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.
Spinescent is a term used for plants that have sharp structures, but these are not necessarily spines in the technical sense.
Some authors prefer not to distinguish spines from thorns because, like thorns, and unlike prickles, they commonly contain vascular tissue.[3]
Many plants commonly thought of as having thorns or spines actually have prickles. Roses, for instance, have prickles.[3]
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It has been proposed that thorns may first have evolved as a defense mechanism in plants growing in sandy environments which provided inadequate resources for fast regeneration.[4][5] However, there is no reason to attribute the development of spiny defences to any particular ecological challenge. In some cases, spines have been shown to shade and insulate the plants that grow them (e.g. saguaro cactus spines shade the apical meristem in summer and glochids insulate the apical meristem in winter).
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),[6] a tiny point at the tip of the leaf (mucronate leaves) (e.g. in Sansevieria),[7] leaves fully converted to spines (e.g. in Opuntia),[8] 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).
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.[9]
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.[10]
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.[11]