A further issue is whether there is an intrinsic control phenotype, generated in the absence of signalling and often assumed to be a growth‐room phenotype. This perception is compounded by the relatively uniform appearance of many field crops, a situation that has arisen because crop plant breeders have (1) eliminated much signal‐induced behaviour and (2) selected only a few individuals (genotypes) for subsequent breeding out of a much greater range of behavioural and genotypic variation ( Lewontin 2001 ). But all phenotypes are constructed from a complex two‐way conversation between genes and the total environment. Growth‐room environments are perceived and used to specify only one out of a range of phenotypes. All that genes can ever do is specify a norm of reaction; they are not invariant determinants of phenotype or behaviour ( Lewontin 2001 ).

A further complication is the potential lack of reversibility in phenotypic responses raised in this issue by Metlen, Aschehoug & Callaway (2009 ). As they pointed out, behavioural plasticity in secondary metabolite production is reversible. But in contrast, abscission can be used to substantially reverse phenotypic changes, and innate animal behaviour certainly appears irreversible.

There are several problems with equating plant behaviour only with plasticity. The term ‘reproductive behaviour’ is often used to distinguish whether reproduction is sexual or vegetative without regard to plasticity changes. Some species do have separate male and female flowers, and environmental conditions can change the proportions of each, implicating plasticity, too ( Trewavas 2007b ). Tsvi Sachs objected to essential developmental processes like germination being classed as behaviour (quoted in Silvertown 1998 ). On the other hand, there is certainly inherent plasticity in the germination phenotypes of almost any species.

Predation is inevitable for wild plants, but numerous dormant meristems, regrowth and often extraordinary regenerative capacities can diminish but not eliminate the potential reduction in fitness. It is also the reason that plants do not place critical functions in one or two tissues as animals do with heart or kidneys. Such specialization would make the individual extremely vulnerable to even slight predation. However, the phenotype is holistically determined. Excision of either a whole shoot or root inhibits further plasticity changes until regeneration of the lost organs is completed. Moreover, fitness itself is a function of the integrated phenotype, not just the behaviour of individual tissues.

Phenotypic plasticity is not unique to plants however. Plant behaviour can, and indeed should, express a phenotypically local response to local signalling, but so can that of other organisms. For example, human weightlifting specifically increases the development of the muscles most involved. The real difference between plant and animal behaviour was again indicated by Arber (1950 , p. 136). ‘The individuality of the mammalian body is of a much more fixed character; that body consists of a limited number of organs which were once and for all marked out in the embryo. With its parts arranged in an ordered hierarchy there is no such thing as indefinite succession of limbs, of branches of limbs, numerically unfixed and liable to impede one another but this is what we find among plants’. Movement is essential for the higher animal lifestyle. Only with accurate replication of limb numbers and complex coordination among them could this be reproducibly achieved. Thus, crucial embryological tissue specification is limited to the protected environments of the egg or uterus, and subsequent plasticity is constrained to more marginal changes in already specified organs. The potential for plasticity is considered to have a genetic basis, but its realization must be epigenetic.

‘Among plants, form may be held to include something corresponding to behaviour in the zoological field. The animal can do things without inducing any essential change in its bodily structure. When a bird uses its beak to pick up food, the beak remains unchanged. But for most, but not all, plants, the only available forms of action are either growth or discarding of parts, both of which involve a change in the size and form of the organism’ ( Arber 1950 , p. 3). This statement identifies phenotypic plasticity as a form of action in plants, that is, plant behaviour. The Latin word habere , from which the word behaviour is derived, means ‘having’ or ‘being characterized by’. Arber's statement indicates that plant behaviour is action, that is, doing something. Behaviour is then what a plant does, rather than something it is characterized by or has.

The life cycle goal of any individual plant is optimal fitness, usually equated to maximum numbers of viable seedlings or more conveniently, for experimental purposes, the numbers of seeds. Seed yield is known to be dependent on lifetime acquired resources (carbohydrates, minerals and water, i.e. food), extent of predation and success in reproduction. Similar fitness requirements exist for animals – acquisition of adequate food, avoidance of predators (or catching prey) and successful reproduction. In animals, all these behavioural processes involve movement, and movement is recognized as the basis of animal behaviour. Higher plants spend their life cycle rooted in one position and, to the casual observer, exhibit no movement, with only rare exceptions like Mimosa . How then can plant behaviour be described?

This enormous signalling/environmental complexity is best conceived by the reader as a complex but changing topological surface composed of hills and valleys, and the successful plant (from seed to flower) must navigate its way through this topological and hazardous environmental terrain, which keeps changing in structure, with minimal expenditure of energy. Bazzaz (2000 , pp. 91, 168) illustrated striking, complex topological surfaces involving the influences of only two environmental parameters. How much more complex with 20 or more?

What makes for much greater complexity is that many of these signals arrive coincidentally. Decisions among often conflicting signals have to be made and priorities determined on phenotypic change. Leyser (2009 ), in this issue, described the role of auxin in leaf and branch initiation in which a coherent model is beginning to emerge. The abiotic signals of light, gravity, mechanical signals, soil structure and composition, minerals and water availability add to the difficulties for the growing plant because each, like the biotic signals, varies in direction, length of signalling and intensity. This enormous complexity of signalling ensures that no plant behavioural response is autonomic, a kind of behaviour that requires complete replication under all environmental circumstances. Selection will favour individuals that can best assess the probabilities of particular kinds of behavioural action and optimize their fitness.

Other biotic signalling results from competition for soil resources, from mycorrhizal and cooperative bacterial interactions and from allelopathic chemicals, disease, mutualism, trampling and, finally, plant cooperation ( Kelly et al. 2008 ). Some of these signals, like disease and bacterial cooperation, are relatively well understood; the others are less well characterized.

Other kinds of signalling have been detected, but their current molecular basis remains unknown. As both Hodge (2009 ) and Novoplansky (2009 ), again in this issue, indicated, root systems are not only able to sense the soil volume in which they grow but can recognize and discriminate against the roots of adjacent conspecifics and thus possess self‐recognition. [Astonishingly bacteria have self‐recognition ( Gibbs, Urbanowski & Greenberg 2008 ), indicating perhaps the ubiquity of self‐recognition processes.] Potentially, the roots of any individual plant avoid each other as far as possible to improve the extent of soil space occupied and exhibit a kind of territoriality ( Schenk, Callaway & Mahall 1999 ). Furthermore, the root system exhibits holistic responses to the patchy environment experienced. These observations do imply complex signalling below ground; could these root signals be presently unknown volatiles, too ( Erb et al. 2008 )?

In this special issue on plant behaviour, many articles deal with particular kinds of environmental signalling. Foraging is described as the behavioural alterations that enhance the uptake of essential resources and De Kroon et al. (2009 ) highlighted both the local and integrated signalling that underpins these vital processes. Mott (unpublished data) described the systems behaviour of complexes of stomatal cells that are crucial for foraging for carbon dioxide. Forde and Walch‐Liu (2009 ) also reviewed the important role of amino acids and nitrate in constructing the root phenotype. The shoot phenotype is dependent on the presence, absence and crucially the identity of neighbours (see pictures in Bazzaz 2000 , p. 114), and these may reflect the ubiquity of competition. Ballaré (2009 ) emphasized the critical role of phytochrome in both light foraging, overall resource allocation, herbivore defence and thus shoot phenotype construction.

What growth rooms cannot mimic is the enormous complexity of the external environment experienced by the wild plant. Behaviour is inextricably linked to environmental signalling. Because plants are sessile organisms, they may perceive more environmental signals and with greater sensitivity and discrimination than the roaming animal. ‘If etiolated seedlings are placed between two sources of light differing so slightly that the differences cannot be detected by ordinary photometric methods, the seedling always bends promptly towards the source of the more intense light’ ( Palladin 1918 , p. 246) is certainly indicative.

BEHAVIOUR, MOVEMENT, PURPOSE AND INTENTION

McDougall (1924) described behaviour in the following way. Animals are behaving if they actively resist the push and pull of the environment, exhibit persistence of activity independently of the impression (signal) that may have initiated it and exhibit variation in the direction of persistent movements. This definition would characterize plant behaviour, too.

Movement and behaviour Movement would seem to be the simplest criterion of behaviour, and movement has always been an essential part of the animal lifestyle to find food, avoid predators (or catch prey) and find mates. Predator–prey relations among animals accelerated the evolutionary specialization of sensory organs and muscles to respond to signals. The nervous tissue, a rapid information transmission system, then evolved, to link these two together. The faster the prey responded and moved, the faster any effective predator had to evolve in turn. Animal behaviour tends to be equated with movement because we ourselves are animals, because our perception/response system works at the rate of transmission of the nervous system (like most other animals) and because we regard our own movement as behaviour. Multicellular organisms that lack a nervous system can be expected to operate on a very different timescale, and higher plants are no exception. This change of timescale creates problems for recognition of behaviour. Pfeffer (1906, p. 2) early on recognized the problem. ‘The fact that in large plants the power of growth and movement are not strikingly evident has caused plants to be popularly regarded as still life. Hence, the rapid movements of Mimosa pudica were regarded as extraordinary for a plant, and the same applies to the spontaneous movements performed by Hedysarum gyrans (telegraph plant). If mankind from youth upwards were accustomed to view nature under a magnification of 100 to 1000 times (seeing streaming or lower plant sperm swimming) or to perceive the activities of weeks or months in a minute as is possible by the aid of a kinematograph, this erroneous idea would be entirely dispelled’. Pfeffer (1906) thus predicted time‐lapse facilities that brings plant behaviour, in some sense, to a more familiar human timescale. The web site (http://plantsinmotion.bio.indiana.edu/plantmotion/starthere.html) constructed by Roger Hangartner contains many excellent time‐lapse examples that show behaviour that complies with the aforementioned McDougall (1924) definition. One fundamental difference between plant and animal behaviours is therefore in their respective time frames. The real value of time‐lapse records is to uncover behaviour either difficult to record or missed by previous recording procedures. For example, the Attenborough (1995) time‐lapse films record a kind of rapid vertical/horizontal shaking behaviour by a growing bramble stem that so far has no explanation. Other revealing time‐lapse movies are to be found in Massa & Gilroy (2003) on root behaviour encountering soil obstructions and Runyon et al. (2006) on Dodder locating its prey by detecting host volatiles. Time‐lapse recording needs to be focused on the behaviour of wild plants as well, because the timescale difference with human observers implies that much novel behaviour may simply have never been seen.

Purposeful behaviour In a seminal paper, Aphalo & Ballare (1995) indicated how plants were commonly perceived as ‘passive organisms’ undergoing a predetermined programme whose culmination was occasionally slowed by poor environments. They argued instead that plant behaviour is both active and predictive. The ‘passive plant’ attitude almost certainly results from experimental experience of plants in which signals are imposed by the investigator to make plants perform in controlled conditions, perhaps, similar to the way circus animals are made to perform. But in the wild, it is plants that must compile environmental information and make active decisions to change development, in order to optimize life cycle behaviour and eventual fitness. Active behaviour may be more simply defined as a dependence on metabolic energy (Rosenblueth, Weiner & Bigelow 1943). True passive behaviour is then simply limited to processes, like the explosive distribution of seeds, that depend only on unequal drying of dead tissue or the floating of seeds in the wind. Active behaviour is most easily defined as purposeful when it is goal oriented (Rosenblueth et al. 1943; Russell 1946). The goal is often achieved by some complex form of negative feedback, and obvious examples (out of many) are the adaptive responses of tropic bending to gravity or light. In negative feedback, an information loop is constructed from the signal to the responding cells to indicate the margin of error from the goal and adjust behaviour accordingly (Trewavas 2007a). The clearest indications of a kind of negative feedback control are the damped oscillations around the goal that can sometimes be observed in tropic bending (Trewavas 2003). Other examples of more complex and less understood, purposeful (goal‐directed) behaviour are (1) the stem thickening that accompanies wind sway; (2) leaf abscission that rebalances the water relations of a whole plant when water supplies are diminished; (3) the (indeterminate?) elongation of the leaf petiole in water plants like Nymphaea, which only stops when the leaf breaks surface; and (4) the seasonal, average tree‐leaf temperature that remains remarkably uniform at about 21 °C from trees ranging from the subtropical to the arboreal (Helliker & Richter 2008). This unusual form of long‐term homeostasis, which undoubtedly benefits photosynthetic processes, is suggested to result from an interaction among internal leaf cooling, leaf structure, branch structure and leaf distribution among others, all important behavioural traits that have clearly been optimized. Russell (1946) included several other good plant examples. The molecular mechanism underpinning goal‐directed behaviour is clearly dependent on growth substance involvement (auxin, ethylene, etc.) that is only partly understood. Certain forms of purposeful behaviour seem overwhelmingly controlled by one signal; the extreme sensitivity of etiolated seedlings to unidirectional blue light that can override opposing gravity signals is an obvious example. The most obvious purposeful behaviour, however, arises from an integration of different signals. Charles Darwin (1880) showed experimentally how seedling roots sensed the signals of touch, light, moisture and gravity simultaneously resulting in sensory integration (Trewavas 2007c). Furthermore, he showed that growing roots could distinguish between these signals and decide which was the most crucial to respond to. Both touch and humidity can override the gravitational signal if applied in a different direction (Eapen et al. 2003; Massa & Gilroy 2003), in a recent excellent expansion of Darwin's observations on soil obstructions, indicating how the root response is integrated between touch and gravity. Natural soil is very heterogenous both in texture and in the distribution of resources (Bell & Lechowicz 1994). Signal integration is therefore necessary. The successful plant must more correctly assess the probabilities of appropriate action in constructing the root phenotype.