Method A: Evolving a True Plant with Neural Tissue
This method describes how a traditional, plantlike organism could evolve a "brain" while maintaining plantlike characteristics in every generation.
First of all, brains can come in all shapes and sizes!
- Jellyfish style: Nothing resembling a brain is present but a distribution of nervous tissue around the body maintains control. Think "reflexes" and not "thoughts".
- Cockroach style: Clumps of tissues resembling a brain exist but are spread out in the body.
- Spider style: The brain tissue literally spills out of the head and around the body
- Human style: One central brain controls the body through a network of tissues
- Colossal squid style: The cognetive organ is wrapped around the esophagus in such a way that a large bite of food means permanent brain damage :)
A plant could plausibly evolve and survive with any of these systems in place, but it would likely evolve clumps or spread tissue. This is because plants tend to lose parts, and this method would be most likely to preserve most intelligent thought if said plant were to lose said parts.
Now how would a plant evolve a web of neural tissue?
Well to spread tissue to every part of the plant, it could potentially evolve from a single-celled, photosynthetic organism. As this organism grows larger over time, becoming two or three-celled, one cell mutates to have neuron-like properties. If this three-celled organism can
- Sense light with Cell A (photo-receptor, which isn't uncommon and could also be a precursor to the evolution of eyes)
- Remember what or where light is with Cell B (precursor to neuron; single neurons are capable of doing simple tasks like this)
- Photosynthesize with Cell C (precursor to leaves; must contain chloroplasts or another organelle capable of photosynthesis)
- Travel to the light by moving all three cells
Under these conditions, an organism could survive to reproduce, grow larger, and eventually become an organism comprised of two separate, main tissues: "traditional" plant matter, and a web of neural tissue. Eyes could additionally evolve from the original organism's photo-receptors, and these would aid the neural tissue in collecting information.
Method B: Adapting an Organism with a Brain to Photosynthesize
In this method, an organism that is not a plant, but has cognitive capacity, evolves photosynthesis and other plantlike characteristics.
An organism that dwells in sunlight mutates a form of chloroplast, and passes the genes that create them to its offspring. Individuals that spend more time in sunlight will survive to reproduce because they will produce more of their own food (sugars) and will not have to forage or decompose other organisms. Additionally, organisms with larger and more efficient photosynthetic cells or tissues will also survive to reproduce.
What could evolve in this way? Something:
- Mobile, to make use of sunlight throughout the day
- Intelligent enough to meet the "has a brain" requirement
- Relatively flat to make efficient use of sunlight
- That can fulfill the chemical needs of photosynthesis for its own system. For example, an organism whose process requires carbon dioxide and releases oxygen would make great use of porous flesh to filter in gases.
- That makes contact with the ground, such as a slug, to absorb nutrients from the soil without having permanently anchored roots (for now)
- That doesn't contain extremely rigid structures such as bones. Relying on the soil, water, and sunlight won't provide the nutrients to form skeletal structures, so only a soft organism will be able to survive using photosynthesis alone and evolve into a plantlike organism from there.
If you aren't satisfied calling green slugs "plants", here's what comes next: The organisms' metabolisms could slow drastically, or they could find a place such as an open field to proliferate. Either way, they will move less. This is important, as the next stop on our journey is roots!
In order for a plantlike organism to be truly convincing, even if all tissues except its brain now resemble that of a plant, you might want roots. These could plausibly develop if the organism doesn't need to move to find sunlight. The organisms that can reach further into the soil for nutrients while still being stationary in this environment will survive to reproduce, making roots increasingly common and efficient. With the energy these organisms would now have, they could be capable of sprouting upward and forming chutes or stalks. This would put the brain at the base of the plant, between the roots and stems, with all other surviving organs from the original species.
Reproduction for a stationary organism will be problematic, and will likely have to evolve before said organism becomes stationary. One solution could be the development of asexual reproduction, which is not uncommon among plants on Earth. New plants would sprout at the base of parent plants, from the organs that used to facilitate reproduction, but daughter organisms would contain only parent DNA. Another method could be the exchange of genetic information through external contact, such as stems or roots.
Finally, you have a plantlike creature that evolved from another organism through natural selection and a whole lot of unlikely plausibility.
Method C: Symbiosis Between a Brain Species and a Plant
I won't go into much detail on this one, but I will lay a foundation.
Lichens (LIKE-ins) are a symbiotic combination of two or more organisms, always a photosynthetic organism to produce energy, a fungus to anchor to a surface, and sometimes some other things, but let's ignore those other things for now.
Theoretically, your plants with brains could evolve similarly to lichens - a photosynthesizer could produce sugars for a host with a brain, which would in turn house the photosynthetic organism safely.