completely transformed. The sharp Long thorns of the hawthorn, the needles of a cactus, and the prickles of a shrub like the rose are all spines. Although spines generally serve as a passive defense mechanism, in some species they can be hollow and contain poisonous substances that cause lasting pain or even paralysis, and in others, may be barbed and detach readily, sticking to whatever brushes against them.

Botanists use several terms somewhat loosely when referring to spine- or needle-like structures on plants; however, the following differences are typically distinguished:

• Prickle – a sharp outgrowth from the epidermis, also called an emergence and usually involving some subdermal tissue as well.
• Spine – a modified stipule or sharp branchlet found in a leaf axil or on the margin of a leaf.
• Thorn – Sharp outgrowth from a stem other than at a node; a modified stem.