Annals of Botany 93: 345±351, 2004 doi:10.1093/aob/mch058, available online at www.aob.oupjournals.org VIEWPOINT Plant Intelligence: an Alternative Point of View

RICHARD FIRN* Department of Biology, University of York, PO Box 373, York YO10 5YW, UK

Received: 9 September 2003 Returned for revision: 24 October 2003 Accepted: 8 January 2003

The concept of plant intelligence has been advanced by Trewavas as a potentially useful framework to guide those seeking to understand plant growth and development. In this short critique, the validity of this concept is critically assessed. Central to this critique is the proposition that the concept of the individual, to which intelli- gence and behaviour are intimately linked, cannot usefully be applied to plants. It is argued that the adaptive responses of plants are best appreciated if the importance of the autonomy of the individual organs is acknow- ledged. Although Trewavas does acknowledge the autonomy of organs by describing an individual plant as being `a democratic confederation', that terminology implies a complexity to the interaction between organs which would demand a cogitative ability beyond that actually demonstrated in plants. It may be more appropri- ate to consider a plant as operating normally as a simple economic federation of many specialized economies (organs and cells). Occasionally, there can be a dramatic, and sometimes complex, reshaping of the economic balances, with the result that the fate of some or many of the individual cells will change. However, such major changes in growth and development are driven by a few simple events in an individual organ and cells. These driving events are more akin to small local revolutions in individual states than they are to democratic decisions in a sophisticated confederation. ã 2004 Annals of Botany Company

Key words: Phenotypic plasticity, adaptation, development, individuality, intelligence, plant behaviour.

INTRODUCTION the overlaying of a very in¯exible pattern of growth and In a stimulating, thought-provoking and wonderfully wide- development with an extremely variable behaviour (both ranging article, Trewavas has argued that `the use of the `hard wired' and learned). (This short article focuses on term intelligence with regard to plant behaviour will lead to comparisons of unitary `higher' animals and plants while a better understanding of the complexity of plant signal recognizing that equally important comparisons might be transduction, the discrimination and sensitivity with which drawn between `lower' and colonial animals and plants.) In plants construct images of their environment and raise contrast, the concept of the individual in the case of a plant critical questions how plants respond at the whole plant is much harder to de®ne. It is easy to think of a very young level' (Trewavas, 2003). Trewavas argued that evidence of seedling (the commonest experimental object in plant learning and memory in plants, both considered necessary science) as being an individual, but which is the individual components of a truly intelligent system, has largely been bracken plant in a huge clonal patch of bracken? Are all ignored with the result that there is little appreciation of Cox's Orange apple trees the same individual? Maybe, intelligence in plants. While there is a danger that any along with the injudicious use of statistics and the excessive discussion about intelligence in plants becomes distracted constraints that tight experimental control impose on studies by semantics, beneath any such linguistic debate there are of plant functioning (Trewavas, 2003), we need to add that a important issues that need to be appreciated by anyone blinkered use of `individual' young seedlings as model attempting to understand how plants work. systems may also be misleading. Interestingly, the wide- spread adoption, in the 19th century, of the very young seedling as the model for much experimental work INTELLIGENCE IS A PROPERTY OF coincided with the exploration of the proposal that, as in INDIVIDUALS? animals, certain plant organs could control the growth and development of other parts of the individual (for some A key issue in discussing intelligence is the concept of history, see Went and Thimann, 1937). In a very young individuality. The word individual leads us to the ®rst seedling with only two, distinct elongating organs, each semantic trap. The word in English derives from Latin, in- possessing an apical region pioneering the exploration of meaning not and dividuus meaning divisible. However, their own very different environments, it is maybe reason- dictionary de®nitions (e.g. Chambers, 1959) stretch this able to consider the seedling as an individual. However, de®nition to encompass meanings from subsisting as one to having an independent existence. Animals are highly once the two young axes lose their monopolies when other individualistic in the sense of subsisting as one. Each organs grow and develop, the sense of what is the individual animal has an individual identity which is a consequence of becomes less clear. Consequently, while there are some general principles governing the ways in which plants grow * For correspondence. Fax 01904 328505, e-mail [email protected] and develop, principles that may be most easily identi®ed by Annals of Botany 93/4, ã Annals of Botany Company 2004; all rights reserved 346 Firn Ð Plant Intelligence: an Alternative Point of View studying very young seedlings, the experimentalist needs to response to a stimulus' and that allows the term to be be conscious of the need to explain how similar processes applied rather easily to plants. Plants do indeed show might be controlled in more complex, multi-organ plants. A responses to stimuli and clearly those responses are largely 4-d-old arabidopsis seedling might look like an individual adaptive, hence one comes to a point that Stenhouse's but a strawberry plant with attached runners, a coppiced de®nition of intelligence could be applied to plants. But has willow tree or a shallot plant present dif®culties for those this semantic exercise helped us identify new, productive seeking to identify the individual. Those experimentalists ways of observing, studying, manipulating or understanding who con®ne their studies to young seedlings are not plants? Several generations of scientists have never doubted confronted with one of the key differences between animals that all organisms have evolved to show some ability to and plants. In animals, growth and development is the adapt to their environmental circumstances. A bacterium enlargement and modi®cation of the individual. But, in can monitor its environment and instigate developmental plants, growth and development also includes the adding of processes appropriate to the prevailing circumstances, but is new members, or the discarding of some old members, of that intelligence? Such simple adaptation behaviour might what Trewavas calls the `democratic confederation'. This be bacterial intelligence but is clearly not animal intelli- key difference is least apparent in young seedlings; but even gence. Hence we have reached a de®nition of intelligence in young seedlings the main organs can be grown in that has no meaning unless combined with another word or isolation, hence in the short term such organs largely depend used in a precise context in a sentence. This is not a new on their neighbours in the `confederation' as suppliers problem, as shown by the debate about whether machines (McIntyre, 2001) rather than governors. The `confederation' can be intelligent. The term machine intelligence can mean has evolved with economic dependence being the main something other than the intelligence of a machine. We can driving force behind specialization, hence the seedling is certainly start using the term plant intelligence but only if it really an economic union rather than democratic confeder- is agreed that the term has nothing to do with intelligence ation. (Plant biologists might more usefully read Adam (the ability to discern, comprehend and choose) in the more Smith than Thomas Paine to appreciate their experimental widely accepted sense. (To add to the opportunity for subjects.) The capacity of the seedling, and more evidently confusion it should be pointed out that the term `plant the capacity of the mature plant, is largely the summed intelligence' is already in use to mean the utilization of capacity of the component parts, with many of those data about the performance of factories to optimize capacities being interdependent largely in `economic' performanceÐbut plants could be considered as factories, terms. As will now be argued, any `intelligence' that so maybe that is OK; http://www.gefanucautomation.com/ might be ascribed to `the plant' could only reside in organs, plantintelligence/default.asp.) However, why do we need a tissues or cells because the concept of the plant as an new term? If the discussion of a plant's adaptive responses individual is a misleading one. is less ambiguous than a discussion of the plant's intelli- gence, why adopt the new term? Indeed, might not the adoption of the term plant intelligence begin to distort, INTELLIGENCE rather than clarify, our perception of the way in which plants `Intelligence is not a term commonly used when plants are function? To illustrate this danger, some of the key terms discussed' (Trewavas, 2003). Maybe this is because plants that are part of Trewavas's view of plant intelligence will are clearly not intelligent in the sense that most people now be placed under scrutiny. understand the term `intelligent'. The word intelligence comes from Latin (from intellegere to discern, comprehend and, literally, `choose between', from inter- and legere,to LEARNING choose). The key words in this de®nition are discern, Trewavas (1999, 2003) considers learning to require two comprehend and choose, all of which are terms that are faculties. The organism must be capable of having a goal, meaningful in the context of human behaviour. These terms, and the organism must be able to assess its current and the concept of intelligence, were adopted by those behaviour and adjust that behaviour to make the attainment participating in the development of the English language to of the goal more likely. As part of the latter ability, he describe actions, and to express thoughts, about their own considers the need for an error-detection mechanismÐa behaviour. The terms discern, comprehend and choose each method for judging current state against a reference state imply a considerable degree of mental processing of more (goal). Each of these elements needs to be considered from basic sensory information. There is rather little evidence the perspective of the adaptive responses of plants. that plants (or maybe more accurately plant cells) do anything other than rudimentary processing of sensed information and that alone should caution us against Goals adopting the term `intelligence' when discussing the Trewavas de®nes a goal as a plant's ®tness objective. The abilities of plants. Trewavas moves to ®rmer ground by main problem with the concept of a single plant possessing a adopting Stenhouse's view (Stenhouse, 1974) that intelli- goal is that each mature plant is made up of ever-changing gence is `adaptive variable behaviour within the lifetime of components (roots, leaves, ¯owers, etc.), each of which an individual', hence directing attention at the term occupy their own temporally and spatially variable environ- behaviour. This is convenient for Trewavas's thesis because ments. When a plant is at its most individualistic, as a seed, many biologists use the term behaviour simply to denote `a there are clear goalsÐdispersal, survival and germination at Firn Ð Plant Intelligence: an Alternative Point of View 347 the right time. However, shortly after germination each ational control of the organs is devolved to local sensing, so extending meristematic region, such as a root tip or an apical that the development and functioning of an organ can match bud, may encounter local conditions that are very different the local conditions and hence can contribute most effect- from those encountered by other organs on the same plant ively to the union. Selection in plants has favoured the exploring other localities. Furthermore, the conditions evolution of ¯exible modular organisms where the ®nal experienced by one generation of organs are likely to differ structure is just one of many different possible outcomes. from those experienced by later generations. The need to The developmental ¯exibility which enhances plant ®tness optimize the performance of each organ in its own variable, is in contrast to the adaptive ¯exibility that behaviour gives unpredictable environment has given rise to a selective to animals. process that has favoured specimens with sensing and response capacities, which are largely devolved to the organ or sub-organ level. This devolution of sensing and response Error detection to functionally specialized organs results in most goals As outlined in Box 1, a simple on/off switch linked to a being themselves functional and local. Thus it is hardly sensor is all that is needed for `error detection'. The surprising that the great majority of sensing and response examples of oscillations that Trewavas takes as evidence of events in plants are localized to the cell, tissue or organ, in `trial-and-error learning' could equally be used as evidence that order. Furthermore, because most organs are multi- for simple automaton behaviour. Examples of homeostatic functional and grow in heterogeneous environments, any mechanisms in plants and animals, which are readily overall goals must be ¯exible and necessarily based on a modelled in terms of a sensor and a reaction to sensor conservative compromise. To cope with the variability that state, provide robust, if simple control systems, suf®cient for an organ may encounter, it is likely that evolution will have many purposes. The fact that many examples of oscillatory selected for variants that can make approximately the behaviour occur in ephemeral organs also causes one to optimum number of structures (be they organs, cells or cell doubt whether any bene®t would accrue from the possession components), but where there is some variable functional of an ability to learn. For example, a root undergoing capacity. The variable functional capacity of each unit gravitropism is actually a different root at the start and end enables the overall current and short-term functional needs of the process. If a root is exposed to an extreme to be met. For example, the gas exchange capacity of a leaf gravitational stimulus twice, 12 h apart (which would be is in part determined by the number and the aperture of the an event so rare in nature that one cannot imagine selection stomata, and varying the latter is the most effective way of optimizing a mechanism that included a learning stage), the giving operational control to the leaf in a varied environ- second stimulus will initiate a new response in cells that ment. Furthermore, the environmental conditions that a have not necessarily experienced the ®rst stimulation, and developing leaf experiences might differ considerably from hence they cannot usefully have learned. This is in contrast the conditions that a mature leaf experiences. So for an to an animal where learned behaviour resides at the organ there may be one or more developmental goals and organism level ± the hand has no memory of being burned, some operational goals. The end result should provide but the brain controlling the hand might possess the suf®cient functional ¯exibility to cope ef®ciently with appropriate knowledge after an adequate previous learning whatever changing environment it may have to operate in. experience. Another level of control might have evolved to cope with rarer, more extreme conditions that push organs beyond the limits of their operational controls. Such `emergency' Choice shutdown routines at best protect all the members of the Trewavas considers the evidence that plants make economic union and at worst ensure the survival of choices, or decisions, in reaching his conclusion that plants suf®cient members to enable a successful recovery have a kind of intelligence. But can plants really make programme to be initiated. The graded responses of plants choices in any meaningful way? A thermostat can `make a to increasingly severe water stress would be an example of decision' (Box 1) but few would claim that it makes a real such `emergency' recovery routines (Hsiao, 1973). Even in decision in any sense of free will. There would seem to be a these circumstances, the response of `the plant' is actually confusion here between approaching a state with a limited the sum of the responses that take place in cells, tissues and number (often two) of predetermined outcomes and real organs. In such circumstances, each responding cell need decision making. A more demanding criterion for choice or have no information as to the location or scale of the decision making is whether there are several variable problem elsewhere, simply that a programmed response is outcomes, e.g. action or inaction, and different ways of needed to contribute to the ®tness of the union. taking action. When `choices' are made in plants they are In conclusion to this consideration as to whether a plant made at the organ (or cell) level and not at the plant level. can possess a goal, the most important means by which the Even for the most fundamental `choice' that `a plant' might economic union that is the plant achieves the fundamental makeÐto initiate ¯oweringÐone could argue that the goal common to all living creatures (survivorship and switch to reproductive development in the apex is a simple reproduction) is to possess both a ¯exible way of controlling two-state system. It is also surely signi®cant that the control the number and types of organs (organs can be initiated and of ¯owering is not determined centrally by `the plant', discarded) and allowing each organ considerable oper- indeed the photoperiodic sensing can occur in many leaves ational autonomy. Most of the developmental and oper- or even within a small part of one leafÐa very robust 348 Firn Ð Plant Intelligence: an Alternative Point of View system (Zeevaart, 1976). The plant does not make a choice plants is referred to Weyers and Paterson (2001).] It might to ¯ower, instead a leaf in¯uences the predetermined fate of also be signi®cant that some of the best examples of long- some cells in the apex. There is no choice made by `the distance signalling (e.g. ¯owering or tuberization) are plant' rather events occur within the plant that dramatically actually ones where spatial information is not importantÐ change the balance within the economic union, with many leaves can detect day-length changes and it matters consequent changes to the fate of some members of the not which one does. Are there any examples of spatial union. information derived by plants that is not used mainly locally? In conclusion, there is no convincing evidence that plants SPATIAL MAPS possess the ability to convey suf®cient information from Does a plant build up a spatial map of its environment? To sensors to any processing units to enable a meaningful answer this question one needs to think more carefully about spatial map to be constructed. The only map that a plant as a the type of plant, the type of environment and type of spatial whole makes of its environment is recorded in its morph- map that might provide a ®tness bene®t. Once again, the ology, anatomy and chemistry. It is a map of past events, not favoured experimental subject of those investigating a of future plans. sensing system in plants, the very young seedling grown in a very controlled environment, might be misleadingly simple. It is very clear that the construction and maintenance of an MEMORY accurate spatial map of a large plant such as a tree would There can be no doubt that the developmental history of a demand huge processing power. Thousands of leaves and plant can determine the outcome of many subsequent roots, each having a need to sense their local environment to sensing events. More accurately, the developmental history serve their own needs, would produce very large amounts of of many organs (or cells) can determine the response that data. The data would be changing by the minute as the results from subsequent stimulation. Is that memory or is it leaves ¯uttered in a breeze and the roots explored their very simply a consequence of developmental progression? A heterogeneous environment. This example indicates that simple washing machine controller possesses `memory' due there is a need for great caution when scaling up knowledge to the equivalent process of developmental progression. from young seedlings to more mature, more complex plants. Developmental progression at its simplest is a linear It might be more pro®table to regard a tree not as a big sequence of events, with one or more steps under the seedling but as a collection of thousands of seedlingsÐeach control of some sensor (Box 1). Once the sequence is potential cutting is a potential new plant, hence the concept initiated, progress is determined by the state of some or all has some validity. This consideration should make us of the controlling sensors. Events occurring at an early stage concentrate on the organizational level at which spatial can in¯uence the outcome of later events and the system can mapping could be meaningfulÐthe organ level. In the main, thus have `a memory' of earlier events. However, a crucial each operational or developing organ can, at best, sense a distinguishing characteristic of memory in animals is that few parameters that suf®ciently characterize their local the processes are not linear and they are not sequence- environment with respect to the function of that organ. Each dependent. organ has some extra information derived from the overall Assuming that `plant memory' is little more than integrative state of the plant (much of which might be developmental progression, it is clear once again that considered to be similar in nature to consequences of developmental progression would be a property of organs or independent, competing economic activities in a human cells and not plants. This fact reveals why it is unlikely that confederation). This extra information is available from the true memory could have evolved in plants. Organs, for summed activities of all members of the federation of example, need to cope with the environment in which they organs, but such exotic information is rarely complex. Some ®nd themselves and, being essentially ephemeral, they extra speci®c information might also be available to certain could bene®t little by retaining and using information of cells from chemical signals emanating from non-adjacent past events. A root growing in a heterogeneous environment cells. Although Trewavas, as Canny (1985) before him, cannot use an experience of its current state to predict its argues that the pathways of inter-organ transport could future state. Water (or potassium or nitrate, etc.) which is in allow complex chemical signals to ¯ow between organs, it is short supply now may be in ample supply later. The memory unlikely that the complexity of the signalling molecules of animals is aimed at using the experience of one organ to implies a complexity of message. (It should also be noted the advantage of othersÐa hand that is burned can provide a that the existence of signalling systems is not the issue at lesson to the whole organism so that in future other limbs stake, rather it is whether complex signalling is a good can avoid experiencing the same problem. Where is the indicator of the existence of an `intelligent' adaptive equivalent collective organism experience (= memory) of response.) The complexity of signalling molecules is only bene®t in a plant's growth and development? necessary to ensure that the right recipient receives the simple message. A signalling molecule is akin to a postcard bearing a tick or a cross as the message, with the much more CONCLUSION complex information of the address ensuring that the simple Adaptive behaviour takes many forms in different organ- message reaches the right recipient (Firn, 1985). [The reader isms. By using terms and concepts of learning, memory and interested in the complexities of hormonal signalling in intelligence that derive from human experience one can Firn Ð Plant Intelligence: an Alternative Point of View 349 build up a view of each type of adaptive behaviour in other organisms, but maybe only at the expense of clarity. Indeed, BOX 1 there is a danger that the adoption of concepts and terms Intelligence: switches, transistors, logic gates and from higher animals distracts attention away from the key computers adaptive processes evolved by organisms that have not been How much complexity must a system possess before selected to learn, memorize and think. In this article it has it is regarded as intelligent? The way in which been argued that the `intelligence' perceived in the adaptive relatively simple machines can be built to carry out response of a plant might best be considered to be the sum of quite complex tasks can help us answer this question. the collective adaptive responses of its cells. A general In a simple refrigerator, a temperature-controlled theme running through this article is that the human sense of switch (thermostat) simply turns the cooling system the individual easily distorts the ability of humans to on when there is deviation from a prede®ned appreciate how a plant operates. Even in the previous temperature. The refrigerator has been set a goal and sentence the two simple words `a plant' feel, to the author, achieves it by what could be considered to be error inappropriate because these words emphasize the individual detection. However, such a simple control system is entity. Plant population biologists have adopted terms such very prone to the kinds of oscillations that impress as genet and ramet (Harper, 1977) because they appreciate Trewavas. Due to the thermal mass of the thermostat that descriptors beyond the concept of the individual are and the thermal mass of the cooling coils, once the needed to understand plant populations. However, many thermostat removes the power from the compressor, plant biologists studying the functioning of cells or organs cooling continues beyond the value set by the don't look to plant population biologists for inspiration but thermostat. So the oscillations that Trewavas con- to animal biologists. Such plant biologists might bene®t siders evidence of sophisticated `learning' can also be from shifting their focus to other types of plants rather than characteristic of simple on/off controls, lacking other types of organisms. An appreciation may then develop hysteresis. More sophisticated controls can eliminate that clonal plants are simply demonstrating an extreme form temperature oscillations in a refrigerator. A controller of developmental adaptation, but it is a form of adaptation (known as a proportional controller) that can measure that can be seen in simpler terms in a 4-d-old arabidopsis the magnitude of deviation from a set point, and can seedling. In clonal plants, the ramet is a device to provide an modulate the magnitude or duration of the restoring ef®cient division of labour in a heterogenious, patchy process, will be able to control a process with much environment (Stuefer et al., 1996; Hutchings and less oscillation. In plants, where the control mechan- Wijesinghe, 1997; JoÂnsdoÂttir and Watson, 1997) and that isms are often devolved to the cell level and where the is simply an extension of the strategy of allowing organs an behaviour of the cells may not be synchronous, it is autonomy to optimize their localized performance. The possible that simple control systems suf®ce and that extent to which those semi-independent units interact, and oscillatory behaviour is to some extent mitigated by how they interact, is easier to understand if the overall the hysteresis capacity given to the system by the fact strategy of plant development is comprehended. that multiple sensors/regulators are aggregated at the Finally, the author, during the preparation of this article, organ level. has become very aware that human language guides and By combining a few simple sensors with a timing greatly limits our thoughts when trying to appreciate the mechanism, more sophisticated, but still simple, functioning of plants. It might ultimately be unproductive to control systems can be built. A well-known example debate whether a plant has memory, whether a plant can is the domestic automatic washing machine. These learn or whether a plant can possess a spatial map, because appliances have sometimes been given names or the key terms come from an organism that is highly descriptions by their manufacturers that imply an individual, an organism that ®nds it hard to appreciate the intelligence. However, in most washing machines, the faculties of organisms that lack individuality and all that that `intelligence' actually resides in the user and the implies. Our language lacks appropriate words. However, if designer of the controller. The user selects a goal new words are needed to describe how plants function, (from a choice of goals actually determined by the maybe we should invent new ones rather than trying to design engineer) and the controller is designed to start and stop certain processes in order to progress towards rede®ne existing ones. the chosen goal. To achieve this, several concentric, plastic pro®les are rotated by a motor and the peaks and troughs on each pro®le activate one on/off switch. ACKNOWLEDGEMENTS Each switch controls one of the main power circuitsÐ the main drum motor, the draining pump, the water Tony Trewavas is thanked for many years of provocative ¯ow valves, the timer motor and the heater. A few and stimulating comment. John Raven is thanked for simple on/off sensors (water temperature and water sharing his wisdom and knowledge. depth) interact with the motor that drives the spindle on which the pro®les rotate. In other words, complex temporal sequences, tied into several physical param- eters, can be achieved by no more than a few on/off 350 Firn Ð Plant Intelligence: an Alternative Point of View

switches linked to some timing mechanism. Such a ing that is characteristic of animals and it is a mechanical timer even has `memory' in that if its characteristic that distinguishes animals from plants. cycle is interrupted, it starts again where it has Where are plants on this continuum of control stopped. It also has `memory' in terms of being able to process sophistication? As argued elsewhere in this proceed to one process only after another, prede®ned, article, plants are modular organisms with some process has been ®nished (e.g. it won't spin your information ¯ow between organs. Quite sophisticated clothes until the drum is free of water). Thus a modern control systems can be elaborated by using a hierarchy washing machine controller could be analogous to of controls ± some local and some more regional. developmental progression in a plant ± processes Consider the operation of a heating system in an happen in a sequence and there are sensors that are apartment block with a central hot water heating made active for de®ned periods and which can system. The control for the temperature for most in¯uence the rate of progression itself. rooms is devolved to the user of that room (via a crude The machine analogies becomes more interesting if thermostatic radiator valve) but each apartment can one considers what happened after the transistor was control when heating should be available (time switch invented ± the evolution of complex mechanical controlling an on/off diverter valve connecting the controls eventually ceased. Mechanical controllers main heating supply pipe to the apartment's heating persisted for decades (indeed, they are still used in system). The apartment occupier can also set a lower some washing machines) because they were reliable, minimum level ± a simple thermostatic switch that robust and cheap. However, the invention of the bypasses the timer. The source of overall heat for the simple transistor opened up an evolutionary path that apartment block is controlled by sensors on the ¯ow would lead to controllers with more precision, more and return pipes and the output of the heat source is versatility and lower costs. The transistor can be proportionally regulated to meet demand. To antici- regarded as an electrical on/off switch. Within 10 pate demand somewhat, sensors for the outside years of its invention it had become possible to temperature are also built in to the control system of produce devices that incorporated several transistors the central heat source. In other words a devolved, as cheaply as producing one transistor. By connecting hierarchical system of simple controls meeting local several on/off transistor switches, such that speci®c needs, overlaid by a few layers of controls operating at inputs and outputs were connected, it was possible to a higher level, largely by monitoring the state of major produce electrical circuits that displayed Boolean ¯ows into and out of the system, can provide an logic. The development of logic circuits which adequately regulated, robust and versatile system retained information (memory) and which could which would work in most environments. If you add measure time, enabled the development of machines to such a hierarchical system some speci®c opportun- that could monitor many inputs, process the informa- ities for the communication between some local tion in complex ways and produce a variable output control systems, then even more complexity is dependent on the outcome of the result of the possible. The modern alternative to such a system of processing of that information ± the simple computer. linked, local, hierarchical controls (which may be The myriad of electronic devices currently available is what is found in plants) would be a central controller simply the result of an evolutionary process in an with a capacity to ®ne tune each control element via industry that has had to learn new ways of connecting learning and memory (which would seem to be up transistors in order to survive. analogous with animal control systems). 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