Armed by Nature: Thorns, Spines, and Prickles

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"We're not talking about one hungry plant here; we're talking about world conquest!" (excerpt from ​​​Little Shop of Horrors)



I’ve never seen a plant throw a punch when it’s feeling threatened in a dark alley. Rather, their defense mechanisms are a bit more underhanded in nature.


A lethal dose of a poisonous alkaloid to an unsuspecting browser? Sure! (Just ask Socrates.)


Luring in an unwary insect after drawing a bath of digestive juices? Why not?


Creating a fortress to house an army of very angry, herbivore-assaulting ants? Absolutely.


In many cases, plant structures that tend to be a bit more obvious play the first line of defense against threats posed by insects, mammalian browsers, and gardeners, such as myself, who have recklessly forgotten to don gloves. And I would bet my pawpaw harvest that you too have been victim to this passive confrontation. Of course, I am referring to those pointy protrusions that produce temporary, yet sometimes excruciating, pain (and likely a couple of 4-letter words): thorns, spines, and prickles.


My fellow flora fanatics would agree, what plants lack in biceps, they make up for in some pretty remarkable adaptations including the trait of being “armed”. An example of convergent evolution, plants in many different families have, in fact, independently evolved to grow sharp, pointy structures as a means of defense against browse. Though thorns, spines, and prickles all function similarly, there is a distinction to be made between these terms as they are produced by plants in different ways.


You may be asking, “Carrie, why do I care what part of the plant is jabbing me? Just make it stop.” It’s a fair question (and request). But true Plant Geeks will appreciate the fact that knowing what part of the plant the stinger is derived from is an important clue about how plants are (or are not) related.


Thorns: “Stem”ly spiky


Thorns are defined as modified shoots. What does that mean? Basically, it’s a branch that has crossed over to the dark side. Thorns are made from the stuff of stems!


According to Missouri Botanical Garden’s Grammatical Dictionary of Botanical Latin, a thorn is defined as “a short, rigid process of a plant, developed from a bud typical of a leafy branch.” Many hawthorns (Crataegus spp.), as well as honey locust (Gleditsia triacanthos), are examples of commonly found plants in Ohio that possess thorns. As the definition implies, thorns occur in the axil of a leaf where a branch would normally grow, but instead, conclude in a sharp, durable, woody point. As such, they contain internal vascular tissue (phloem and xylem) that the rest of the plant’s branches possess.


cockspur hawthorn branch with leaves and thorns
Cockspur hawthorn, Crataegus crus-galli


honey locust leaf and thorn
A new thorn emerges from a honey locust, Gleditsia triacanthos


Spines: "Leaf" us alone


Rather than being made from the stuff of stems, spines are modified leaves or parts of leaves. Sometimes, spines occur in pairs, as is the case with barberry. The Grammatical Dictionary of Botanical Latin defines a spine as “a stiff, sharp pointed protective outgrowth or plant process, as a modified leaf, leaf part, petiole or stipule.” Cacti (family Cactaceae) and some holly species such as American holly (Ilex opaca) and English holly (Ilex aquifolium) are armed with spines. (Interestingly, many holly species lack this defense mechanism, including winterberry, Ilex verticillate, and Ilex paraguariensis, the plant that makes up yerba mate.)


The most vicious spines I have personally encountered stemmed from a leguminous tree species called Prosopis kuntzei (referred to as Neltuma kuntzei in some literature). Inhabiting the Gran Chaco of Argentina, Bolivia, and Paraguay, arguably one of the harshest environments in the world to be a plant, this tree seems to be a worthy opponent with a canopy of innumerable, long spines that can extend past a foot in length. Though it does form some leaves in the spring, they quickly fall off and allow the spines to be the principal photosynthesizers. Fortuitously, I was able to come out alive on the other end.


And since spines are formed from a plant’s leaf tissue, they also contain internal vascular tissue.


cactus with yellow flowers
Our native cactus: Eastern prickly pear, Opuntia humifusa


holly with red fruit
American holly, Ilex opaca


mass of Prosopis kuntzei, a South American species with long spines
From a distance, Prosopis kuntzei resemble a mass of conifers...


Prosopis kuntzei up close to show long spines
...but up close the pods reveal that we're looking at a flowering, leguminous plant.


Prickles: Sharp skin tags


Though prickles can definitely pack a punch in terms of pain (as every rose gardener and florist will surely tell you), they differ from thorns and spines in the fact that they are derived from a plant’s skin tissue, thus do not contain phloem and xylem. Prickles tend to be short and sharp and (if you dare) easier to break off. According to the Oxford English Dictionary, a prickle is “a rigid, but comparatively short and slender, sharp-pointed outgrowth that is an extension of the epidermis of a plant.”


Roses (Rosa spp.), many brambles (Rubus spp.), greenbriers (Smilex spp.), and Aralia spinosa, the fierce Devil’s walkingstick, (if that common name isn’t a warning not to touch with bare hands then I don’t know what is) are all plants that possess prickles.


“Every rose has its thorn
Just like every night has its dawn
Just like every cowboy sings his sad, sad song
Every rose has its thorn
Yeah it does…”

Sorry, Bret Michaels. Turns out it doesn’t. Roses have prickles.


the prickles on a multiflora rose
 Multiflora rose (Rosa multiflora) is an invasive plant in many states throughout the U.S. It can be distinguished from other species by the fringed stipules that are located just underneath the prickles.


common greenbrier growing in a woodland
Smilax rotundifolia, also known as common greenbrier or roundleaf greenbrier, is a woodland species that is not to be messed with.


devils walking stick has lots of prickles
Aralia spinosa, Devil's walkingstick....enough said.