The Light Eaters: Book Review

The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth, Zoe Schlanger, 2024. Schlanger is a journalist who interviewed botanists all over the world studying plants and writing technical papers. With this added knowledge, does it mean that plants communicate, make decisions, and are intelligent? That requires a paradigm shift that’s still in progress. [Schlanger described the researchers making the discoveries, but for this short review, I ignore them—sorry.]

Prologue: “In the Pacific Northwest … are complex adaptive systems. Each creature is folded into layers of interrelations with surrounding creatures. … The plants with the soil, the soil with its microbes, the microbes with the plants, the plants with the fungi, the fungi with the soil. The plants with the animals that graze on them and pollinate them. The plants with each other” (p. 1). She’s viewing biological creativity. As a former academic I understand “publish or perish” and staying within accepted paradigms.

Chapter 1: The Question of Plant Consciousness.

Ferns are ancient and can have up to 720 pairs of chromosomes compared to our 23. They absorbed CO2 and released oxygen and fixed nitrogen. 19th century naturalist Alexander von Humboldt noted: “Everything is interaction and reciprocal. …The planet is a living whole, with climate systems and interlocking biological patterns bound up as a net-like, intricate fabric” (p. 11). This was ecological thinking. Ferns can dehydrate and remain in a dried state for a century. They reproduce using spores (this was initially before seeds).

“Science is a conservative institution for a reason. Conservatism is a crucial backstop against false knowledge. … Science has no agreed-upon definition for life, death, intelligence, nor consciousness. … Science is constrained to the sort of questions that can be answered using the scientific method” (p. 18). Definition of intelligence and consciousness remain human-centric. One problem is networks of knowledge in an era of specialization.

“Nature is chaos in motion. Biological life is a spiraling diffusion of possibilities” (p. 21). Plant sensing is okay, plant behavior so-so, but plant intelligence is dangerous (especially to a scientific career). Plants can remember and can have kin recognition—but how?

Chapter 2: How Science Changes Its Mind. “An alga-like cell swallowed a cyanobacteria. That alga-like cell was the early organism from which both animals and fungi would later evolve, and the cyanobacteria is an ancestor of the unthinkably diverse bacteria that flood our world. … It took sunlight, and alchemized the spare materials of its environment—water, carbon dioxide, maybe a few trace minerals—into sugar. … Plants have evolved and proliferated into a half-million species. … They created the atmosphere we now enjoy. … A leaf is the only thing in our known world that can manufacture sugar out of materials—light and air—that have never been alive. … Chloroplasts in the leaf cell convert the particle of light into chemical energy. … Leaf … stomata suck in carbon dioxide. … Water and carbon dioxide molecules are ripped apart. Half the oxygen molecules from both particles float away from the meeting, passing back out. … The carbon, hydrogen, and oxygen that remains is spun into strands of sugar glucose” (p. 25).  

In Hawaii indigenous plants are killed off as invasive species were introduced. They did not develop the resilience over time to survive. Some 238 species have 50 or fewer individual plants.

Flowers and seeds (embryos with nutrients) appeared 200 million years ago. Plant roots have a couple days to locate water and nutrients, then stems and leaves appear to photosynthesize. “A plant is modular; snap off a leaf, and it can grow a new one. Without a central nervous system to protect, the plants’ vital organs are distributed and come in duplicate. That also means a plant has evolved remarkable ways to coordinate its body and defend itself” (p. 32).

Plants can defend themselves, like growing thorns and stinging hairs. They might send airborne chemicals to activate their immune systems or communicate to distant branches.  

“Virtually all indigenous groups around the world have a more intimate relationship with and recognition of plant life. Many cultures ascribe personhood to plants” (p. 35). Plato thought only men had superior reason: not women, children, or slaves, meaning a men-in-charge hierarchy. Aristotle thought only humans had rational souls. Aristotle student Theophrastus was willing to consider plants as autonomous beings with desires. Descartes viewed animals as “machines.”

Up to the 20th century, vivisection was practiced on the belief that animals felt no pain, until animal welfare societies were formed. Philosophy and ethics change over time. What was animal awareness and when was it taken seriously? Scientists found that animals had problem-solving abilities and no one found the human brain unique, just relatively large. The “mind” was based on studies by behavior, but also true of, say, dolphins, parrots, and dogs.

Charles Darwin was interested in plants. He noted the “root-brain,” the ability of roots to sense moisture, useful nutrients, and potential harm in the soil. Darwin was ignored for 125 years. Thomas Kuhn noted the rare paradigm shift to new ways of thinking of systems. Louis Pasteur faced skepticism with his germ theory of disease. Plants produce electric impulses with glutamate and glycine (common in animal brains) present.

“A plant is a multidimensional organism in constant biological conversation with its surroundings, the bacteria, fungi, insects, minerals, and other plants” (p. 49). A Society of Plant Neurobiology was created (changing its name to the Society of Plant Signaling and Behavior). Humans have neural networks. What about plants?

Chapter 3: The Communicating Plant. “Communication implies a recognition of self and what lies beyond it—the existence of other selves” (p. 53). A paper by David Rhodes in 1983 described a forest devastated by caterpillars; after several years they began to die. He believed the trees were communicating to each other and creating toxic chemicals: nature in a self-organizing system. Presumably each cell from the beginning understood its purpose relative to other cells.

Dormant seeds have a “decision-making center” to decide when the plant should emerge, producing the right hormone from cell to cell. “Through the chatter of their cells, plants are self-organizing systems. … Plants are tremendous at chemical synthesis” (p. 56). The idea of “induced resistance” where plants chewed by insects altered their chemistry.  

Ian Baldwin and Jack Schutz at Dartmouth College: “Baldwin and Schultz placed pairs of sugar maple seedlings inside the sterility of a growth chamber. The seedlings shared the same air but didn’t touch. Then the researchers ripped the leaves of one and measured the response in the other. Within 36 hours, the untouched maple seedlings loaded up its leaves with tannin. In other words, despite not experiencing damage itself, the untouched maple went to work making itself extremely unpalatable. … The scent of cut grass is the chemical equivalent of a plant’s scream” (p. 60).

In South Africa kudus (an antelope) dropped dead. Apparently, they were forced to eat acacias that increased tannin in their leaves, which poisoned them—“coordinated poisoning.” “Wild tobacco, when it begins to be damaged, can summon predators to eat the caterpillars that feed on it” (p. 65). Sagebrush used “private communications” to communicate insect attacks on family groups. They also used “public communications” for heavy attacks. Birds do similar “emergency calls.” “Their communication is not just rudimentary but complex and layered, alive with multiple meanings” (p. 66). Individual plants may have different tolerances for risk—as do animals including humans (a “shyness-boldness continuum”). Plants thus have judgment as perception of raw information and decision-making.

German chemist Baron von Liebig in 1840 noted that plants needed nitrogen, phosphorus, and potassium, but plant health needs more like soil complexity with microbes and fungi. “Agricultural researchers have warned of the dangers of monocultures—planting a single genetic variety of crop over large swaths of land” (p. 73). One result was the potato blight of the 1840s, devastating in Ireland. “The economics of modern agriculture values yield above all, many of the world’s food staples continue to be grown in vast, undifferentiated fields. … Huge quantities of pesticides and fertilizer are often needed to sustain them” (p. 73).

 Chapter 4: Alive to Feeling. “Electricity has its own vibrancy. … A plant is a sack of water—or slightly more specifically, a skin-like sack of cells, each inflated by a coursing watery liquid. … Electricity pulses move through the plant body” (p. 75).

In Humans: the membrane potential of our cells, when at rest, is ever so slightly negatively charged. Positively charged elements—sodium, magnesium, potassium, and calcium ions—are afloat in the plasma between those cells. These are your electrolytes. … With the influx of ions, the cell’s charge flips from negative to positive. … This sudden burst triggers the ion gates in the neighboring cell to open too, electrifying that cell in turn … sending information via the electric current. … Our brains are fantastically electrical. … When humans are put under general anesthesia, they stop responding to touch” (p. 76). Ditto, Venus flytraps—they don’t snap closed. One perspective is electricity creates feeling of reality—consciousness. That could mean all living organisms have consciousness.

Touch triggered hormone response in plants. “When touched, a plant will activate its immune system” (p. 80). A “death spasm” can happen as a spike in electrical activity, such as a “convulsion” in a cabbage dropped in boiling water. Plants may have nervous systems. However, “plant cells are different from animal cells: they have cell walls and things like chloroplasts. Plus, plants simply don’t have synapses” (p. 82). Photosynthesis is an electrical process.

“Our understanding of how electricity governs human nerves and muscles began in plants. Electricity moves in waves in plants. The researchers Hodgkin, Huxley, and Eccles won a Nobel Prize for figuring out the electrical nature of human neurons in the 1950s. … based on electrical impulses in the giant cells of Chara algae, a common pond weed. … the first definitive evidence for a link between an electrical signal and a biochemical response in plants. … Voltage-activated ion channels are the basis of nerves” (p. 84).

A problem for researchers was funding, which was mainly for patterns in genetic codes, not electricity—although this is a major field of study. The basic view was Darwin evolution was based on random mutations, a passive view, ignoring how plants focus on their environment.

Humans perceive gravity based on fluid and trigger hairs in the inner ear. Plants have similar systems, with hairs in their cells similar to the inner ear. How information is transmitted is not known, although electrical impulses are likely. “Up and down” are important to roots and stems. “That is quite literally the essence of the entire question of plant intelligence: How does something without a brain coordinate a response to any stimuli at all?” (p. 91).

Chapter 5: An Ear to the Ground. A certain vine corresponds with bats, who pollenate them. Plants can detect caterpillars chewing by their distinctive sound. Perhaps they interpret vibrations. Some plants used chemicals like tannins or “insect repellant” against the bugs. Bites trigger hormone changes possibly based on electricity, “ecological relevance.” Hairs on stems could act like antennae. Roots also can be sensitive to sound. “Cavitation clicks” are plant sounds as water travels in them, including drought stress. “Tomato and tobacco, when they’re feeling well, they will emit very few sounds” (p. 112).

“Climbing vines are known to circle the air as seedlings, looking for an upright staff the climb—and seem to locate the position of an appropriate climbing surface long before actually coming into contact with it. … The vines may be using echolocation to sense the position of the pole” (p.  114). Some funders support the concept of “diverse intelligence.”

Chapter 6: The (Plant) Body Keeps the Score. Plants seem to have a memory, like time intervals of bumblebee visits for pollination. They also orient themselves in time and space—“elastic memory” that can be used to manipulate pollinators.  

“Plants … take information from the outside world. They process. They make decisions. And they perform. They take everything into account, and they transform it into a reaction—intelligence” (p. 124). To sprout, garlic needs the memory of winter (“vernalization”), as do apples, peaches, tulips, and so on. Plants adjust to the cycle of day and night.

Dodder plants are parasitic, with spikes placed into host plants to suck the juices. They afflict farms globally.

Plants have self-organizing systems exploiting local environments in the context of the entire plant. Plants build their bodies for the specific environment. Did sensing the environment and building memories start with the first multicellular organisms?

Chapter 7: Conversations with Animals. Plant-to-plant communications is based on the exchange of chemical signals, “biocommunication.” Research was done using corn, caterpillars, and wasps: the corn determines the species of caterpillar based on saliva, then releases specific chemical gas to attract the wasp to kill the caterpillar. Plants schedule blooming to have bees around to pollinate them. Plants can send the right scents to attract bees. It’s a battlefield. Some plants attract ants, provide food and nesting; in turn, the ants protect the plants (“ant plants”) from harmful insects.

Legumes gather bacteria in their roots to keep a supply of nitrogen for fertilizer. It’s nitrogen fixing in exchange for sugars. Plants can synthesize an array of chemical compounds, often to be excreted as gases.

Asters and goldenrod bloom together in September (see Braiding Sweetgrass—coming soon). This is associated with attracting more bees for pollination.

Many plants clone themselves, or have sex, or do both. They can be bisexual, or switch sexes. Research in Finland focuses on aspen clonal colonies. Birch trees defend against weevils when growing near rhododendron. The rhododendron senses the weevils and birch compounds defend both.

“The way we grow food appears to be getting in the way of plant communication” (p. 155). Pesticides are used because the plants cannot defend themselves against predators or weeds. Some plants survive and grow better with other plants, like the “Three Sisters” (corn, beans, and squash) of the New World.

Chapter 8: The Scientist and the Chameleon Vine. In Peru: “a common vine in this rain forest was capable of something no other plant was known to do. It could, quire spontaneously, morph into the shape of almost any plant it grew beside “adaptive plasticity,” the ability of plants to adjust their behavior to suit a changing environment” (p. 158). This was one of the first observations of mimicry in plants. Weeds can mimic crops to a point. Rye went from an inedible weed to a separate crop, taking a form similar to wheat. A similar story for vetch in lentil fields. Some mistletoe (a parasitic plant) can morph to look like the host plant. Plant plasticity is the capacity to change body or behavior as new conditions appear.

“Crop science is typically seen as the domestication of scrawny wild species to turn them into plump, useful food machines” (p. 165).

Darwin studied vines, discovering they use different techniques to hoist themselves up, like coiling around objects, secrete sticky substances, or grow tiny hooks. They revolve their growing tips in a circle to bump into something solid.        

Vision in animals is based on light-sensing. Plants need light because they “eat” it. Too much light causes leave scorching and root damage because roots need near-total darkness. Roots can sense it and grow away from it. “An ancient cyanobacteria, an early ancestor of plants, had (and still have) the smallest and oldest example of a camera-like eye” (p. 167). “Botanists have thus far found fourteen types of light receptors in plants” (p. 169). Plants are phototropic: they orient toward sunlight. Plants with sunlight expand and increase surface area of leaves. Those in shade make small, tough leaves.

“Microbes are the only thing presently known to be capable of altering genetic expression in plants” (p. 182). They may signal molecules between plants. Perhaps plant appearance is controlled by microbes. Some 2,600 types of micro-RNA are in humans, which probably regulate some of our genes. Humans have microbial clouds surrounding them. “Our microbes influence our immune systems, our smells, and our attractiveness to mosquitoes” (p. 186). Microbes probably influence autism, depression, and anxiety.

“A memory-based immune system may have evolved in vertebrates because of the need to recognize and manage complex communities of beneficial microbes” (p. 188). Bacteria can swap genes with other bacteria even in different species.

Chapter 9: The Social Life of Plants. Insects like ants and bees evolved as part of larger groups with specific roles (“eusocial behavior”). Researchers focus on “social intelligence.” There may be plants equivalents. “Staghorn ferns grow in round, hive-shaped agglomerations of many staghorn individuals, … [some] good at directing rainwater to their base” (p. 194). Only some of the plants reproduced.

How some plants grow depends on what grows around them. They are less likely to block out kin with stems or roots. “Sunflower farmers get up to 47% more oil yield from their plants if they grew them in rows with kin closely packed next to one another” (p. 200).

Pollinators like big flower displays, the “magnet effect,” which takes effort from the plants. It creates a tradeoff with pollinators versus number of ovules. Plants for crops are picked by looking vigorous, but these are the most competitive. More “altruistic” plants likely would increase yields. “The sweetness of tomatoes, the aromatic qualities of basil, and the properties of the essential oil in mint depend on the species of fungi the plant grows with” (p. 207). Fungi don’t photosynthesize and get nutrition from plants. “The concept that roots actively forage for their food … implies intentional, directed behavior. … When you stress a plant by damaging its leaf veins, it will make bad foraging decisions. … Coexistence is a powerful concept; it doesn’t quite have a place in Darwinian schema of ruthless competition” (p. 209). “We hurt ourselves in ecology … by relying a lot on super simplified models” (p. 213). Plants adapt and make spontaneous decisions.

Chapter 10: Inheritance. A Brazilian plant (spigelia) “bends down to plant its own fruit is quite rare, though it is shared by the common peanut” (p. 216). Radishes suffering from caterpillars can make babies with bristly leaves and defensive chemicals. There seems to be generational memory based on the environment. “Plants—and every other living thing—have agency over their own development. … Genes are not exact instructions” (p. 219). A plant will develop differently in one environment versus another, like in a sunny versus shady patch or well-watered versus drought. A small number of foreign plants thrive and become invasive, like the Japanese knotweed which is one of the most invasive plants in the world. “We’re changing the planet faster than many plants can evolve to keep up” (p. 237).  

“Science is not objective, and scientific paradigms come and go, each with their own blind spots and biases” (p. 221). A problem for experiments: “there is no control environment” (p. 225).

Chapter 11: Plant Futures. One theory is biology was interconnected networks, with a brain only one way to build a network. Plus, ambiguity and complexity in nature.