*** Excursus - Plants

Back to Chapter 2 Back to Case studies
Previous case study - bacteria Current case study - protoctista and plants Next case study - animal cells
References *** SUMMARY of Conclusions reached

There is scientific disagreement over the definition of a plant. All plants are eukaryotes, and they all develop from embryos. For this reason, they are all multicellular. Their cell walls are made of cellulose. Plant cells typically contain a membrane-bounded chloroplast, where the products of photosynthesis are stored, with chlorophyll. The question of what makes a plant is dicussed at further length in an Appendix.

The possibility of cognition among plants should not be ruled out a priori. As Di Primio, Muller and Lengeler forcefully argue,

How can a plant be capable of (cognitive) behavior? The interesting point about this question is that it concerns the relationship between behaviour and movement. Plants are believed to change only their structure and form and not to behave at all, because "behavior" is identified with "locomotion". But based on his studies with plants, Pfeffer concluded already in the 19th century: "life is movement" (Pfeffer 1884). Plant movement may not be as spectacular as animal movement because it is either reduced to the cellular level or rather slow. Their abilities to perform undirected (so-called nasties) and directed movements (taxes, tropisms), however, is almost as complex and diverse, and certainly as purposeful, as those of animals. Plants have to solve the same problems as other organisms. In brief, they have to survive by sensing properties of their environment and responding appropriately, individually, as well as collectively (like bacteria, they release chemicals into the environment and communicate chemically).

...[T]he fundamental thing [for an organism to behave] is not the ability to move to a new location, but the ability to modify itself (by developing effectors as needed), i.e. to respond appropriately to changing conditions (2000, p. 10).

Whereas a taxis is defined as a movement of a cell in response to a stimulus, a tropism is the directional growth of a plant organ in response to a stimulus such as light (phototropism), water (hydrotropism), touch (thigmotropism) or gravity (geotropism), while a nastic movement (nasty) is a movement of a plant organ in response to stimuli, that is independent of the direction of the stimuli (e.g. the opening of flowers in response to changes in temperature or light, or the folding up of the leaves of the Mimosa plant when touched) (Isaacs, Daintith and Martin, 1999). Clearly, these movements indicate sensitivity in plants, but by themselves, they do not seem to be qualitatively distinct from the phototaxis, chemotaxis and magnetotaxis displayed by bacteria.

Do plants have senses?

The sensory capacities of plants are more deserving of attention. According to a recent report in "New Scientist" magazine (Coghlan, 1998), plants have analogues to each of the traditional five senses found in human beings. They are sensitive to light, and some plants can work out the quality of light and compete with neighbouring plants, as well as sensing whether it is night or day, the length of the day, the quantity of light, and the direction it is coming from. Certain plants can also "taste" the soil and find out where scarce nutrients are most plentiful. Additionally, there are plants which can "smell" signals from neighbouring plants that are being attacked by predators or insects. Many plants are also sensitive to touch. Some plants are even sensitive to certain sound frequencies. These findings are discussed at further length in an Appendix, where it is concluded that plants meet Aristotle's defining criteria (reception of form without matter and the existence of a mean) for the presence of sensory capacities, but that this does not give us any warrant for saying that they have mental states (Conclusion S.5). The ability of plants to discriminate between beneficial and harmful stimuli can be described using a mind-neutral intentional stance.

If plants prove to be inflexible in their response patterns to sensory stimuli, and unable to learn new ways of responding to unforeseen events via an internally generated mechanism, then the ascription to them of cognitive mental states would seem to be redundant: it would tell us nothing useful about their behaviour (see Conclusion N.11). The question we have to ask is: can plants learn to change their patterns of behaviour?

Is adaptive behaviour in plants evidence of cognition?

It is certainly true that plants are capable of adaptive behaviour. For instance, according to Godfrey-Smith (2001, p. 7), many plants can sense not just that they are being shaded, but that they are being shaded by other plants. They do this by detecting the wavelengths of the light reflected by these plants. The plants respond by growing longer shoots. However, this kind of behaviour does not qualify as "flexible" according to the definition given above: all that happens here is that the behavioural "output" (the length of the plant shoot) varies as one of the input values (the wavelength of light) fluctuates. The underlying action pattern is still fixed and therefore non-cognitive (see Conclusion S.7).

Plants, like other organisms, also exhibit phenotypic plasticity, in which organisms with the same genotype can vary their developmental pattern, phenotype or behaviour in response to an environmental cue (Ancel and Fontana, 2002). For instance, plants can adjust their phenotype, according to altitude or available moisture (Godfrey-Smith, 2001, p. 14). Godfrey-Smith considers this behaviour to be "proto-cognitive", because the plants are responding flexibly to their environmental conditions. As well as this kind of within-individual, non-genetic variation, certain plants are also capable of undergoing within-individual, genetic changes in their lifetimes, which are transmitted to their offspring. For example, nutrient deprivation can cause mutations within the DNA of some cells in the flax plant Linus usitatissimum. The mutant cells can enter the seeds or alter the size and branching pattern of the plant. Random variation between individuals in response to environmental changes also occurs in plants: the phenomenon of bet-hedging, discussed in connection with bacteria above, is an example (Ancel Myers and Bull, 2002, pp. 552-555).

Should we regard these kinds of flexibility as evidence of cognition in plants? My own position, following that of Kilian and Muller (2001) discussed above, is that these are not cases of genuine cognition: the plants are not learning a new response pattern, but simply responding in to environmental cues, either by making a hard-wired "selection" of the best of a built-in set of possible phenotypic responses, or by making a random response to their environment which happens to be adaptive. Neither hard-wired nor random behaviour constitutes evidence of cognitive mental states (see Conclusions N.11 and N.8).

Are plants capable of learning?

Most studies of plant learning have examined habituation in Mimosa, a small shrub whose leaves are sensitive to stimulation. There is reliable experimental evidence (see Appendix) that Mimosa plants are capable of habituation. In most of these cases, the habituation appears to be non-associative, although one intriguing apparent exception is discussed in detail. An experiment in 1965 showed that Mimosa could discriminate between different types of stimuli: it could be "trained" to stop closing in response to water droplets, but still retained its response to the touch of a finger. Clearly, more research needs to be done here.

It was argued above that non-associative "learning" (e.g. habituation in its simplest form) does not require a mentalistic explanation, and that true learning involves an in-built mechanism for acquiring information that allows organisms to modify their response to a stimulus. Without such a mechanism, they cannot "tailor their own responsive dispositions to their particular surroundings" (Beisecker, 1999, p. 298). Before we can attribute learning to plants, we have to ascertain whether they are capable of associative learning (classical and/or instrumental conditioning), in which an organism "does learn to do something new or better" (Abramson, 1994, p. 38).

Research into the possibility of classical conditioning in Mimosa has produced negative or conflicting results, and the methodology of studies which found conditioning has been criticised (ibid., pp. 175-176). Attempts to condition Mimosa with light touch as the conditioned stimulus (CS) and electrical or mechanical shock as the unconditioned stimulus (US) failed. Other attempts, using light as the CS and touch as the US yielded positive results in two (arguably flawed) studies, and negative results in another study. In keeping with my methodological constraints, these studies will be ignored here, as will unsubstantiated claims of plant "telepathy".

A more refined plant learning experiment by Abramson, Garrado, Lawson, Browne and Thomas (2002, pp. 173 - 185) is described in an Appendix. The study used a new method: different groups of Philodendron cordatum plants were exposed to a six-hour training period of light only, darkness only, or alternating one-minute periods of light and darkness, and subsequently exposed to a ten-minute testing period in darkness, when their bioelectrical potentials were recorded, using EEG equipment. The researchers looked for differences in the amplitudes recorded for plants in the last group, corresponding to the switching of the light in the training period. The results were negative. There are several possible explanations for this failure: the intervals of light and darkness may have been too short for the plants to adjust to changes, or the plants may have been poor at timing (like most invertebrate animals), or it may not be possible for true learning to occur in the absence of a nervous system. Follow-up research with longer intervals is recommended by the authors.

The upshot of all this is that although plants are highly adaptive, multicellular, eukaryotic organisms, there is no good evidence to date that they have any cognitive mental capacities. Further research into the alleged learning abilities of plants is warranted, but the research to date is consistent with Kilian and Muller's contention (2001, p. 4) that true learning cannot occur in the absence of a nervous system.