PDF Summary:The Light Eaters, by Zoë Schlanger

You might think of plants as passive organisms that simply exist in the background of our lives. But in The Light Eaters, Zoë Schlanger challenges this assumption by exploring the complex ways plants perceive, communicate, and respond to their environment. Plants use electrical and chemical signals similar to animal nervous systems, transmit information through sound and airborne chemicals, and can even remember past experiences to inform future behavior.

Schlanger examines how plants demonstrate agency by adapting their physical forms to environmental conditions, recognizing their relatives, and engaging in cooperative relationships with fungi and other organisms. She also discusses the broader implications of plant intelligence, including potential agricultural applications and the ethical questions that arise when we consider plants as entities deserving of moral and legal consideration.

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The cells share information about the levels of two hormones in the seed: one that encourages dormancy and another that encourages germination. The cells gather data on the soil's temperature fluctuations to adjust the levels of each hormone. The cell cluster determines when to make the shift and come out into the world. It's vital to get the timing right for deciding when to emerge. This risky choice becomes more precise by depending on the aggregate responses of several cells. Deciding by evaluating two contrasting factors—the levels of two temperature-sensitive hormones—boosts the plant's odds of making the right call in an ever-changing environment.

(Shortform note: The two hormones in the seed are abscisic acid (ABA) and gibberellin (GA). ABA keeps the embryo in a dormant state, preventing it from growing until conditions are right. This hormone helps the seed survive by making it tolerant to drying out. GA, on the other hand, encourages the seed to start growing. It activates processes that weaken the seed coat, making it easier for the root to push through. The balance between these two hormones determines when the seed will break dormancy and begin to grow.)

This kind of intercellular signaling is similar to some formations in the brain. The human mind also transfers opposing hormones among cells to enhance our ability to make decisions in an unpredictable world. Instead of stimulating a muscle with a lone input, our brains decide by gathering hormonal data from multiple cells and filtering out unnecessary information in the process.

(Shortform note: This may not be entirely accurate. In Behave, Robert Sapolsky explains that the human brain makes decisions by sending electrical signals between neurons, which is much faster than the hormonal signaling that Schlanger describes. Hormones are slower and affect the whole body, not just specific cells. They help regulate things like stress and energy levels, but they don’t carry detailed information like electrical signals do.)

Plant Agency: Behavior, Ecology & Ethics

Schlanger argues that plants have agency, which is their ability to adapt to their surroundings. This agency allows them to assess their environment and change themselves to suit it. For instance, vegetation raised in dim lighting can expand two to three times to broaden their surface and capture more photons. Plants in excess water will produce hair-thin roots at the soil's top layer to obtain oxygen, even when the soil is saturated with water. Those deprived of water will grow less tissue overall but will increase their roots' surface area, making them extra long and thin to extend over a wide area.

(Shortform note: Schlanger uses the term “agency” to describe plants’ ability to adapt to their environment. In Vibrant Matter, Jane Bennett defines agency as the ability to make a difference in what happens around you. She argues that agency doesn’t require consciousness or a brain, but rather the ability to exert effects in ecological relationships. For example, a plant’s ability to adapt to its environment by changing its physical structure or behavior demonstrates its agency, as it actively influences its own survival and the ecosystem around it.)

Plants can also pass adaptations to future generations. For instance, yellow monkey flowers faced with threats will develop offspring that have defensive spines on the foliage. Wild radishes that have survived a caterpillar infestation will create offspring with leaves that have additional bristles and defensive chemicals. Additionally, plants raised in arid ground will produce offspring with deep, long roots to access water. Plants can even replicate the form and tactile quality of other species' leaves, including their hue, vein pattern, and texture.

Epigenetic Inheritance

Biologists have long debated how to interpret the phenomenon of plants passing adaptations to future generations and replicating other species’ leaves. In Evolution in Four Dimensions, Eva Jablonka and Marion J. Lamb argue that these phenomena are best understood through the lens of epigenetic inheritance. They explain that environmental factors can induce molecular changes in plants that are then passed on to offspring, allowing for rapid adaptation to changing conditions. This perspective challenges the traditional view that inheritance is solely based on DNA sequence, suggesting that plants can transmit acquired characteristics through epigenetic mechanisms.

In the following sections, we will explore the mechanisms and manifestations of agency in vegetation, along with the ecological, agricultural, and ethical implications of this agency.

Mechanisms & Manifestations of Plant Agency

According to Schlanger, plants can control their destiny by adjusting their forms to match shifts in their surroundings. They can also pass on these adaptations to their offspring, directing their species' evolution.

(Shortform note: In Evolution, Douglas J. Futuyma and Mark Kirkpatrick explain that plants can change their growth patterns in response to environmental conditions, but they can’t control their destiny or direct their species’ evolution. Instead, the evolution of plant species is governed by natural selection, which acts on genetic variation within populations.)

Schlanger adds that flora can interact and have personalities. They can remember past experiences and use that knowledge to make decisions. They can perceive their surroundings and perform complex assessments of their environment. They can even identify their relatives and modify their actions accordingly. For example, some plants will invest more energy in producing flowers when growing among their relatives, which helps attract pollinators to them collectively.

Plants can also interact through chemical signals both underground and aboveground. They can alert one another about threats like herbivores or diseases, and they can even coordinate their growth and development with neighboring plants.

Plants Have Personalities

In Plant Behaviour and Intelligence, Anthony Trewavas explains that individual plants exhibit consistent behavioral differences over time and across situations. Some plants are more competitive and exploratory in their growth, while others are more conservative and stress-tolerant. These repeatable, correlated patterns of response form stable behavioral syndromes that can be analyzed in exactly the same way that animal biologists analyze “personality” in animals. This shows that members of a single plant species are not behaviorally identical but display enduring individual styles of behavior.

In the following sections, we will explore plant agency on relational and individual levels.

Relational Agency: Communication & Cooperation

Schlanger explains that vegetation and fungi engage in complex symbiotic relationships. Fungi live underground and can't perform photosynthesis, meaning they get carbon from plants. In return, fungi provide plants with minerals from the soil. These relationships are not always equal, as fungi may demand more carbon from plants when minerals are in short supply. Similarly, plants might direct more carbon to fungi that provide more minerals. Some plant species provide no carbon to fungi at all.

(Shortform note: Many scientists use the concept of “biological markets” to understand the relationship between plants and fungi. In a biological market, organisms exchange resources or services, like carbon and minerals, based on supply and demand. This means that fungi might demand more carbon from plants when minerals are scarce, and plants might give more carbon to fungi that provide more minerals. This flexible exchange system helps both plants and fungi adapt to changing environmental conditions.)

Additionally, Schlanger discusses how plants can identify their relatives and adjust their behavior accordingly. They can detect their kin by perceiving the light quality reflected from their leaves or by detecting chemicals excreted by their roots.

(Shortform note: Some scientists, like the plant biologist Lincoln Taiz, argue that plants don’t actually identify their relatives. They claim that the results of experiments that seem to show that plants can identify their relatives can be explained by other factors.)

Individual Agency: Perception & Adaptation

Schlanger describes how Boquila plants adjust to their environment by mimicking neighboring plants. Boquila is a vine capable of mimicking the leaves of as many as four separate trees at once, such as their form, hue, surface, and vein design. This mimicry lessens the vine's consumption by herbivores. By camouflaging itself among the foliage of a tree, every Boquila leaf is at a lower risk of being eaten.

Boquila can adjust to any nearby plant, without needing to physically touch them. It detects nearby plants immediately and alters itself to imitate them. It can even change its leaves to resemble the foliage of several trees simultaneously, without physical contact.

How Do We Know That Boquila Can Mimic the Leaves of Neighboring Plants Without Physical Contact?

To support the claim that Boquila can mimic the leaves of neighboring plants without physical contact, researchers have used a combination of field observations and statistical analyses. In a study, researchers photographed Boquila leaves alongside the leaves of their host plants in their natural environment. They then used image analysis software to extract quantitative data on leaf shape, size, color, and vein patterns. By comparing these traits between Boquila leaves and the leaves of their host plants, they found that Boquila leaves closely resembled the leaves of their host plants, even when they were not in direct contact.

Ecological, Agricultural & Ethical Implications

Schlanger suggests that plants' ability to respond to sound might provide significant agricultural benefits. If we can make plants produce substances that repel pests by playing them sounds, this might decrease or even eliminate the necessity for synthetic chemicals in agriculture. Sometimes, it could enhance the quantities of the compounds the crop is cultivated to produce. For instance, mustard is a crop that's cultivated to harvest the plant's natural pesticide—mustard oil. By using the appropriate sounds to raise the defenses of a lavender plant, you can increase the production of the protective substances we value in lavender oil.

(Shortform note: If you work in agriculture or agribusiness, consider experimenting with sound-based plant responses on a small scale. Set up pilot plots where you expose some crops to specific sound treatments and compare their pest resistance, chemical usage, and yields to untreated controls. This approach allows you to assess the practical benefits and challenges of integrating sound-based methods into your existing operations without risking your entire crop.)

In the following sections, we will explore the community, ecosystem, ethical, and legal implications of agency in plants.

Community & Ecosystem Impacts

Schlanger notes that ecosystems are intricate and unpredictable, with many factors influencing their dynamics. The surroundings are constantly changing, and so are the characteristics of the individuals within it. This constant change prevents any single species from dominating the ecosystem for long. The intricacy of ecosystems also makes it difficult to predict how they will respond to changes.

(Shortform note: Ecologists have found that some species can dominate an ecosystem for long periods of time. Scheffer et al. (2001) describe “alternative stable states” in which one or a few species can dominate a habitat for decades or even centuries. These states are maintained by feedback loops that reinforce the dominance of the prevailing species. For example, a forest dominated by a particular tree species can persist for centuries, with the dominant trees shaping the environment in ways that favor their own continued success.)

Ethical & Legal Considerations

Schlanger notes that giving plants legal rights is being considered. In 2021, the White Earth Band of Ojibwe granted legal status to wild rice, acknowledging its "inherent rights to exist, flourish, regenerate, and develop." However, in 2022, the case was dismissed by the tribe's court, citing an absence of prior legal decisions. At the same time, Ecuador's Indigenous Kichiwa of Sarayeku are campaigning for the UN to acknowledge that their forested land in the Amazon rainforest is a sentient entity with universal rights.

(Shortform note: The idea of granting legal rights to elements of nature is part of a broader movement in environmental law and philosophy known as the “rights of nature.” This movement argues that natural entities—such as rivers, forests, and ecosystems—should have legal rights similar to those of humans and corporations. The concept was popularized by Christopher D. Stone’s 1972 article “Should Trees Have Standing?” in which he argues that natural objects should be granted legal personhood, allowing them to be represented in court by human guardians.)

Schlanger adds that ethical considerations regarding flora are gaining attention. Some Indigenous philosophies regard plants as kin or forebears, deserving of respect and autonomy. This perspective challenges the traditional view that only people and non-plant creatures deserve moral consideration. The idea of plant personhood is gaining traction, with some arguing that plants possess autonomy, will, and an inherent right to existence. This shift in perspective could transform our ethical and legal frameworks, altering how we inhabit the planet.

Counterpoint: Plants Lack Moral Standing

In Practical Ethics, philosopher Peter Singer argues that only beings capable of experiencing pleasure and pain have moral standing. He contends that plants, lacking a nervous system, cannot experience suffering or well-being and therefore do not have interests of their own. Singer maintains that while we should protect plants for environmental reasons, they do not possess personhood or intrinsic moral value. This view challenges the idea that plants have rights or deserve moral consideration independent of their role in supporting sentient life.