Plant neurobiology explores the idea of plant intelligence. Jagadis Chandra Bose, a plant electrophysiologist in the early 19th century was the first to provide direct evidence for electrical signaling between plant cells for coordinating responses. Bose concluded that plants have a nervous system, a form of intelligence, memory and learning capability (Shepherd 2005). This wasn’t accepted well by many scientists back in the day, years later it has received little acceptance, but it is still a controversial topic.
The opinions about plant intelligence are not new. Almost 100 years ago, a Belgian poet Maurice Maeterlinck wrote an essay titled “Intelligence of Flowers” in which he described intelligent decision-making behavior in plants, especially the way roots grow by finding a way through a complex maze of rubbish (Cvrcková et al., 2009). Von Hartmann in 1875 published a report about leaf behavior stating ‘If one sees how many means are here to attain the same end, one will be almost tempted to believe that here dwells a secret intelligence which chooses the most appropriate means for the attainment of the end.’ In the early 1930s, Frits Went discovered an important plant hormone auxin and concluded that ‘In tropistic movements, plants appear to exhibit a sort of intelligence; their movement is of subsequent advantage to them.’ Von Leibig from Germany who discovered the mineral requirements of the plant for growth, once said ‘Plants search for food as if they had eyes’ (Trewavas 2017).
Information on past experiences and behavior, the presence of an alternate, and judgment of immediate future are needed for making a beneficial decision to increase fitness (Trewavas 2017). Although a little immediate impact may be seen when a less beneficial choice is made, it wastes essential energy that could have been used to improve fitness. Plant intelligence is defined as an intrinsic ability to process information from abiotic and biotic stimuli in the environment to reach optimal decisions for future activities (Brenner et al., 2006).
Many studies argue that plants do not have intelligence rather they are just adaptable to their surroundings, and simple adaptive behavior cannot be compared to animal intelligence. David Stenhouse wrote a book about evolution and intelligence in which he defines intelligence as “adaptively variable behavior during the lifetime of the individual” which fits plant intelligence. Mia Molvray wrote an essay on the criteria for recognition of intelligence in a non-human entity. She describes intelligence as not a quality that is present or absent rather it can be present to a varying extent, forming a series of stages. At the initial stages, intelligence is rudimentary, the minimum ability to respond adaptively to the environment. Followed by the ability to learn from novel stimuli and adapt to the changed environment. Eventually, higher cognitive functions like self-awareness and identification of objects are achieved. Stenhouse defines intelligence as a system capable of showing developmental or observable behavior, individual variability and adaptivity in the form of learning and memory while Molvray’s defines intelligence with emphasis exclusively on learning. The ability to learn and use novel experiences for the benefit of the individual is what truly defines an intelligent system (Cvrcková et al., 2009).
There have been many different descriptions of intelligence published till date (Trewavas 2017). For example,
- Intelligence is a property of an individual that enables it to interact with its environment. Plant behavior is expressed through phenotypic changes or molecular response to the external environmental or internal signals. For example, plants in the wild interact and respond to their environment through competitive, biotic and abiotic signals.
- Intelligence is an individual’s ability to profit from a goal or objective. The most successful and fit plant will provide more offsprings by producing a fit and healthy seed.
- Intelligence is how the individual can adapt to its different environment and objectives.
- Intelligence is the mental ability or capability for problem-solving and benefitting from the experience. Experiences through learning and memory can lead to behaviors that are beneficial for survival and fitness.
Plants form an inner representation or a cognitive map of their surrounding environment by continuously assessing and recording external stimuli. Plants that are able to cope with competition, abiotic and biotic stressors more rapidly or by showing greater plasticity at a low cost are said to be fitter and hence more intelligent. Plants with better skills that help them best adapt to their environment throughout their life will be more fit. The fitness of an individual depends upon the sensitivity of the organism to the resources available in the surrounding environment and its ability to gain maximum energy from the resources while spending minimum energy. Resources are then stored to provide food for seeds. The number of seeds acts as a proxy for fitness in the wild. Apart from the available resources, environmental conditions can interfere with the contracting resources and can be damaging to the individual. So for the plant to show intelligent behavior it needs to be highly sensitive to all the different types of signals (biotic/abiotic) in its environment to optimize its fitness chances (Trewavas 2017).
One of the aspects of intelligence is plasticity described as the reversible changes in behavior that enables an individual to dominate its immediate environment and aid in optimizing fitness (Trewavas 2017). Conditioning is when plasticity is developed through previous experiences by the individual and preconditioning is when plant responses are shaped by its parent’s experiences rather than its own (Karban 2008). Plants show developmental and physiological behavior plasticity to cope with the constantly changing environment. For example, deciding where and when to look for nutrients, how to use nutrients, which organs to grow or mature and age, when to reproduce, how many offsprings to produce, how to defend against an attack and where to produce such defending response in the plant (Brenner et al., 2006).
Plants have organizational skills as described by Trewavas in his book “Plant Behavior and Intelligence” (Trewavas 2016). Such abilities allow the plant to interact with its surroundings to increase their chances of survival and to identify external signals. Plants have spatial awareness especially when it comes to roots as these are organized as per the neighboring plants to maximize absorption.
The judgment of plant intelligence has always been debatable as compared to in animals. Swarm intelligence is used to describe the behavior shown by social insect colonies where the entire colony runs on complicated messaging and feedback mechanisms between individuals similar to plants, where such communication and feedback takes place between cells, tissues, and organs. Hence, swarm intelligence in social insects is analogous to the description of intelligence in plant behavior. Swarm intelligence is a system made up of multiple homogenous individuals that interact with each other and the environment forming a complex dynamic network based on feedback processes for change and control of the function. The overall swarm behavior is flexible as each individual can perform different function in parallel to each other. The individuals are not aware of the overall behavior, with the behavior coordination requiring no overall controller. The system is fault proof as there is very little difference in behavior observed in case of any loss. Plants show swarm intelligence as they are self-organizing with no overall phenotype and development controller. Leaves and roots although having different functions work parallel to each other to obtain essential nutrients. Trimming of roots and shoots does not produce any behavior change showing that the system is fault proof. Feedback processes are used by the plant for controlling growth and phenotypic plasticity. The intelligent behavior of swarms and plants is indicated by a quote ‘Indeed it is not too much to say that a bee colony (individual plant) is capable of cognition in much the same sense that a human being is. The colony (plant) gathers and continually updates diverse information about its surroundings, combines this with information about its internal state (assessment) and makes decisions that reconcile its well-being with the environment’ (Trewavas 2017).
An improvement in the behavior later on due to experiences in a lifetime of an individual is adaptation. Adaptive behavior is a form of intelligence as it is more rapid, occurs at a lower cost with a higher probability thus overall improving the efficiency of the individual by increasing their fitness. Plants have a wide range of sensitive signals similar to the five human senses. The adaptively variable phenotypic changes that occur in the individual are by context intelligent in nature in the correct environment. The signals can be regulated by the intensity of the stimulus or by other signals interaction received simultaneously. Such observations led to the Darwinian analogy of root tip and the brain. Development is important for an individual plant’s survival and reproduction but is not the same as behavior. For example, a plant cannot develop without seed germination, but at what time the seed will germinate is the adaptive and intelligent behavior (Trewavas 2017).
Behavior is the combination of responses, reactions or movements made by an organism to a stimulus. It is a rapid and reversible response to a stimulus and is considered a form of phenotypic plasticity. In plants, the behavior is visible usually due to changes in the growth. However, without measuring the growth, it is hard to see such behavior as it is very slow to occur. Plants show three different types of behaviors which are associated with cognition (Karban 2008)
- Plants usually anticipate future environmental changes before they occur.
- Plant behavior is conditioned by experiences that either the individual itself or their parents have had. Hence, plants have a memory of past experiences which can influence their responses.
- Plants use signals to communicate with other organisms that can alter their behavior.
Some scientists argue that plant behaviors are reactive as they are automatic and does not change in many different situations, this is based on the observation that plants respond to thresholds, gradients, or changes in the magnitude of the environmental variables rather than the more complex variation in these variables (Karban 2008). Plants show behavioral spontaneity which suggests that it can control its own behavior and information flow. This is recognized when individuals behave differently from others in the same settings or conditions (Trewavas 2016).
Movement and visibility are also considered signs of intelligence and behavior. When plant cells, through evolution, acquired chloroplast they started using light energy for photosynthesis preventing the need for the movement to find nutrients as animals do. In order to contain the osmotically active molecules made during and by using the products of photosynthesis inside the cells, plants cells developed rigid cell walls which further prevents flexibility and movement. This also limits the growth of the plant to certain regions called meristems found at the tips of roots and shoots. There is competition among plants similar to seen in animals but for slightly different factors like light, water, and minerals. The way plant deals with competition, as they cannot move around, is by fighting over space using a branching structure with growing tip to occupy maximum space, obtain resources over a wide area and deny other nearby competitive plants from gaining resources. Similarly, competition for light has led plants to grow further upwards in height and increase in width using cambium tissue. Growth in plants is very slow to occur and is not readily visible as a change in the plant which is the reason for often ignoring plant behavior. Apart from growth, plants use motors cells in specific areas to change phenotype by using turgor pressure. This pressure does produce visible movement and behavior which are too slow for visual observation (Trewavas 2017).
There are two points of views according to N. Tinbergen (Cvrcková et al., 2009) in order to study any aspect of behavior
- Functional aspect that focuses on the survival value
- Casual aspect that is concerned with searching mechanical, developmental or evolutionary roots.
Most of the opposition of plant intelligence is based on the casual aspect of the phenomenon while the functional aspect seems to be neglected even though it allows for in-depth testing of the essential requirements of intelligence in plants.