*** Appendix - Sensory capacities in plants

The following is a summary of a report in "New Scientist" (Coghlan, 26 September 1998), describing plants' sensory capacities:

Research on a humble weed known as thale cress (Arabidopsis thaliana, a plant with a relatively simple genome) reveals that plants can "see". They have proteins attached to light-sensitive compounds. Proteins called phytochromes enable plants to work out the quality of light and compete with neighbouring plants. Other proteins called crytochromes enable plants to work out whether it is night or day, the length of the day, the quantity of light, and the direction it is coming from.

A particular gene in Arabidopsis allows its roots to "taste" the soil and find out where vital nutrients are most abundant, thereby saving energy, as the roots grow towards the source of the nutrients, rather than randomly. Additionally, an enzyme called apyrase, found on root surfaces, allows various plants to "taste" ATP, a useful source of short-term energy. The leaves of corn, beet and cotton plants can also "taste" the saliva of caterpillars and respond by secreting volatile compounds which attract parasitic wasps, which kill the caterpillars.

Tomato plants exude a substance called methyl jasmonate when wounded. When neighbouring plants "smell" this signal, they prepare for battle by producing chemicals that repel insects or attract predators. Recent research also suggests that plants can smell smoke, and that this triggers forest generation after a fire, as buried seeds germinate.

The responsiveness of certain plants (the Venus fly trap, or Mimosa) to touch is well-known, but these abilities are simply amplifications of what all plants can do, according to researchers. In particular, plants respond to the buffeting of the wind by strengthening tissues that are being swayed. Within a tenth of a second after being pushed around, calcium ions flood into the plants' cellular fluid, and activate genes that strengthen their cell walls.

Some plants even respond to sound. Prolonged exposure to 2 kilohertz frequencies (about the same as a human voice), at 70 to 80 decibels (a bit louder than speaking) can double the growth rate of dwarf pea plants and quadruple the germination rate of radishes. (However, talking to plants will not work: "You'd have to sit there and talk to them for days", says one researcher.) It is believed that sound induces the biosynthesis of gibberellic acid, which increases growth and germination. Inhibiting the synthesis of this hormone blocks the enhancing effects of sound.

What the above research shows is that plants have a wide range of discriminatory abilities that allow them to fine-tune their responses to their environment. Should these abilities be called senses? Earlier, we examined Aristotle's argument that a sensory capacity should not be defined merely as a capacity to respond to environmental changes. Aristotle's defining criteria (reception of form without matter and the existence of a mean) were interpreted to mean that an organism with sensory capacities had to be able to encode information (form without matter) about environmental changes, within an organ which could exist in different information "states" (allowing it to serve as a "mean" between them).

It will be recalled that for Aristotle, the existence of sensory perception (aisthesis) was both a necessary and a sufficient criterion for being an animal, while locomotion was a sufficient but not a necessary criterion. Although Aristotle denied perception to plants (De Anima 2.3, 414a31; 3.13, 435b1), it is by no means clear from his own writings why the discriminatory abilities of some plants (e.g. thale cress, discussed above, which uses proteins to identify the quality of light, as well as whether it is night or day, the length of the day, the quantity of light, and the direction it is coming from) are fundamentally different from the sensory capacities of animals, and it has been argued that according to Aristotle's own criteria, bacteria also possess "senses" which enable them to detect food and noxious stimuli.

Does the possession of "senses" by plants carry implications of mental states? It has already been argued that the possession by an organism of sensors which encode information about its surroundings is an insufficient warrant for saying that the organism is capable of cognitive 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, which explains their behaviour in terms of their information relating to built-in goals. We do not have to resort to an agent-centred or subject-centred intentional stance here. For this reason, we should refrain from ascribing mental states to plants unless we have other, independent grounds for doing so (Conclusion S.2).