Plant Sentience
Keith sometimes uses the fact that plants have complex chemical signaling systems to infer sentience:
“Plants are in constant communication with each other. “Each plant, plant neighborhood, plant community, ecosystem, and biome has messages flowing through it constantly—trillions and trillions of messages at the same time.”122 Any place that roots touch other roots or their shared mycelial network, they can also exchange chemistries, medicines. One plant will send out a chemical distress call. The others respond with precise antibiotics, antifungals, antimicrobials, or pesticides to help. Like my chickens when they sight a hawk, plants will give out an alarm call when a predator is near. Lima beans will release chemicals that warn other lima beans when they are being attacked by spider mites.123 When something ambulatory brushes past a plant in the woods, not only does the affected plant respond by stiffening as best it can, it also sends out a chemical warning that allows all the plants nearby to stiffen their branches in preparation.” (88)
“When the soil is ready, the nurse plant sends out the chemical message: join me.” (88)
“Plants produce millions of chemicals to attract, repel, immobilize, or kill animals. It’s how some of them reproduce. And it’s how they fight back: nature, red in phytochemicals. Just because they can’t locomote doesn’t mean they’re passive.” (27)
While it is true that plants have fascinating, complex stimulus feedback systems, modern science knows that these are not sufficient to give rise to consciousness, just as similar systems in spermatozoa do not imply spermatozoon sentience. What plants and spermatozoa both lack in this regard are the types of complex nervous systems (including the limbic system) that are understood to give rise to sentience wherever we see it expressed in life on Earth.
To this fact we can then apply the scientific principle of parsimony, which forces us to conclude that because we have no good reason for mistaking the kinds of hormonal, electrical and chemical responses exhibited by plants as indicators of any degree of sentience, it is highly unlikely that they are.
As Dan Cudahy puts all this in his excellent article on plant sentience (http://unpopularveganessays.blogspot.com/2009/06/plant-sentience.html):
“Another source of confusion regarding plants that leads some people to speculate that they are sentient is that plants are highly evolved and complex organisms that ‘react’ to their environment in surprising ways, especially in larger time scales than we perceive in everyday life. Some plants ‘react’ to insects by releasing deterrent or poisonous chemicals. Some plants release chemicals to deter other plants from growing near them. Some plants are either aggressive or passive in root development depending on whether or not they are around their own species. The Venus Flytrap catches and consumes insects when insects come in contact with tiny hairs that trigger the trap to close.
The confusion arises when the assumption is made that such plant ‘behavior’ is caused by the plants “subjectively experiencing the world through sense data” rather than by insentient hormonal, electrical, mechanical, and chemical processes.“
Finally, evolutionary biology also weighs in here: being, by any reasonable measure, immobile, plants lack a good evolutionary reason for having developed a subjective experience of themselves in the world.