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Do Plants Feel Pain? February 13, 2011

Posted by Metabiological in Ecology, Ethics.
Tags: , , , ,


Okay I’ll expand on that.

A common though ultimately misguided argument against veganism that often crops up whenever the subject is being discussed is that vegans are hypocrites because they don’t care about the suffering of plants.  In practical terms this isn’t so much a logical point as it is an ad hominem but even were it not a fallacy it’s still based on a rather shaky assumption; namely that plants feel pain.

Let me say this right up front.  There is no empirical evidence of plants experiencing a sensation even remotely close to what we call pain.  Pain has a very specific definition that is often lost in the colloquial use of the word.  Simply put, pain is a negative sensation that occurs when an organism has suffered some sort of physical injury.  Pain signals travel from specialized sensors cells in the body, called nociceptors, through the sensory peripheral nervous system and to the brain.  Depending on the nature of the pain the response could be either unconscious, as when you pull your hand off a hot plate without thinking about it, or conscious, as when you stick your hand under freezing ice water for as long as you can on a dare (I know I’m not the only one to do this.)

I’m not sure where the idea that plants feel pain first originated.  The Jain religion of India teaches that all living things have a soul and preaches ahimsa (non-violence) towards all life.  An unusual (for western vegetarians) result of this philosophy is that Jains will abstain from eating root vegetables such as potatoes or onions because the death of the plant is required to harvest it.  However this practice has less to do with reducing suffering than it does respecting life, two very different things.  It may have had some influence on the idea but does not seem to be where it originated.

Sometimes cited as support is the fact that plants are certainly capable of responding to outside stimuli.  Sunflowers are able to move their flowers and track the sun across the sky while tress have been shown to shift nitrogen (important in photosynthesis) to leaves in heavier sunlight.  However from an anatomical and physiological point of view the idea that plants feel pain finds little support.  Plants lack pain receptors that would allow them to experience negative stimulation in the first place as well as lacking both a brain and a nervous system that would be required to analyze and respond to such sensory information.  There does seem to be evidence that plants release hormones after being damaged much in the same way animals will but plant hormones are quite different from those found in animals.  Furthermore this would not be evidence of a pain response in plants but rather an automatic reaction to stimuli.

More damning for the position though is the total lack a reason for plants to have evolved a pain response.  Pain is a useful adaptation for animals because it helps them avoid dangerous situations.  A perfect way to illustrate this is to look at humans born without the ability to feel it.  The condition is called congenital analgesia or congenital insensitivity to pain.  A rare genetic condition the prime symptom of which is the total lack of any ability to experience a pain response.  As you may have been able to guess suffers often do not live long and those that do must be carefully watched.  Broken bones may go untreated or infections unremarked upon if only because they have no reason to be concerned about them.  Without pain there is no reason to pull your hand out of the fire.

So if pain helps animals survive can the same be said for plants.  Of course not.  Lacking any ability to avoid danger if it arises, such as retracting a branch being munched on by an herbivore or uprooting to a new spot during a fire, the development of a pain response system would be an evolutionary waste.  It would add nothing the fitness of the organism (the ability to reproduce) and the resources that would be used to grow it could instead be put into growth and reproduction.   Plants certainly can respond to threats, many species of algae increase their production of defensive chemicals after suffering damage from herbivory, but such things only indicate a response to outside stimuli not pain.

Some objections, though none of them good, can be raised to the position I’ve offered.  One is that any negative experience qualifies as pain.  Since plants are able to respond to stimuli, and therefore must experience it in some way, and some stimuli are certainly negative plants then would be said to experience pain.  This however is conflating two separate issues; the question of panpsychism and the question of suffering.  As shown above it is quite possible for a human to possess the ability to experience while simultaneously lacking the ability to feel pain.  A similar situation is found in those lacking photoreceptors in the eyes (blind) or who have suffered damage to chemoreceptors in the nose (lack of smell).  One could hardly say that a blind person is able to experience sight or a deaf person able to experience sound even though they are certainly able to experience other stimuli for which they have the proper tools.  In a similar vein a computer is clearly able to respond to stimuli such as the introduction of a virus into its software but very few would say that such an event causes the computer any sort pain.

As it currently stands it makes little sense to extend our moral sphere to encompass plants but that does not mean we may ignore the role they have to play.  As primary producers all life on earth ultimately derives from the ability of plants to provide energy for the rest of us.  The continued happiness and prosperity of both us and the rest of the animal kingdom requires the continued health of the ecosystems we share.



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