The roots of the Ceropegia juncea plant. Adapted from an illustration in ‘Plants of the Coast of Commandel’ (source).

Many thinkers start to think loftier thoughts as they get older. They zoom out; they try to take in the bigger picture, the 32,000-foot perspective. Charles Darwin went in the other direction: after giving the world far-reaching theories about the nature of life and mind, he spent the last stretch of his career with his nose near the soil, straining to make sense of the minute. His final book, published in 1881, was about earthworms. Before that, he wrote a series of monographs — four in all — about plants.

Darwin’s most provocative proposal about the vegetal world would come in the final paragraph of his final work on the subject. The book was called The Power of Movement in Plants. It was published in 1880, with the help his son Francis. The book drew on a wealth of experiments the two had conducted together on beans, peas, corn, and other species grown in a hot-house in their garden.

In the paragraph in question, Darwin wrote: “We believe that there is no structure in plants more wonderful, as far as its functions are concerned, than the tip of the radicle.” (Note that “radicle” is another word for root.) After sketching some of the remarkable root-tip behaviors they had observed, he concluded:

“It is hardly an exaggeration to say that the tip of the radicle thus endowed, and having the power of directing the movements of the adjoining parts, acts like the brain of one of the lower animals; the brain being seated within the anterior end of the body; receiving impressions from the sense-organs, and directing the several movements.”

“Like the brain of one of the lower animals” — that’s the key part. That’s the startling claim that contemporaries of Darwin would dismiss in a huff. That’s the claim that would languish for more than a century, all but forgotten. And that’s the claim that — more than a century later — would experience an unexpected revival.

The idea is now known as the “root-brain hypothesis.” And according to a 2009 paper by František Baluška and colleagues, it really has two parts. The first part is that the roots are best thought of as the anterior of the plant, not the posterior. This is a bit counterintuitive. Being upright creatures ourselves, we naturally imagine that the upper parts of the plant are the more animate, essential parts and that what is down below is somehow lesser, more menial.

But maybe we’ve got this backwards. As Baluška and colleagues sum it up, Darwin is actually suggesting that plants are “anchored in the soil by their ‘heads,’ exposing their sexual organs to the air and to prospective pollinators.” Their nethers are fluttering aloft, in other words, and their noodles are down in the dirt.

The second part of the idea is that the tip of the root acts like a brain, that it serves as a kind of command center, “directing the several movements.” How could Darwin have arrived at such a provocative proposal? What could have led him to so bold and implausible claim? The answer, as we’ll see, is a lot of close — some might say tedious — observation.

As the book’s title makes clear, Darwin’s main interest was movement — how plants move, which parts of them move, and, of course, why. This may seem like a strange focus. Plants, in our everyday experience, are profoundly still creatures. They’re paragons of placidity. It’s probably for this reason that, long ago, Aristotle wrote that while life in animals is “clearly manifest, in plants it is hidden and not evident.”

But if we look closer and, more importantly, longer — as the Darwins did — a different picture starts to emerge: Plants are, in fact, moving all the time. They are engaged in what Baluška and colleagues describe as a “veritable whirl of activities — but at their own slow pace.” Darwin coined a word for all this commotion — “circumnutating” — a kind of spiraling, searching movement made by all plants parts, at all life stages.

Darwin emphasized that even the roots, tucked out of sight, are in fact circumnutating. He wrote: “If we could look beneath the ground, and our eyes had the power of the microscope, we should see the tip of each rootlet endeavoring to sweep small ellipses or circles, as far as the pressure of the surrounding earth permitted.”

Much of this slo-mo circus seems to be unprovoked — just part of plants being plants. But at other times the movements are clearly in response to external forces. Naturally, the Darwins wanted to understand these forces and how plants were able to sense them and respond.

And so father and son improvised some rather clever experiments. In one, they tested the sensitivity of just-emerging grass seedlings to light. They found that the seedlings would bend toward a light source in one part of their dark chamber — but not when their tops were covered with little hats made of tin foil. The experiment showed that photosensitivity seemed to be concentrated at the seedling’s tip.

The Darwins’ investigations covered all parts of the plant — nose to tail, as it were. But they were especially enamored of the root-tip. Through a variety of interventions, they were able to show that the root-tip exhibited what they called “diverse kinds of sensitiveness.” It responded to gravity, contact, moisture, and light. It could distinguish softer from harder objects. It was more sensitive to touch, they found, than even the “most delicate tendril.”

Crucially, these sensitivities were localized to the tip, which then transmitted information to upper portions of the plant. They showed this, not with foil hats, but by interfering with the radicle in other ways — by cauterizing or even amputating it.

Perhaps most remarkable of all was the fact that the root-tip seemed to weigh the different forces acting on it. In a sense, it had too. It was constantly being subjected to varying pressures, to gradations of illumination and dampness, and so on. It would take in this information in and choose the best way forward (or, more often, downward).

So the putative braininess of the radicle can be summed up as follows: it senses a range of information about its environment; it integrates this information and arrives at a “decision”; and then it transmits signals to other parts of the plant. In this way, it seems to direct how the plant behaves.

Not everyone was impressed with these findings. One rival, the German plant physiologist Julius Sachs, dismissed Darwin’s book as the work of amateurs, of country-house botanists. He blasted the root experiments in particular as “unskillfully made and improperly explained.” This brusque dismissal — by a leading figure in the field, no less — is a big part of why the root-brain hypothesis went underground, and stayed there for more than a century.

But in the time since, the Darwins’ work has largely been vindicated. Key observations reported in the book have been replicated. A number of their findings have been extended. The experiment with those tiny tin-foil hats, for example, is now credited with leading to the discovery of auxin, a hormone of major consequence in the so-called “green kingdom.”

In fact, plant biologists in recent decades have gone far beyond what the Darwins observed, demonstrating a host of further sensitivities. The authors of a 2011 paper note that plants are responsive to “oxygen, chemicals, trauma, fluctuations in water flow, geomagnetic fields, electric fluxes, shade, darkness, and parasites.” And this list of environmental variables, they stress, is non-exhaustive. At least 22 have now been identified.

The list of impressive plant behaviors is growing as well. We now know that some plants can tell their own roots from the roots of other plants — even when those other roots belong to members of the same species, even when they belong to genetic clones. The researcher Monica Gagliano and her colleagues have presented evidence that plants learn by forming associations — much as animals do — and that they use sound cues to find water. There’s even reason to believe that plants may sense their surroundings use something akin to echolocation.

So might there be something like a brain behind these diverse sensitivities, these dazzling displays of what certainly looks like intelligence? Some think so. In 2006 a group of researchers inaugurated a new subfield that they termed “plant neurobiology.” They noted the etymological curiosity that the word “neuron” originally referred to vegetable fiber. And they noted that many of the signatures of animal nervous systems are also present in plants. Some plant cells, for instance, traffic in electrical signals, just like neurons.

But what about the root-brain proposal, specifically? Is there anything to the idea that such brain-like properties are somehow concentrated in — or coordinated by — the tip of the radicle? Quite possibly, say some plant neurobiologists. They’ve zeroed in on a particular portion of the root tip known as the “transition zone.” It sits a tiny ways back — a millimeter or so — from the tip. And it appears to have special properties relating to the hormonal and electrical circuitry of the plant. But admittedly, its precise functions remain to be understood.

Needless to say, not everyone is on board — either with the root-brain idea or with “plant neurobiology” in general. The researchers who christened this field in 2006 were met with a phalanx of resistance: a letter signed by 36 plant biologists. (The original report had only six authors — they were outnumbered.) The response letter rebuked the plant neuro camp for trading in what it called “superficial analogies and questionable extrapolations.”

If this seems harsh, it’s downright collegial compared to criticisms of plant neurobiology lodged elsewhere. In an article in The New Yorker in 2013, journalist Michael Pollan quotes the eminent plant biologist, Lincoln Taiz, as saying that plant neurobiologists are given to “over-interpretation of data, teleology, anthropomorphizing, philosophizing, and wild speculations.” Other critics that Pollan spoke to were terser. One called the whole business a “foolish distraction.”

The debate often seems to be less about specific controversial findings and more about the gloss we give to those findings. While opponents of “plant neurobiology” call out what they see as “superficial analogies,” defenders of the movement appeal to the generative power of metaphor in science. One such defender, Anthony Trewavas, argues that metaphors spark questions that would not otherwise have been sparked. To him, “plant neurobiology” is clearly metaphor and clearly a useful one.

More than a decade on from this flashpoint, it’s still not clear the debate was settled. But talk of plant cognition — or something like it — seems to be inching slowly toward the mainstream. In June of this year, researchers in Italy reported a remarkable finding about pea tendrils. The tendrils move more slowly towards a distant support rod if it’s thicker and thus harder to grasp. It’s as if the plants are sensing the rod from afar and exercising care in how they approach it. As remarkable as the findings themselves was where they were published: in the Psychonomic Bulletin & Review, a long-standing psychology journal, and a mainstream one at that.

The debate still flares up occasionally, and no doubt will for some time yet. “Should we really be talking about cognition and brains when we’re talk about plants?”, someone will ask. Hackles will rise, tempers will flash. But, in a sense, metaphors like the root brain and plant neurobiology have already done their work. They’ve circumnutated their way into the soil of the scientific imagination and taken root. It took awhile. But some ideas — like plants — move at their own pace.

– Kensy Cooperrider

Note: This post originally appeared in audio format on the ‘Many Minds’ podcast (@ManyMindsPod). You can listen to the episode here.