I think plants, and also us as humans, do some energy grounding towards the earth... channeling energy from the sky or something (from the sun too if you think about it, but also starts and cosmos).
When you think about trees, they have a structure alike to an anthena.
We are all connected to mother earth in a way, or gaia. Plants are constantly moving these energy and we benefit from eating it. Is like, they are happy with a boner and "high" on energy... breathing constantly.. "universe breathing you"
Also, I´m not sure the plant itself is so intelligent.
There´s a hiererchy of intelligences... like mineral-plant-animal-human-angel-planet-else
Plants and animals have like a collective mind or elemental. An elemental genius that might be the smart guy (also a forest or mountain could have a even higher elemental genius keeping it all together, in peru they call them apus). Archetypal spirits that have great wisdom and power (but a human with soul is even more complex than that, closer to angelic intelligence).
In white magic, each plant has an elemental of a certain type, that can be used as helper for a specific task... healing, protection, etc. (animals have it too, like cats).
So, for instance, ayahuasca is a special case cause you might connect with 2 elemental spirits (from caapi and chacruna, for instance) each of them is a snake. Look at amaringo paintings and that book in english.
Finally, about the function of DMT. Which happens to be in a lot of plants...
You have to think about whether we used to be more connected and have higher states of perception, and somebody dumb us down by inhibiting the dmt levels in the blood... though that famous enzime.
You can actually produce dmt for short periods... doing those weird greathing exercises like Wim Hof style... or meditation.
Why do we have limited access to a substance that we ourselves produce? (pineal gland?)
Mythology now: Perhaps before being expelled from the garden of eden... man could talk to animals and plants and communicate telephatically, also live longer.
Now, we are dumb, not connected, and live short incarnations.
Well, just some thoughts on you post, hope you appreciate another edge.
"...after five seconds I was no longer a marxist, no longer a materialist, no longer a rationalist.
It killed those things, it cauterized them..."Terrence McKenna