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.

Peter Wohlleben’s “The Hidden Life of Trees”

Until recently I was quite sure that a broad difference between animals and plants was that animals, because they are mobile, readily interact with each other (flocking, pursuing, etc.) while plants, anchored to the ground, don’t do so because they can’t. Except to attract insect pollinators, plants, I thought, live a life of exquisite solo struggle, seeking only the sun and water.

I’ve been steadily learning how far off I was. German forester Peter Wohlleben’s popular book, The Hidden Life of Trees: What They Feel, How They Communicate, is the most compelling lesson yet.

Among his many descriptions of communication and mutual assistance is Wohlleben’s account of how trees defend not only themselves but also each other. Observers have noted, for example, that umbrella thorn acacias in the African savannah pumped toxins into their leaves when they felt giraffes nibbling on them. “The giraffes got the message and moved on to other trees in the vicinity. But did they move on to trees close by? No, for the time being, they walked right by a few trees and resumed their meal only when they had moved about 100 yards away.” They passed by the nearest trees because the trees being nibbled, in addition to pumping a repellent, “gave off a warning gas that signaled to neighboring trees that a crisis was at hand.” The giraffes knew these trees would not taste any better and kept walking.

hidden life of trees (pri.org)

pre.org

Many trees also have the ability to call in the air force. Reacting to bites from hostile insects, such trees emit scents that attract predators that devour the pests. “For example, elms and pines call on small parasite wasps that lay their eggs inside leaf-eating caterpillars.” The growing larvae devour the caterpillars from the inside.

The book brims with information and appreciations of this kind. Three more examples:

  • Trees that spend their lives in the forest fare much better than trees raised in one place and then transplanted to the forest. “Because their roots are irreparably damaged,…they seem almost incapable of networking with one another.” Like “street kids,” they “behave like loners and suffer from their isolation.”
  • Time for trees is slow and long. Internally, they, like animals, send alerts to parts of their body via chemicals and electrical impulses. But in a tree the electrical impulses move only about a third of an inch per second. (In our bodies, pain signals move  through our nerves about two feet per second, muscle impulses a hundred times faster.) No wonder it seems to us that plants are unresponsive.
  • Conifers (evergreens) “keep all their green finery on their branches” throughout the winter and have been doing so for 270 million years. Then deciduous (leaf-bearing) trees came along 100 million years ago, growing and discarding annually millions of delicate green solar panels. Was this an improvement? Why go to all that trouble? Wohlleben asks. Because “By discarding their leaves, they avoid a critical force—winter storms.” Between high winds, muddy soil, and a surface area equivalent to that of a large sailboat, tall evergreens take a battering in European winters. Growing and then dropping their huge surface area every year proved well worth while for the leafy new comers.

Wohlleben’s liberal use of human descriptors to explain the actions of trees delights many readers and annoys others. Andrea Wulf, in her review of the book, has both reactions.

I’m usually not keen on anthropomorphizing nature—and here trees are “nursing their babies” and having “a long, leisurely breakfast in the sun” while…fungus mushrooms are “rascals” who steal sugar and nutrients. These cutesy expressions make me cringe….But I have to admit that Wohlleben pulls it off—most of the time—because he sticks with scientific research and has a knack for making complex biology simple and thoroughly enjoyable.

I agree. While the vocabulary may bestow on trees a dignity and affection that we usually reserve for our own kind, it is scientists’ growing understanding of trees that creates the real story here. At a time of rapid environmental change, the book is as fascinating a revelation as one could ask for that life is even more intricate and purposeful than we knew.