Spines are leaves that have been modified into cylindrical, hard structures with sharp ends. They are occasionally called thorns, which is incorrect (thorns are modified branches or stems). Similar to prickles and thorns, they are used by plants to protect themselves from herbivores.
References
Esau, K. 1965. Plant Anatomy, 2nd Edition. John Wiley & Sons. 767 pp.
Llamas, K. A. 2003. Tropical Flowering Plants. Timber Press, Portland. 423 pp.
