Ask Mr. Smarty Plants

The sharp projections found on plants, which are used mainly as a defense against animals, have several different names and origins. Botanists identify three major kinds: thorns, spines, and prickles. The thorns are modified stems ending in a short point. They originate from axillary buds at nodes (where leaves or branches arise). Texas hawthorn (Crataegus texana) is an example of a plant with thorns. Spines are modified leaves or parts of leaves. The American holly (Ilex opaca) and agarita (Mahonia trifoliolata) have leaves modified with sharp projections from the edges. Cactus spines are modified sharpened petioles (leaf stalks) without the leaf blades. Prickles are woody outgrowths of the epidermal tissue or bark and occur in irregular patterns between the nodes. Prickles are often recurved and can serve as support for vines as well as a defense mechanism. Prickly ash, or Tickle-tongue (Zanthoxylum hirsutum), in the Family Rutaceae, and raspberries and other members of the rose family (Family Rosaceae) have prickles.

Additionally, many seeds and seed cases have hooks and barbs that enable them to attach to clothing or animal fur and help to disperse the seeds. Two examples are the Canada cockleburr (Xanthium strumarium) and the devil's claw (Proboscidea louisianica).

You can read more about "Prickles, thorns, spines and their relations" on the National Council for the Conservation of Plants and Gardens web page.