Thorns and Roses

Something is odd about the thorns on roses. I noticed it when I was trimming the plants in the yard. To get at the inner twigs, I reached in and down, avoiding most of the barbs, snipped away, and began to withdraw my hand. Yow! The little meat-hooks, pointing down along the stems at various angles, grabbed hold of any bit of glove, shirt or skin that grazed them on the way up and out. I thought of those “Don’t Back Up” signs at entrances to parking lots guarded by spiked grates that lay flat when you drive forward but are otherwise aimed upwards towards any tire coming out.

Thorns of the rose
(Flickr)

The most common explanation for thorns is that they discourage plant-eating creatures from nibbling. Are rose thorns in particular any less efficient at this because of their angle down the stem, instead of straight out, like, say, cactus spines? It’s difficult to say. True, on an untrimmed, mature rose bush with stems growing in every direction, the thorns seem to deter a hand or animal mouth moving in any direction. Moreover, an animal poking its snout in for a nibble may get snagged as it withdraws and then intensify that pain dramatically as it struggles harder to pull back and escape.

But maybe these slightly backward thorns serve other purposes beside deterrence. The Wikipedia entry under “Rose” cites a different advantage up front: “Rose prickles [“Prickles” are the proper name for such thorns that grow from the skin of a stem; true “thorns” and “spines” are  sharp, modified leaves or stems sprouting from the woody core]…Rose prickles are typically sickle-shaped hooks, which aid the rose in hanging onto other vegetation when growing over it.” “Sickle-shaped hooks!” And “grappling hooks” as well.

A third factor in the rose thorn question is whether thorns and spines successfully deter caterpillars from climbing up to eat leaves and flowers. It seems that they do; at least, they slow them down, according to Christie Wilcox’ reporting on the work of entomologist Rupesh R. Kariyat in Zurich (“The Thorny Truth About Spine Evolution,” Quanta magazine June 14, 2017).  The current evidence suggests that while the first thorns and spikes “evolved against mammalian herbivores” a couple of hundred million years ago, many plants’ tissues gradually became toxic and repelled animals, while thorns stuck around (not intended) as the most effective defense against caterpillars.

Scientists agree that they have more to learn about thorns, spines, and prickles than they thought. Wilcox concludes that such under-research “illustrates our own species’ limitation and preconceptions. ‘When we go in the garden, we get cut by roses, so we perceive those thorns to be a defense against mammals,’ [British scientist Mick] Hanley said. ‘In almost every manifestation of understanding biology, we’re always putting our own human view on it.’”

Such biases may hold especially true for thorns and roses thanks to truisms about the pain that is said to accompany the search for beauty or love. The function of needle-sharp thorns in particular seems self-evident and unchanging. But the truer wisdom is that the capacities of hunters and hunted, seeker and sought, steadily evolve.

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