My question is:
- Could a macroscopic (e.g. visible) plant grow on a living human, provided that the human is completely immobilized?
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First off let me say yuck!, that is a series of Google searches I can never take back.
But as to your question, apparently plants often germinate inside the human body, specifically inside the lungs, as the warm moist environment is good for sprouting a seed. There are multiple reports of this happening with a variety of plants, this was widely covered when in 2010 a man in Massachusetts had a pea plant removed from his lungs, and in 2009 a Russian man had a small tree removed. Also found a case of a girl in china with a dandelion growth in her ear.
There is however not a consensus on this issue, as a group of chest physician published an article claiming the news stories about large plant growth are likely hoaxes, since extended plant growth would require light, which is lacking inside the lungs and other body cavities.
So, it seems very plausible to assume seeds could sprout inside the human body and that given a restrained subject, the plant could exit the body cavity and receive some sunlight and continue growing.
It should be noted that all the subjects mentioned in the news stories had their plant growths identified by doctors as they were suffering pain and discomfort from the growths, so it could likely work as a very slow torture method.
Man, and I wanted to be a smartass and recommend Chia helmets, but you had to go and use parasite.
Farther south, in the sweltering Mişymaşy jungles, the indigenous Wişywaşy tribe have a far more cruel method of punishment. Criminals are tied down in special tents where they are fed and tended, and protected from attack by the yhettis. There, a small wound is made and allowed to fester and necrotize. Administrations of special potent combinations of antibiotics and histamines form cystic walls surrounding the necrotic tissues so as to protect the victim from spread infections and to foster the necrosis. Putrefaction is allowed to continue for several days.
Eventually, special sanctified soil is mixed in with the wound, and a seed of the sacred Redikkulos vine is allowed to germinate. When the vine has grown to a length of 2 or so ficties, the criminal, now ready to be admitted back into their society, is unbound and allowed to go free.
Maintaining a Redikkulos vine in such a manner is thought to help purify the criminal.
― from the journals of Dirk Dark, adventurer extraordinare.
Personally, I doubt the veracity of Dirk's documents. Later scholarly snobbishness has postulated that he was little more than a spindoctor for the so–called Glorious Republic — which was, as we well know, one of the most despotic, totalitarian oligarchies to ever arise on the Sunken Continent.
― “My Thoughts on a Diverse Variety of Things” by Sire Clyde Shortstoff, prime minister to the people of the royal Hoytitauyts.
To grow plants need
The first excludes growth inside the body, the second excludes growth solely on the body. Normally plants rely on microbes present in the ground to decompose organic matters into elemental components they can absorb (Nitrates, Phosphates, Sulfates, etc) which are then used for their growth. Direct assimilation is not possible.
You may change a bit the aim and go for fungi. They (some of them, at least) already grow directly on rotting trees. Not sure if you need some handwavium, too. Normally in nature a corpse doesn't last undisturbed long enough for a fungus to grow, like a rotting tree does.
The human body tends to be pretty good at rejecting foreign materials. Plant or not, often if you put something that isn't supposed to be in a human body the body will try to remove it. This is part of the body's immune response. You can see it happening with something as simple as a splinter in your finger.
This can even happen with surgical implants. I had a friend who had a joint in his foot/ankle replaced with an artificial implant, his body rejected the implant and effectively tried to push it out. I've never seen anyone in so much pain...
So, realistically an implanted seed could sprout inside a human body, but the real pain/torture comes when the body starts to reject the seed/plant and push it back through the tissue and skin.
Perhaps it could be possible if an already-rooted plant was transplanted, somehow without killing the host, onto the human, with its roots in their stomach (so it could parasitically steal minerals like a tapeworm) but with the leaves outside? Birds have lived with arrows sticking through their bodies, for example: https://www.youtube.com/watch?v=2KCUpB_f5hI Maybe if the plant was surgically put in your navel or in a fashion so there was no "gaping hole", it could stop you from bleeding out.
This is a really gruesome question!
As a PhD in biology I want to add my 3 cents: plants do NOT need light to grow IF they can absorb sugars from the media on which they grow. Which mainly happens in controlled laboratory cultures, but hey, we're on worldbuilding, right? The plants grown without light do look a lot different tough, for starters they are not green, because, of course, they do not produce chlorophyll needlessly.
So, if You want to, You can make up a species of plant which is an opportunistic pathogen and grows inside a human body if the situation is advantageous for it. Of course, the evolution of such a species would be a tale of its own (evolution of pathogens typically is).