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 while avoiding most of the barbs, then 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.

Thorns of the rose
(Flickr)

The common-sense 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 they angle down the stem instead of straight out like cactus spines?

It’s difficult to say. On an untrimmed, dense, mature rose bush with stems growing every which way, the thorns would seem to discourage a human hand or animal mouth that is poking around. And a snout that gets  snagged as it withdraws would hurt even more when animals pulls hard to back out.

But is it possible these backward thorns serve other purposes in addition to deterrence? (Incidentally, such thorns are actually prickles. Prickles grow from the skin of a stem; true thorns and spines are modified leaves or stems growing out of the stem’s woody core.) The Wikipedia entry under “Rose” cites up front a different and interesting advantage to prickles: “Rose prickles  are typically sickle-shaped hooks, which aid the rose in hanging onto other vegetation when growing over it.” So…grappling hooks!

Do these prickles/grappling hooks perhaps deter caterpillars from climbing up to munch on leaves and flowers? If so, how effectively? Effective enough to help the plant survive, according to Christie Wilcox’ reporting on the work of entomologist Rupesh R. Kariyat in Zurich*. 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.

So, the rose’s downward pointy spikes do help the plant climb higher and do slow down caterpillars trying to walk around them, but do they discourage a deer looking for a snack? More likely the animal would be sickened by the first bite of plant itself.

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.’”

* “The Thorny Truth About Spine Evolution,” Quanta magazine June 14, 2017.