Something is odd about the thorns on rose bushes. 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 point down along the stems, not up, as I’d expected. They grabbed hold of any bit of glove, shirt or skin that grazed them on the way out. I thought of those “Don’t Back Up” signs at entrances to parking lots. When you drive forward, the spiked grates fold down flush with the asphalt, but the rest of the time they stick up at an angle sure to puncture any tire moving the other way.
The usual explanation for thorns is that they discourage plant-eating creatures from nibbling on too many leaves and flowers. Are rose thorns in particular any less efficient at this because of their angle down the stem, instead of straight out — like cactus spines, for instance? It’s difficult to say. True, on an untrimmed, dense, mature rose bush with stems growing in every direction, the thorns seem to deter a human hand or animal mouth that is 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 the harder it struggles to pull back and escape.
But maybe these slightly backward thorns serve other purposes in addition to 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.] are typically sickle-shaped hooks, which aid the rose in hanging onto other vegetation when growing over it.”
As for thorns and prickles deterring caterpillars from climbing up to munch on leaves and flowers, just how effective a deterrent are they? Effective enough to help the plant survive, 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, but thorns remained useful 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.’”