Thorns, spines, and prickles

In botanical morphology, thorns, spines, and prickles are hard structures with sharp, or at least pointed, ends. In spite of this common feature, they differ in their growth and development on the plant; they are modified versions of different plant organs, stems, stipules, leaf veins, or hairs. In nontechnical usage, the terms may be synonymous.

Smooth, featureless Citrus thorn

Gymnosporia buxifolia thorn, its leaves, nodes, and emergence from an axillary bud demonstrating its nature as a branch

Carissa bispinosa showing characteristic branched thorns

The spines of Fouquieria splendens (Ocotillo) develop from the leaf petioles

Stipule spines on Acacia xanthophloea

Raised prickles on the stem of Caesalpinia decapetala

Rose prickles

Prickles on the leaves of Solanum viarum

Function

Agrawal et al. (2000) found that spines seem to have little effect on pollinators, which the plants need in order to reproduce.

Definitions and technical distinctions

• Thorns are modified branches or stems. They may be simple or branched.

• Spines are modified leaves, stipules, or parts of leaves, such as extensions of leaf veins.

• Prickles are comparable to hairs but can be quite coarse (for example, rose prickles), i.e. they are extensions of the cortex and epidermis.

Spinescent is a term describing plants that bear sharp structures that are not necessarily spines, thorns, or prickles in the technical sense. It also can refer to the state of tending to be or become spiny in some sense or degree, as in: "...the division of the African acacias on the basis of spinescent stipules versus non-spinescent stipules..."

Some authors prefer not to distinguish spines from thorns because, like thorns, and unlike prickles, they commonly contain vascular tissue.

Technically speaking, many plants commonly thought of as having thorns or spines actually have prickles. Roses, for instance, have prickles.

Evolution

It has been proposed that thorny structures may first have evolved as a defense mechanism in plants growing in sandy environments that provided inadequate resources for fast regeneration of damage. However, the suggestion was unsupported by any argument to discount the likelihood that spiny defences might have been developed as means of defence in say, resource-rich environments where herbivory might have been more intense than in the hypothesized sandy environments.

Not all functions of spines or glochids are limited to defence from physical attack by herbivores and other animals. In some cases, spines have been shown to shade or insulate the plants that grow them (e.g. saguaro cactus spines shade the apical meristem in summer and in members of the Opuntioideae glochids insulate the apical meristem in winter).

Morphological variation

Thorns, spines, and prickles occur in a wide variety of ecologies, and their morphology also varies greatly. They occur as sharpened branches (e.g. in Carissa, Citrus, Crataegus), spiky inflorescences (e.g. in Tylecodon reticulatus), a tiny point at the tip of the leaf (mucronate leaves) (e.g. in Sansevieria), leaves fully converted to spines (e.g. in Opuntia), stipules converted to spines (e.g. in many Acacia), prickles on stems (e.g. of Rosa, Erythrina and Ceiba speciosa), uriticating (i.e. stinging) hairs, bristles, and finely barbed spines called glochids. Some thorns are hollow and act as myrmecodomatia, others (e.g. in Crataegus monogyna) bear leaves. Thorns of some species are branched (e.g. in Crataegus crus-galli, Carissa macrocarpa).

In human culture

Primitive humans are known to have used thorns as tools. Human history records a variety of cultural references to sharp-pointed plant defensive mechanisms. The Book of Genesis recounts the creation of thorns as one of the punishments for the sin of Adam and Eve, stating, "Thorns also and thistles shall [the ground] bring forth to thee; and thou shalt eat the herb of the field." Genesis, 3:18. One of the most enduring cultural images is the Crown of Thorns described in the Bible as having been placed on the head of Jesus before his crucifixion. It is mentioned in the Gospels of Matthew (27:29), Mark (15:17), and John (19:2, 5) and is often alluded to by the early Christian Fathers, such as Clement of Alexandria, Origen, and others. For example, John the Evangelist states that "the soldiers plaited a crown of thorns, and put it on his head". (KJV, ch. 19). The biblical account does not specify what kind of thorns were involved, and leaves no clues from which to determine whether these were biologically classifiable as thorns, spines, or prickles. In modern times, the oxygen isotope composition of spines from saguaro cactus have been shown to record changes in local rainfall and can be used to reconstruct climate and plant ecophysiology over the plant's lifetime Acanthochronology.

An early popular myth involving a thorn is that of Androcles, a fugitive slave in ancient Greece who was said to have befriended a lion by pulling a thorn from the lion's paw.

The status of the Scottish Thistle as the national emblem of Scotland is founded on the story (recounted here) that an invading Norse army attempting a night attack was betrayed when they encountered a thistle in the dark.

Plants bearing thorns, spines, or prickles are often used as a defense against burglary, being strategically planted below windows or around the entire perimeter of a property.

They also have been used to protect crops and livestock against marauding animals. Examples include hawthorn hedges in Europe, Agaves in the Americas and in other countries where they have been introduced, Osage Orange in the prairie states of the US, and Sansevieria in Africa.

See also

• Areole
• Glochid

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.

External links

• Texts on Wikisource:
  • Reynolds, Francis J., ed. (1921). "Thorn (plant)". Collier's New Encyclopedia. New York: P. F. Collier & Son Company.
  • "Thorn". The New Student's Reference Work. 1914.