Spines are the ends of branches or leafs, that have been modified into cylindrical, 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 and Fouquieria are leafs that have been completely transformed. The spines of cacti also help in saving water by reducing the rate of transpiration. In Black Locust the spines are modified stipules. 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.
Plant spines and thorns
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; see also hair.
- 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.
- The seta (bristle) is a similar plant structure.
There are a number of different terms used to describe spines and plants with spines:
- Spinescent - Meaning spiny or tapering like a spine.
- Spinicarpous - Having spiny fruit.
- Spiniferous - Bearing thorns.
- Spiniform - Like a spine or thorn.
- Spiniger - Producing spines or thorns.
- Spinose - Spiny.
- Spinule - A very small spine or prickle.
- Spinulose - Having small spines or thorns.
Thorns and prickles, most notably those on roses, are common literary symbols for the hidden dangers or woes of something beautiful or pleasant, as in "Every rose has its thorn". Roses lack true thorns since their prickles emerge from the epidermis rather than the pericycle. Growth from the pericycle would make it a modified stem and therefore a thorn. Some roses have been bred not to have prickles.
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.
