Plant Brains and Root Architecture
What began as an engineering comparison of soils under native plant communities and culivated land has turned into an investigation of what it means to be alive and have a mind.
Plant Brains and Root Routing
To try to do justice to the “terra-forming” capabilities of plants, I want to start by describing how, in relationship with their environment, plants form their own bodies. I want to talk about what plants do and how they do it to help re-consider who they are. By asking you to re-consider who plants are I am also asking you to re-consider fundamental notions of identity, like who has a mind and where does it reside?
Talking concisely about roots in isolation from their intimate relationships with the earth is a challenge. I will start with some terms and definitions to set a common baseline but will have to gloss over other details for now. I will touch on the “context” that helps shape the root system – climate, geography, geology, soil and living community, but will return to plants’ specific interactions with their milieu in more detail later.
Plant architecture – the three-dimensional organization of plant parts - is highly variable (mutable), but highly organized; Plants continuously synchronize multiple resource acquisition, allocation, growing and sharing activities. The growth of plants, the particular architectural model of the species, is strongly constrained by genetics but is intimately influenced and changed by the environment. The degree to which plants are affected depends on their plasticity – their mutability and the particular set of circumstances.
Plant rooting architecture is primarily engaged in managing water. Water is the conduit of life. Plants manage water, to the extent that they can, in the ground and in the sky. The ground and sky are like big water reservoirs that the plants help regulate. Sometimes there is too much water from the sky and sometimes not enough and plants plan for and execute responses that better prepare them for both extremes.
The other aspect to re-consider about “plants that plan”, is our hard-wired idea of what and where a brain resides. While roots may reside below the ground, they should be considered the anterior - the front or top when it comes to where the brain resides. It was Charles Darwin, who first suggested that root tips act like a diffuse brain. In “The Power of Movement in Plants” (1880) Darwin wrote: “….the tip acts like the brain of lower animals; the brain being seated within the anterior end of the body, receiving impressions from other sense organs and directing several movements.”
Primary plant development is driven by stem cells located in the tips of stems and roots. As the shoots and roots grow, these cells sense, accommodate and adapt their morphologies and behavior to meet and manage their surroundings. The plant root and shoot system are interconnected by long, continuous strands of highly elongated vascular cells. Among the many materials transported along these cells is auxin, a plant hormone that directs plant growth and, as research increasingly demonstrates, acts like a plant neurotransmitter, facilitating long-distance communication among all plant parts.
Auxin is largely made by leaves and shoot tips and most of it is transported to the root tips. Cell division and elongation rates in root tips are more than ten times faster than shoot tip rates. Roots pare back unsuccessful routes all the time. The faster growth of roots means more exploration opportunities can be had in the uncertain earth than in the sky.
In the root tips there is a transition zone of “brain-like” tissue between the very tip of the root (aka: the apical meristem) and the cells that elongate during root growth (basal elongation region). This zone has thousands of active synapses that receive and respond to external signals and developmental cues and synchronizes cellular activities and electrical responses. This zone also exhibits rhythmical oscillations in the uptake of oxygen, potassium and calcium, just like a brain (Baluska, et.al., 2004). The root tip is also the first part of the plant body to emerge from a seed, like the emergence of the head is the first part of the body to emerge from an animal’s fertilized egg.
Types of roots and rooting architecture
Plant root systems are a challenge to classify because of their high degree of plasticity. Root architectural analysis attempts to tease out those forms that are inherent to the plant and those that result as a reaction to environmental influences. These reactions to external stimuli are called tropisms for plants.
A tropism is the directed movement of a part or the entire plant in response to these external stimuli. These stimuli induce differential cell and tissue growth that deform the plant parts and change its orientation to stimuli dynamically and interactively. Tropisms are classified by the stimuli including, among others, phototropism (response to light), geotropism (response to gravity), chemotropism (response to particular substances), hydrotropism (response to water), thigmotropism (response to touch), traumatotropism (response to wounds), and galvanotropism, or electrotropism (response to external electrical fields). Typically a plant is responding to multiple stimuli from different locations at the same time and the resulting movement integrates multiple signals and results in complex behaviors.
Root morphology and positional information are best represented by a root map or root “architecture” in three dimensions. From an experimental point of view, classical methods for observing the root system – excavation, soil cores, trench walls and rhizotrons (underground laboratories with glass walls) give the most direct and comprehensive access to rooting zones as they exist naturally (Bohm, 1999). I will endeavor here to show the most comprehensive 3-D root maps that are mostly the traditional ones.
The root systems of all plants generally have the same major functions: water and nutrient uptake, anchorage, carbohydrate storage, deposition and excretion of biochemical compounds, the production of young, and associations with other plants and organisms. Acquiring multiple resources, when they can be haphazardly distributed in patches underground can mean complementary growth of different root classes, e.g., shallow vs deep, at the same time. The plant must direct the symphony of growth so that all the aspects of growth make sense together on a continuous basis.
.Root systems of woody plants have a framework of large, rigid woody roots that support the finer roots and the stem. Most tree and shrub roots grow horizontally and radially. The upper one to two feet of soil typically contains 60 to 80% of tree roots by weight and the top 3-ft more than 95% of the tree root mass. Contrary to common images, root architecture is not a mirror of canopy architecture. The diameter of tree roots often extend at least two times beyond the edge of the canopy (or “drip line”) and in some instances can be five times larger than the canopy diameter.
Tree rooting patterns fall into three major categories: 1) taprooted, 2) heart-rooted and 3) flat or plate rooted. However, these can be fluid categories. Some trees start with a tap root but evolve into flat or heart rooted trees. Some flat-rooted and heart root trees can send out deep sinker roots that resemble tap roots. Generally speaking, oaks, walnuts, hickories, hackberry, honeylocust along with some cottonwoods and some maples are or can be tap-rooted. Heart-rooted trees include beeches, hackberries and European larch. Flat-rooted trees include some evergreens like Norway Spruce and cedars, willows, hophornbeam and some maples.
Herbaceous plants tend to have fleshy roots, although some are woody-like roots, particularly the starch-fllled tap roots. Most perennial grasses are composed completely of thin, fibrous roots. Perennial forbs can be both tap-rooted and fibrous-rooted. Tap-rooted plants include carrots, pokeweed, thistle, pigweed, burdock, mullein, dandelion, chickory, blazing star, false boneset, prairie clover, coneflower, compass plant and plants with compound flowers like tansies, daisies and some asters.
Root plasticity
Plants compensate for being stuck in place (sessile) with multiple, modular systems that determine their shape based on coordinated, environmental responses (aka: reciprocity). Developmental plasticity allows the plant to collect signals and information from its environment and incorporate them into decisions about growth and development. This allows plants to optimize the positions of their structures for resource acquisition and adjust their forms to conditions as they change. Plants can self-prune and initiate shoot and root growth from many locations in the plant body. This is called adventitious rooting; exemplified by roots adapted to continuous flooding or roots that turn into shoots (suckering).
Suckering refers to shoots that grow up from lateral roots. Rhizomes are modified stems that grow underground and can produce shoots. A stolon is a branch that bends down to touch the ground and becomes a root. Sprouts are shoots that come off the from the base of the trunk or root crown. A major difference between plant and animal development is that positional information rather than lineage determines cell fate in plants (Malamy, 2005).
The plant’s abilities in the short term, to meet immediate needs and future plastic responses are the key to adapting to environmental conditions. This “metaplasticity” can impact the plant’s current life, while avoiding the costs of maintaining a high degree of plasticity throughout life. For instance, Wang, et.al. (2017) found that early exposure to inundation or drought, alters how plants respond to later conditions. They suggested that exposure to extreme events early can induce physiological or morphological changes that improve tolerance for extreme conditions later.
Plants also show a high degree of plasticity in response to their neighbors, and even the specific identify of their neighbors, Callaway, et.al., (2003) suggest that phenotypic plasticity may allow species to adjust to the composition of their communities, potentially promoting coexistence and community.
Calvo and Friston (2017) propose that plants respond in a fast yet coordinated manner to environmental contingencies. Plants sample the environmental information that will have an adaptive value and they attempt to anticipate the correct response. This anticipatory response is an attempt to minimize uncertainty, to minimize entropy over time. As they put it, “….one can cast biological self-organization as minimizing surprise over time.”
An example of minimizing surprise, comes from the investigation of phototropism in Malva multiflora (formerly known as Lavatera cretica. Common names: Cornish mallow or Cretin hollyhock), a native of the Mediterranean basin. Malva leaves not only follow the sun’s path each day,; at night they turn to the position the sun will be in when it rises in the morning and this is a different position each morning. Malva leaves kept in a dark lab will still turn their leaves to the accurate position of the rising sun, without having seen the sun for up to three or four days (Schwarz and Koller, 1986).
One of the primary drivers of this research for me is to better understand the capacity of plants to sense, find and manage potential water sources, even when that water source seems cut-off or distant and against all odds to reach. There are many examples of plants across the globe that are deep-rooted/tap-rooted that penetrate through hard clay or rock not only anticipating the water to be had, but are re-plumbing the soil. When a root exploits a crack or crevice and finds it way through formerly impenetrable layers, it not only reaches a new, potential water source it connects new water movement routes in and out of a formerly inaccessible area.
Plants’ optimization of water use recognized by Robert Horton in 1933 in a forest in the Catskill Mountains, connects rooting architecture, plasticity, anticipatory plant behavior and plant brains. Water use optimization is not a random or completely programmed outcome. It is the result of millions upon millions of conscious decisions by an endless number of individuals. Arthur Reber at the City University of New York theorizes that awareness/sentience is a property of all life.
I believe every living thing has a mind; even as it varies in size, complexity and degree of self-regard. Even cells learn, have cellular memories and make tactical decisions (Reber,, 2018. First Minds: Catepillars, Karyotes and Consciousness); it is sentience, however, rudimentary. As an engineer I’d say these roots, these plants, these communities compose the most graceful set of optimization routines I’ve ever seen; as a living being I see countless interacting individuals from cells to organelles, organs, tissues and bodies of animals and plants all alive, all aware, making things up and making them right as they go.
We never seem to talk about plant predation, the fact that some plants will compete for water light and nutrients is a given but the release of ecosystem changing chemicals to retard competition can also add light to this complex dynamic eg toxic nature of camphor also works against other plants -- camphor from laurel roots and fallen leaves can effectively poison nearby soil . Thanks again
The interdisciplinary questions this study raises are breathtaking. Humans think of themselves as planters. Perhaps we would be better off if we thought of ourselves as plants.