Intelligence is a term fraught with difficulties in definition. In part, the problems arise because of the human slant placed on the use and meaning of the word. However, although as a species we are clearly more intelligent than other animals, it is unlikely that intelligence as a biological property originated only with Homo sapiens. There should therefore be aspects of intelligent behaviour in lower organisms from which our superlative capabilities are but the latest evolutionary expression.
Stenhouse (1974) examined the evolution of intelligence in animals and described intelligence as ‘Adaptively variable behaviour within the lifetime of the individual’. The more intelligent the organism, the greater the degree of individual adaptively variable behaviour. Because this definition was used to describe intelligence in organisms other than humans, it is a definition useful for investigating the question in plants. Do plants exhibit intelligent behaviour? The use of the term ‘vegetable’ to describe unthinking or brain-dead human beings perhaps indicates the general attitude.
However, in animal terms, behaviour is equated with movement, and since plants exhibit little if any form of movement, plant intelligence on that basis does not exist. Although some higher plants exhibit rapid movements (e.g. Mimosa pudica), these are exceptions rather than common-place. Mimosa captures our attention because it operates on a time scale similar to our own, and it is the difference in time scales that frequently makes plants seem unmoving. The use of time-lapse facilities has indeed indicated that plants operate on very much slower time scales than our own, but once observed in this way, movement is quite clear.
In addition, the majority of multicellular plants, including macroalgae, are sessile, the result of a decision several billion years ago to gather energy and reducing potential via photosynthesis. Since light is freely available, movement has never been particularly critical to plant survival. Such movement as has been observed is usually limited to less complex plants such as blue-green algae. Rejection of that (photosynthetic) decision by the primordial animal eukaryotic cell ensured that movement became critical to find food and mates. Once animals started to prey upon each other, the development of highly differentiated sensory systems and specialized nerve cells to convey information rapidly between sensory tissues and organs of movement was an inevitable consequence. The predator–prey relationship has acted as a positive feedback loop to accelerate complex development and equally complex organ differentiation in animal evolution (Trewavas, 1986b). Movement is, however, the expression of intelligence; it is not intelligence itself. Stenhouse (1974) regarded the early expressions of intelligence in animals as resulting from delays in the transfer of information between the sensory system and the motor tissues acting upon the signals. The delay enabled assessment of the information and modification of information in the light of prior experience, and it was that assessment that formed the basis of intelligence. The key difference between plants and animals in the Stenhouse (1974) definition is in the word ‘behaviour’. Silvertown and Gordon (1989) have defined plant behaviour as the response to internal and external signals. In plant terms these are familiar growth and development phenomena, such as de-etiolation, flower induction, wind sway response, regeneration, induced bud break/germination, tropic bending, etc. Thus, a simple definition of plant intelligence can be coined as adaptively variable growth and development during the lifetime of the individual. To add significance to this definition, time lapse shows that virtually all plant movements are indeed the result of growth and development.
It can be objected that animals also grow and develop, but there are important qualitative differences. The sessile plant requires a morphological and developmental pattern that enables exploitation of local minerals, light and water. Since the environment is a variable and often unpredictable quantity for any individual plant, development continues throughout the life cycle and is necessarily plastic if proper exploitation and growth are to be achieved. Plasticity is from all examinations adaptive (Sultan, 2000), by its nature variable between individuals in different environments, and therefore must involve an element of computation if it is to succeed. Since all plants exhibit adaptive plasticity within the lifetime of the individual (Bradshaw and Hardwick, 1989), they must all exhibit intelligent behaviour according to the definition above. In contrast, much animal development and differentiation is confined to a uterus or egg, is minimal in the adult form and, as a consequence, is often described as unitary. Plant development is clearly modular, highly polarized through tip growth, and often exhibits complex branching patterns to enable proper resource exploitation that continues throughout the life cycle.
It is crucial to appreciate that all intelligent behaviour in both animals and plants has evolved to optimize fitness. Plants must then have access to an internal memory that specifies the optimal ecological niche in which maximal fitness, usually regarded as the greatest number of viable seeds, can be achieved. When the niche is sub-optimal, plasticity in growth and development intervenes to counterbalance and to attempt to recover as far as possible the benefits of the optimal niche. The sub-optimal niche can then, in some way, be compared with the optimal niche to specify the necessary extent of plasticity in growth and development.
This article considers various aspects of plant intelligence and attempts to answer some of the inevitable criticisms that will come with the notion of the intelligent plant. The major problem is a mind-set, common in plant scientists, that regards plants basically as automatons. The reasons for this mind-set will be examined later, and counter-evidence provided. Other aspects, such as learning, memory, individuality and plasticity in plants will be reviewed, and the article will finish with some interesting examples of intelligence in action which ecologists are beginning to uncover. The article is long—it has to be when trying to justify a change in attitude. A very short version of this article has been published (Trewavas, 2002b), and see discussion article by Philips (2002).
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