Saturday, October 24, 2009

A Reader Asks: Are There Sensate Plants?


A Reader Asks: You claim that the difference between the genus 'animals' and that of 'plants' is that animals are sensate (i.e., they can sense) whereas plants are non-sensate. But I believe modern botany can prove you wrong, insofar as it gives evidence of sensate plants. Here is a quote from a recent article on the matter.

Plants can't see or hear, but they can recognize their siblings, and now researchers have found out how: They use chemical signals secreted from their roots, according to a new study. The study in question results below. Bais and doctoral student Meredith Biedrzycki set up a study with wild populations of arabidopsis thaliana, a small flowering plant that is often used as a model organismin plant research. They wanted to use wild populations instead of laboratory-bred species, because the latter "always has cousins floating around in the lab," Bais said. In a series of experiments, young seedlings were exposed to liquid containing the root secretions, called "exudates," from siblings, from strangers (non-siblings), or only their own exudates. The length of the longest lateral root and of the hypocotyl, the first leaf-like structure that forms on the plant, was measured. A lateral root is a root that extends horizontally outward from the primary root, which grows downward. Plants exposed to strangers had greater lateral root formation than the plants that were exposed to siblings. Further, when sibling plants grow next to each other, their leaves will often touch and intertwine, while stranger plants near each other grow rigidly upright and avoid touching, the authors say.
This disproves your statement that plants have no senses.

Ite ad Thomam Responds:

This whole issue is all a matter of difference in terminology between scientists and philosophers. Once this terminology is clarified, the whole issue becomes a moot point.

The venus flytrap, perhaps an even better example of a plant that has a so-called 'sensory system', operates on the basis of purely chemical responses; a chemical stimulus (the fly's touching the trap) causes a chemical reaction in the plant that unleashes the mechanism of the closing of the leaves (and there is much scientific literature on this). Scientists call it this type of mechanism a 'sensory mechanism' because, through it, some plants interact with stimuli from their environment. But none of this involves sensory awareness. The plant does not "feel" anything. There is no subjectivity in the plant. The plant is not conscious of it or of its environment. It just is. It sits there and reacts chemically to stimuli. But botanists, of course, are not psychologists, and have no interest in the issue of plant 'awareness' or plant 'consciousness'; they cannot measure it or detect it in any way. They are practical naturalists: they do not acknowledge, qua scientists, the existence of anything subjective, mental, spiritual, or immaterial. That is why they have no interest in making a distinction between sensation properly speaking (with subjective awareness, as in animals) and sensation in the looser sense (reacting to chemical stimuli).

In the case of sensate beings, however, the action potential (the electric sensory signal as it travels through the neurons) is interpreted by the brain or whatever is the central nervous system of the species in question as a subjective experience. Animals do "feel." So whereas it makes sense to ask, for instance, the question of "what it would feel like to be a bat," nonetheless it does not make sense to ask the question of "what it would feel like to be a plant," because being a plant does not 'feel' like anything, anymore than being a rock 'feels' like anything.

But, ultimately, the main point is this: "plant" in botany and "plant" in philosophy have different definitions. In botany, "plant" refers to the clade (taxonomic group) plantae, a rather narrow monophyletic (closely-related) group of taxa that includes only the land plants and green algae (but excludes other things that are commonly considered 'plants' both by philosophers and the common people, such as red algae and fungi). The synapomorphies (the scientific term for what in philosophy we call propria, or essential properties) of plantae are the following:

(a) their cells have the photosynthetic pigments chlorophyll-a and chlorophyll-b,
(b) the accessory pigment beta-carotene, and
(c) membrane-bound sacks called thylakoids.

That's it: only such beings count as plantae. If something doesn't have these synapomorphies, it does not belong to the clade plantae. So the mushrooms that you eat in your salad, and the algae that gets tangled in your toes when you go swimming at the beach are not 'plants' (under the scientific definition).

In philosophy, however, "plant" means something altogether different, something much broader. "Plant" just means "non-sensate living being": a such, it includes a vast paraphyletic kingdom of taxa, many of which are unrelated (e.g., fungi, plantae, bacteria, archaea). This includes not only the plantae, but everything else that does not have sensation properly speaking (reaction to stimuli with awareness).

Hence, nothing prevents something that belongs to the clade plantae (understood in the scientific sense) from being outside of the genus "plant" (understood in the traditional philosophical sense), or vice versa. As my botany professor once told me: if a taxon of plants evolved a nervous system and grew a brain and eyes and ears and a mouth and ate people, it would still belong to the clade plantae. But, of course, we know that, philosophically speaking, that living being would be sensate, so it could not belong to the genus "plant" (understood philosophically).

So the bottom line is this: all plants are by definition non-sensate (and only animals are sensate living beings, by definition), such that if there were a member of the clade plantae that could really sense (with awareness) it would actually be an animal. My reasoning is quite simple and can be expressed through this syllogism.

Major: If a living being has sensory awareness, then it is an 'animal' (understood philosophically).
Minor: For the sake of argument, suppose there is a certain member of the clade plantae that has sensory awareness.
Conclusion: That member of the clade plantae is an 'animal' (understood philosophically).

Proof of the Major: It follows from the definition of 'animal' (understood philosophically).
Proof of the Minor: For the sake of argument, we are supposing an animal that can actually sense in the strict sense (with awareness).
The conclusion follows.

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