By Patsy Cotterill
The Light Eaters, by Zoë Schlanger, 2024. New York, N.Y., HarperCollins Publishers.
“A single plant is a marvel. A community of plants is life itself.”
Zoë Schlanger
After reading this book you are guaranteed to see plants in a new light (no pun intended), one of greater awe and respect. Plants are not just passive life forms, governed by abiotic factors such as soil and climate, and having to take everything that animals can throw at them because they are mobility-challenged. Rather, they are active agents of their own survival, aware of their environment and able to react with decisions that are in their best interests.
A former reporter with the New Yorker, Schlanger traded the climate and environment beat for the happier subject of plant science, for which she has a natural passion. The result is a book I would classify as serious popular science, akin to Suzanne Simard’s The Mother Tree, Meg Loman’s The Arborist and Martin Sheldrake’s Entangled Web. This genre aims for scientific authenticity, relating the information it purveys to named research. The author has used her journalism skills to plumb the research literature and to interview the research scientists working in this burgeoning new field of plant behaviour. The book is well organized and well-written, clearly a labour of love. She cleverly uses the researchers’ experience, as well as her own, to explain and leaven the science and make it personal and human. In addition to a large and useful index, Schlanger’s book includes a hefty section of references, some of which are accessible online to the reader, scientific journal-style.
Plant Communications
In the first chapter she covers the phenomenon, established in the 1980s, whereby plants communicate with each other by airborne chemicals that they synthesize and release; for example, in response to an insect attack. The plants thus alerted respond by producing toxic chemicals in their leaves that deter the insects. Interestingly, this phenomenon was discovered in the 1970s when it was observed that forest tent caterpillars disappeared within a couple of years of an outbreak, attributed to the trees being able to mount defences. (Probably a number of us will remember that: I came to Alberta in 1981 in the midst of the epidemic!) Incidentally, while it may seem altruistic for plants under attack to send warning signals to others so that they can prepare in advance, the author points out that this isn’t necessarily so; it is an advantage for an individual to live in a healthy community.
Plants Communicate by Electricity!
That plants communicate internally, cell to cell, by chemical means is a subject of school textbook botany: for example, by growth hormones that help roots to grow downwards and shoots to maintain their vertical, upright position. But did you know that plants also communicate internally by electricity? This was an aha moment for me, learning that if you attach an electrode to a plant leaf it will register in the same way that a screen will light up if electrodes are attached to your heart! The electricity is generated similarly to that in animals: positively charged calcium ions (potassium ions in animals) pass through a negatively charged cell membrane, creating an electric current that passes as a wave through the plant body. Imagine that! Step on a plant, its cells register the pressure, and a silent scream of electricity runs through the plant. The current can be shut off chemically, apparently. When the Venus flytrap plant is enclosed in a chamber of diethyl ether it can no longer close its leaves (traps) when they are stimulated. The ability to “anaesthetize” such a response has some scientists wondering if plants actually have a form of consciousness!
Touch and Hearing
It has been known for some time that plants are sensitive to touch, as in the Venus flytrap and the sensitive plant Mimosa pudica, but Schlanger describes an astonishing experiment demonstrating this. A green fluorescent substance from a jelly fish, facilitated by glutamate (a neurotransmitter in animals and common also to plant cell networks), can be used to light up the wave of electricity passing through an Arabidopsis plant when its leaf vein is cut.
Touch? It doesn’t stop there. Researchers have shown that plants can “hear”, responding, for example, to “ecologically relevant“ sounds such as the chewing of caterpillars or the buzzing of bees, with responses such as the increased synthesis of defence chemicals or increased nectar production. The roots of pea seedlings have been demonstrated to grow towards the sound of running water.
Can plants “See”?
If plants can feel touch and hear, can they also “see”? Since the turn of the 20th century, light-sensing organs have been postulated to exist in leaves, and modern science has uncovered a number of light receptors in plants, allowing them to discriminate between light and shade and wavelength. (Mind you, wouldn’t you expect this in an organism that makes its living from the sun?)
Complex Symbioses?
A more sophisticated form of “seeing” has been suggested for the Boquila trifoliata vine of Chile, which can change the shape of its leaves to mimic those of the surrounding plants as if it knew what they looked like. (The idea is that the vine benefits by merging into the background and being less conspicuous to herbivores.) An alternative explanation put forward for the vine’s transmogrifying ability, however, is that it has become infected with microbes that have changed its gene expression.
This leads us into the realm of science fiction, or at least the unreal one, that is becoming increasingly familiar: that no organism is an individual but rather a multi-organism ecosystem in which individual autonomy is at least partly lost. Schlanger quotes the evolutionary biologist Lynn Margulis who says that early on in the development of life “these sorts of symbioses may have been more important to our evolutionary history than the slow, random mutations science believed to be the source of all evolutionary change.” Revolutionary talk indeed!
Closer to home, mimicry has long been observed to occur on the part of weeds coming to look and behave like the crops in which they grow, achieving similar heights and setting seed at the same time, for example. The ability of weeds to become herbicide-resistant may also be a case of mimicry.
Mimosa or sensitive plant, Mimosa pudica, being grown as a house plant on a balcony in west Edmonton. It is a member of the pea family, Fabaceae. Its leaves move and fold inwards when touched, but recover a few minutes later. 2024-08-30. Photo: P. Cotterill.
Memory and Learning
Another chapter deals with evidence of memory in plants, relating evidence that suggests that plants are capable of remembering their experiences and therefore learning, and are thus able to discriminate between alternatives. The word intelligence comes from the Latin verb intellegere meaning to distinguish, or recognize. Since plants can apparently make choices, does this mean they are intelligent? The climbing, parasitic plant, dodder, in being able to recognize an appropriate host (presumably on the basis of remembered experience), would seem so.
Plants and Animals
The content of the chapter entitled "Conversations with Animals" is perhaps a little more familiar to us, as considerable popular literature now exists on the adaptations of flowers to attract pollinators in symbiotic relationships or, in some cases, deceptive ones!
Returning to the subject of communication by means of airborne chemicals, Schlanger asks the question: could human-caused pollution be interfering with plants’ ability to communicate using these signals, thereby reducing their ability to mount chemical defences? Some research suggests this is the case and explains the need for a greater use of pesticides to protect crops. And could the accidental breeding out of defence mechanisms in crop cultivars also place greater reliance on pesticides, with all its implications for ecosystem and human health?
Social Interactions in Plants
Observing, for example, that dandelions grow tall when surrounded by equally tall vegetation but short in a mown lawn might suggest to even the casual observer that plants are aware of their neighbours and thus capable of social interaction. (By the way, many of us think dandelions are intelligent!) Underground, plants are able to recognize their own roots and those of others. The chapter "The Social Life of Plants" contains several examples of plants showing preferential treatment to their relatives, for example by restricting root and shoot growth to share resources, while doing the opposite with strangers.
Schlanger reports one experiment that might have relevance for growers. A Japanese scientist working on plantain noted that seeds speeded up and synchronized their germination in the presence of non-relatives, the conclusion being that such a show of solidarity deterred competition.
Root Etiquette
In a section on roots, including how the fungal relationship of mycorrhizae can influence plants, Schlanger devotes some space to our own J. C. Cahill at the University of Alberta, known for his statement that roots actively forage for food. She describes his experiments with sunflowers, which apparently display “social etiquette” in where they place their roots in relation to a nutrient patch. From his and his students’ research in the grasslands of eastern Alberta, manipulating many field variables, Cahill has concluded that different plant species seek each other out and live in multispecies “neighbourhoods” in which competition turns out to be a less important factor determining plant community composition than previously thought. Alas, for those of us who wish to design and manage landscapes for conservation, the processes at work in plant communities are extremely complex!
Plasticity – the Ability to Change with the Environment
In the chapter on inheritance, Schlanger makes the point that plants are not confined by the strait jacket of their DNA, but can change under the influence of the environment, both by their capacity for plasticity and through epigenetics (modification of the expression of genes by the environment). Plasticity, the ability of an organism to change its appearance and performance in response to changed environmental conditions, is now thought to be an attribute of many invasive plants, which adapt to – and thrive in – the environment to which they have been introduced, usually after a lag period. This would explain why species are not invasive in their home environment but can become so in a new one, after having had time to adapt. Will plants genetically programmed to be plastic fare better in a climate-altered world?
Philosophical Implications
In the final chapter Schlanger returns to some of the philosophical questions she has broached; are plants intelligent according to the standard definition of intelligence? Could they even have a form of consciousness, providing this isn’t restricted to the function of the complex neural networks of an animal brain?
So are plants intelligent? (My answer: yes, if intelligence is defined as being aware of one’s surroundings and responding positively; no, if it means the human ability to understand the world at large.) Are plants conscious? (Yes, if it means they are thus aware, but no, if consciousness depends on a brain and its neural networks of great complexity.)
Legal Rights for Plants?
With all these capabilities, do plants deserve more respect, should they even qualify for legal rights? Schlanger clearly believes that we should respect plants on their own terms (not in comparison to animals, although similarities between these two groups of organisms obviously exist). She maintains that to have such a perspective could “revolutionize …the very way we live on this earth.”
Young readers might be inspired to take up a career in plant behavioural science or plant physiology after reading this book; older readers will appreciate a deeper understanding of the plant world they already enjoy!
Addendum
Coincidentally, after reading this book, I came across an article by Jack C. Schultz in a 2002 issue of Nature called “How plants fight dirty.” Schultz is mentioned in The Light Eaters as one of the early proponents of plant communication via airborne chemicals. At the time of the publication he was an entomologist at Pennsylvania State University.
He begins his article with the statement: “Each of the world’s 100,000 plant species is a target for attack from a range of 400,000 species of plant-eating insects. Herbivory is among the Earth’s most important interactions in terms of numbers of taxa, biomass and mass transfer, as well as evolutionary impact on plant traits, community structure and ecosystem function.“
Plants respond quickly and in different ways to physical attacks, insect regurgitant or spit. The chemical compounds that signal genes to turn on defence responses include fatty acids, peptides, phenolics, terpenoids, and plant hormones, e.g., cytokines and ethylene. These fatty-acid molecules are similar to those that occur in animals; in fact, plants and animals have in common many chemical compounds that act as signals and hence have the same biochemical synthesis pathways. Schultz observes that this common code-book would explain how plants identify insects, how plants and insects could jam each other’s signals, and how plants could wreak havoc on insects by interfering with their immune responses, reproduction and behaviour. He concludes: “Perhaps in terms of perception, signalling and biochemical behaviour, plants are really just very slow animals after all.”