The word cognition is derived from the Latin verb “cognoscere” meaning “to know.” Cognition mostly refers to gaining and understanding knowledge through learning or experiences. It is defined as the ability to process information acquired through perception from different senses and experiences, integrate this information to understand and assess surroundings. The idea is to convert all the information gained from all the different sources to knowledge.

Cognitive processes are mental processes used to integrate new knowledge gained and use this knowledge for decision-making. Different processes are considered cognitive in nature like learning, memory, language, perception, and intelligence. Halfway through the 19^th century, interest in cognition and cognitive processes increased, and a lot of research work was undertaken to especially find out how these processes are involved in acquiring information and how does all the processed information influence the behavior of an organism. Plants also use cognitive processes to respond to external stimuli and learn from these responses for making important decisions for survival. Charles Darwin was the first person to acknowledge plants as having a cognitive ability in the 1800s in his book titled “The Power of Movement in Plants.” He said that plant roots acts like the brain of an animal, as it is sensitive to different stimuli in the ground like water, gravity, and light, and responds to these stimuli in a way which is beneficial for the plant (Darwin 1880).

The study of anatomy, physiology, and pathology of the nervous system is called neurobiology. Since animals have a well-developed nervous system, there is no controversy with this branch of biology when studying animals, but the idea of plant neurobiology has been rejected by many researchers because

• It does not increase understanding of plant physiology, anatomy or pathology.
• There are no structures like neurons, synapses or brain in plants.

Plant neurobiology is different from other disciplines of plant biology as it aims to shed light on the structure of the information network within the plant, discovering and understanding the role of systemic signals. The Darwinian idea of root tip acting as brain over time led to the study of neurobiology in plants as a new field of plant biology research. The research is focused on understanding how plants process the information obtained from their surroundings to develop, thrive and reproduce optimally (Brenner et al., 2006). It also studies how sensory perceptions and behaviors are memorized for predicting behaviors for use in the future based on past experiences. Many plant physiologists claim that animals and plants behavior have a similar level of sophistication but are disguised in plants as they take a long time to show certain responses.

Plant neurobiology explores the idea of plant intelligence. Jagadis Chandra Bose, a plant electrophysiologist in the early 19^th 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

1. Functional aspect that focuses on the survival value
2. 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.

Many researchers assume that since plants do not have brains, they are not capable of showing intelligent behavior and restrict the property of intelligence to animals. However, if single cells like bacteria are capable of showing intelligent behavior even in the absence of nervous system, so are plants as they are made up millions of cells that use electrical signals for communication (Trewavas 2017). The question that can be then asked is how these millions of cells interact with each other to produce intelligent behavior.

Initiation and control of behavior depend upon communication either between molecules, cells, tissues, organs or systems that link the external environment with the internal environment through a continuous dynamic feedback loop. The ‘bit’ is the standard unit of information with a yes or no answer. Research has been conducted to figure out the number of bits involved in information transfer during signal transduction processes at a cellular level. A limited number of outcomes have been shown for received information. Feedback is required for the information released to acknowledge the receiving as well as the correct interpretation to make sure the behavior processes continue on a pathway that will lead to a fitter organism (Trewavas 2016).

Plant communication is thought to involve cues rather than signals. Cues are incidental features that carry a certain meaning for target receivers, not shaped by natural selection, present in the environment while signals are traits that have evolved through natural selection for a certain role in communication. Plant use both cue- and signal-mediated communication by processing information about their surroundings both above and below ground and then sharing information about any resources present in their surroundings.  Plants use communication with its neighbors to identify and prevent competition with relatives; this can lead to facilitating the selection process of kin. Also, plants have been seen to use communication with its neighbors in order to solve a problem as a group (Gagliano et al., 2012).

Signaling pathways in plants use chemicals which provide a biochemical basis for learning, memory, computation, and problem solving as the plant do not have a brain or a neuronal network. Plant cells can exhibit rapid electrical responses or action potentials to environmental stimuli even though they are not neurons. Structurally, plasmodesmata in between plant cells provide an extensive electrical coupling which prevents the need for any cell-to-cell transport of neurotransmitter-like substances. These electrical responses are involved in the production of multiple organic molecules similar to neuroactive substances used by other organisms apart from many others, flowering, respiration, wound responses, tropisms, and photosynthesis. At every stage of development, the plant assesses its changing environment using a feedback system. The developmental change in the plant is coordinated by communication with the entire plant (Trewavas 2017). The type and characteristics of communication signals and feedback mechanisms change as the plant grows in size. The behavior effects depend upon the present state of the plant. Plant responding to one environmental condition may limit and affect its responses to other environmental conditions. This is particularly seen widespread in opposing plant responses to different conditions. Plants use a relatively small number of hormones as a way to sense and respond to different stimuli. The signal inducing one behavior may decrease the ability of the plant to induce a different behavior (Karban 2008).

Perception is defined as the identification, organization, and interpretation of sensory information detected by sensory regions to make sense of the environment. The ability of plants to detect and respond to their surrounding by adjusting morphology, physiology, and phenotype as required is referred to a plant perception. Plants can detect many different stimuli including chemicals, gravity, light, moisture, infections, temperature, oxygen and carbon dioxide concentrations, parasite infestation, disease, physical disruption, sound, and touch. Plant perception is assessed by disciplines such as plant physiology, ecology, and molecular biology.

Consciousness is described as being aware of the outside world. An organism with sensory perception and response is considered aware of their surroundings. On the other hand awareness of the outside world in some way or the other also requires self-awareness or recognition. Each plant is made up of many cells, self-organized in a complex system with different parts having individual control to use their local environment for the benefit of the whole plant. So plants do not have consciousness localized to a certain location like the animal brain, but it is rather distributed throughout the plant (Trewavas 2016).

Memory is considered a prerequisite of learning. Plants can form memories by learning through both positive and negative past experiences and use this information in the future when facing similar situations by modifying their behavior to increase chances of survival. Plants are capable of associative learning as shown by experimental evidence by Gagliano et al., that plants learn to associate one event with the anticipation of a future event known as Pavlovian learning. This has led to plants being recognized as proper subjects for cognitive research.

Priming in plants leads to a more rapid and increased magnitude response after experiencing repeated conditions like herbivory or disease. It can last for years and sometimes survive through the process of meiosis possibly by epigenetic changes to the chromatin. Many other conditions like repeated heat, drought, cold or salt stresses have shown to induce priming in plants. Such experiences are ae learned and stored for subsequent use in the future, eventually impacting the fitness of the individual (Trewavas 2016).

Animals and plants through evolution have developed a way to exploit their internal memory to remodel their behavior in order to optimize fitness. This shows that both animals and plants share some forms of memory and learning. When studying such behavior at a molecular level, not many differences are seen across the eukaryotic kingdom. All the research points out to a common ancient starting point for mechanisms involved in such behavior. The anticipatory behavior of time-estimation has been proved to be vital in phylogenesis. Due to different needs and requirements, animals in time have evolved to have nervous systems as compared to small plants. However, this deviation in terms of computational sense of information-processing is neutral (Garzón 2007). Plant biologists have not yet extensively studied the accuracy of information gathering from the environment or surrounding cells or from areas of shot and long-term memories in the cell, how this information is used or disapproved for future use even though such knowledge is important for understanding behavior, intelligence, and fitness.

Cognition, when looked at from the perspective of the whole organism and its interactions with the environment, is termed as Embodied Cognition. This has led to questioning of the concept of an individual as the fundament of intelligence. For an individual’s behavior to be accounted for, its surrounding environment has to be taken in context (Garzón 2007). Hence embodied cognitive science rejects the idea of cognition as a centralized process and rather an emergent and extended self-organizing phenomenon requiring an understanding of interactions between neural, body and environmental factors in real time. Plants show such decentralized signal-integration behavior when they detect and integrate information from multiple sensory areas simultaneously in real time.

Embodied cognition refers to organisms as adaptive and flexible in their behavior rather than being rigid, automaton, hard-wired and reflexive. In cognitive psychology, “all processes by which the sensory input is transformed, reduced, elaborated, stored, recovered and used” is cognition. Such a definition can include non-living things like tape recorders. There are five different limitations to cognition interpreted from an embodied and biological perspective (Calvo et al., 2009).

• Metabolism is a normal biochemical process for cognition
• Initially, cognition involves exploiting the spatial and temporal characteristics of relevant metabolic environment features by moving organisms.
• The movement takes the form of various sensorimotor organizations.
• Nervous system may or may not be used by offline control structures in expanding a basic online sensorimotor organization.
• The sensorimotor organization is single coherent unit rather than a collection of individual stimulus-response relations.

A plant is considered a modular organism with each module looking at its own survival goals that are not centrally controlled as some researches argue that the individual plant cannot have goals once it has passed its developmental seedling stage. Such views adapt a possibility of trees made up of multiple individual intelligent modules that cooperate, compete and influence each other to influence the behavior from the bottom upwards. Universally, not all plant species develop into larger organisms with modules dealing with different environmental conditions and stressors. Small plants may be exposed to same conditions across their bodies when above and below ground parts are considered separately. Apical dominance shown by plants proves the statements claiming that plants do not have central control of development as untrue (Trewavas 2016).

Animals grow and develop by means of changes that affect the individual rather than specific parts of the individual as compared to plants that do not have a central processing unit, grow and develop through individual parts that can be removed as the plant grows. Hence, any intelligence in plants is said to reside in cells, tissues or organs. Plants show morphological plasticity as they are made up autonomous modular units that arise from active meristems. Meristems are made up of undifferentiated stem cells that have the capability of growing into any type, size, shape or number of organs of undetermined characteristics. Hence, plants can develop in different ways according to the environmental stimuli. As animals rely on mobility for their behavior, plants rely on the modular organization for responding to environmental heterogeneity (Karban 2008).

Language is a tool that can be used to communicate, organize and transfer information. Plant language is not readily noticed because to humans it is a silent language of shapes, colors, and scents (Gagliano et al., 2015). One of the important ways by which plants communicate is through a chemical language using Volatile Organic Compounds (VOCs). Plants use VOCs as a defense mechanism against herbivores or insect attacks. The range of VOCs emitted by a plant varies from species to species and is even different between individual plants of the same species. Plant fitness is benefitted especially by emitting of VOCs by flowers and fruits. Scientists suggest that VOCs acts as a plant language with individual VOC representing words and the VOC signature representing sentences (Trewavas 2016). A sentence is a string of words put together. Similarly, individual volatiles put together as a VOC signature is important for eliciting responses rather than individual volatiles on their own. They emit these VOCs which can be interpreted differently depending upon the target receivers. Sometimes plants can add a bitter word in the form of a bitter chemical along with other VOCs like nicotine to deter unwanted visitors from stealing nectar on the other hand same chemicals can be used to attract visitors to visit more flowers increasing the chances of successful reproduction (Gagliano et al., 2015).

Plant language is versatile as it can use one chemical which can induce multiple different responses. This is possible as a result of experiences and learning to adapt to cope with the ever-changing environment to increase the chances of survival and growth in a particular ecological setting. Three adaptive systems are thought to be the basis of human language including interactions of individual learning, cultural transmission and biological evolution (Gagliano et al., 2015). Similarly, plant language is also considered to arise from these adaptive systems as well. A better explanation for plants using VOCs for communication is to overcome limitations of the vascular system, as it is not widespread and does not cover all the potential areas that can be damaged by herbivores.

Plant science does not commonly use terms like learning, memory, language, and intelligence because these cognitive processes are considered properties of organisms with nervous systems. Although much research has been conducted to provide evidence in favor of plant cognition, the idea is still not widely accepted amongst the scientific community but has definitely made its place in the mainstream research.

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