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I know the idea seems a bit far fetched but it's something I believe is common is science fiction and fantasy stories. I remember the talking trees of Zelda: Ocarina of time and of course, Tolkien's ents.
Is it possible for plants to evolve and gain the ability to think, take decisions, and communicate with their surroundings ?
If yes, how ? what conditions could make it more likely ?
If plants are already intelligent: I'm sorry and I hope I haven't offended anyone.
They are intelligent today, by your definition
"The ability to think and take decisions."
For this, lets turn to wikipedia.. a great source, I know:
Thinking allows humans to make sense of, interpret, represent or model the world they experience, and to make predictions about that world. It is therefore helpful to an organism with needs, objectives, and desires as it makes plans or otherwise attempts to accomplish those goals.
Ignoring the obviously unfair "human" in that definition, this says thinking is defined to be processing input, making plans, and making decisions/actions. Consider my Orange tree, which went through 2 years of really unfortunate weather. For two years, spring arrived, it decided to begin growing new leaves, and then a snap freeze whipped through and hurt all of the plants. This year, the tree decided to wait for a much more "true" spring, at the expense of shorting the growing season.
"Communicate with their surroundings"
Plants communicate with each other, warning each other of insect attacks. Some shout it over the air, relying on airborne pheromones. Others rely on a dense network of roots inter-tangling each other.
Intelligence is better thought of as a range, not a discrete flag. Creatures are not intelligent or unintelligent. There is a great variety of levels of intelligence, of which science is just beginning to scratch the surface. Any definition you can come up with for intelligence which tries to say "Is _____ intelligent" is certain to have great trouble with the answer.
As posted in several of the great comments below, it seems trivial to define "full-blown" intelligence as the ability to build models and predict the future. However, this definition is far harder (and more exciting) than it seems:
This definition gets tricky when dealing with the mentally disabled. It is very difficult to define an objective measure of intelligence which does not exclude the mentally disabled. Consider the brain of Jake Barnett. From a news article:
When Jacob Barnett was 2 years old, he was diagnosed with moderate to severe autism. Doctors told his parents that the boy would likely never talk or read and would probably be forever unable to independently manage basic daily activities like tying his shoe laces.
But they were sorely, extraordinarily mistaken.
Today, Barnett -- now 14 -- is a Master's student, on his way to earning a PhD in quantum physics. According to the BBC, the teen, who boasts an IQ of 170, has already been tipped to one day win the Nobel Prize.
Now how could doctors have known that Jake was intelligent? We know from ancient history that the shape of the brain doesn't give any suggestions that it is the root of intelligence (The Egyptians discarded it when mummifying the body as "useless"). Nobody knew Jake ever stood a chance of being self sufficient. Prove us wrong: he enrolled in Perdue at age 10!
Did Jake become intelligent through life experiences, or is he intelligent because of his physical makeup
Let's lower the bar a little:
This puts the bar low enough to ensure we don't accidentally insult those like Jake. But now plants start leaking in to the "intellegence" pool. In particular, the plant immune system is just as spectacular as an animal immune system. It must learn faster than genetic memory would support, because plants live much longer than parasites, so they evolve slower. If the immune system did not show signs of learning and modeling, plants could never keep up with the ever-evolving bacteria and insects that prey on the plants.
One of the attempts to quantify sentience is the Sentience Quotient (SQ). SQ does not attempt to quantify sentience directly. Rather, it tries to quantify the capability to be sentient by observing that sentience appears to require processing power. It is defined as:
$$SQ = \log_{10} \left( \frac{I}{M} \right)$$
$I$ is the being's ability to process data, measured in bits-per-second of processing capability. This is similar to how we measure the processing capability of computers. $M$ is the mass of the brain. To create an arbitrary reference point for this equation, instead of trying to put humans at +0 or something like that, the creator of this formula, Robert A. Freitas Jr., decided to fix the units to "bits/second" and "kilograms."
This scale ranges from -70 to +50. -70 comes from "processing 1 bit of information over the current estimated age of the universe using the entire mass of the universe." +50 comes from Quantum Mechanics if you limit yourself to only mass/energy methods of encoding data (all known methods are mass/energy methods).
Humans are roughly at +13 on this spectrum. All animals with brains (neuron based brains) cluster roughly around that point, because our +13 is defined by our neuron, not the rest of the body.
Computers, while currently sitting at +11 to +12, can theoretically achieve +23 using known physics.
Plants do process information. They average about a -2 on this chart. Interestingly enough, carnivorous plants, like the Venus Fly Trap come in at +1, 3 orders of magnitude more "sentient" than their bretheren. This comes from the little bits of learning and modeling they need to do to outwit their prey. Consider a venus fly trap does not close unless 2 hair-triggers are tripped in short succession to avoid false alarms.
So if you took the difference in SQ from normal plants to carnivorous plants, and made 4 more evolutionary leaps of similar magnitude, you could have a plant with a +13. Does that mean they're intelligent? No. It just means the numbers line up such that we think they could be intelligent.
Is a robot intelligent?
Your definition "think, take decision, communicate" sounds like a possible definition for a robot. In the robotics community there's a real debate about what is and what isn't a robot. But there is a consensus around the fact that they aren't intelligent (yet).
What is intelligence?
Also, intelligence is really hard to define. It is said (I cannot remember the original author) that we might not be able to recognize a superior intelligence because it would have preoccupations and manifestations that are beyond our understanding. Similarly, we might not recognize an inferior intelligence for somewhat the same reasons. Actually scientists don't even agree on marine mammals' intelligence. Maybe we just don't get them.
Cognitive science approach
Cognitive scientists tend to think that a brain without a body cannot be intelligent. And if you think about it, (almost) everything that happens in your brain has a final goal of actuating a muscle. If a plant can't act on the world, then it can't stimulate a human-like intelligence. Plants actually can do a few things to their surroundings but their range of action if very limited.
More on the subject: How could humans recognize another species as sentient / intelligent?
Thinking requires energy
The biological process of thought requires a huge amount of energy, thus a big amount of intake for an organism that is just spending its day in the sun without moving. For a single plant to develop the ability to think, it would have to adopt a super efficient energy gathering system.
Collective intelligence
Another route for making plants intelligent without any of the limitations mentioned above is with collective intelligence, meaning that one individual by itself isn't more intelligent than a bacteria, but together they act in a very intelligent way. A good example of this is of course ants.
A single plant can already cover a huge amount of surface and remain connected with a single root system. We also know that plants can communicate with each other through hormones and are able to act on their surroundings either directly by injecting chemicals into the ground or taking control of other animals. In fiction, they could believably develop collective intelligence and become sentient on a human-recognizable level. This type of organism has been described in many fiction book, including one of my favorites: The Swarm by Frank Schätzing.
So YES, plants could gain the ability to think, take decisions, and communicate with their surroundings. They already do all of this, just slowly and very little.
Not if anything like Earth plant life.
It's a matter of energy density. A brain is quite expensive of energy. It pays for itself if the creature can keep itself better fed or safer by thinking. This is far more likely for an animal that can direct its own movement.
WIth a few small primitive and sluggish exceptions we do hot have animals that photosynthesise. There is not enough energy to be gained that way. For air conditioning calculation, a human office worker emits 200 Watts. Sunshine is a kilowatt per square meter for about eight hours per day, say 330 W/Sq.m averaged out. Photosynthesis is inefficient (4% at best?). Weather inserts clouds. Animals just do not have enough surface area!
Plants have a different approach. They are static. They spread in a fractal manner to maximise solar radiation capture per unit of tissue. They have high tolerance of being partly eaten. Apart from poisons and thorns, their main defence is storing energy underground. Many plants can survive even total destruction of their parts above ground. They regrow from the roots. Many also scatter myriad seeds with long viability. There will be a next generation, even years after a catastrophe!
Finally, there has been much co-evolution. Fruits are there to be eaten, which gives seeds mobility inside animals. Flowers feed insects, which give pollen flower-targeted mobility. All animal life depends on plants for food and oxygen.So we have the animal and vegetable kingdoms separated but mutually dependant.
Plantlife: high surface area to passively absorb energy and CO2, high tolerance of being partly eaten, growing in a fixed location, with no need to make fast decisions and no ability to enact any. Animal life: small surface area, low tolerance of being partly eaten, mobile, uses thought to outwit predators or to prey. Eat plants directly or indirectly, mostly do not kill the whole plant, spread the plants seeds. The only common ground is monocellular life.
As the question asks whether it could be possible for plants to evolve intelligence, there is no question about it. DNA manipulations allow for almost anything physically viable. Whether that survives or not is a question for natural selection to answer. Thus it seems possible for a brain to evolve in a plant through natural selection. This would, of course, have to be accompanied with a host of other physiological changes, such as a skull, though I hesitate to note that a central nervous system need not require a humanlike physiology.
The simple answer comes in the form a of a question?
Does intelligence cause make a plant's situation worse off?
Intelligence is always a benefit to the organism that develops it. The limiting factor is not, therefor whether it can hypothetically come about from evolution given enough time and energy.
The question is whether or not a plant can get the needed energy for a complex neural system that can act on our time scale (Plants do move and such responding to their environment, it's just slow.)
The 2 ways available to them is Solar absorption and carnivorous plants.
1. I'm not an expert on Solar Absorption rates of plants but I'm pretty sure the answer to this is that there is no way to get the required energy out of this, but it may be possible using something we don't understand as of yet.
2. Eating meat is a lot closer to what it takes to create a neural net. Eating in the same way most animals eat can get a plant to having a neural net, however, if we're talking human level intelligence that requires a digestion technique or cooking, but if you're at the highest level an animal can get you could make the leap, so the answer is yes... plantlife could develop "thinking" because they can replicate animal behavior, which some already do.
