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Could a plant, with the right adaptations, have the right speed and strength to capture an unprepared human without harming them? The plants would be bound by real rules of biology, and couldn't use muscles or other traits exclusive to groups other than plants. The plant would also be capturing the humans with a vine-like structure that coils around the victim

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  • $\begingroup$ I find it unlikely you could do it with vines and be able to restrain a strong adult while not harming a kid when that trap springs. But if thorns are acceptable as well, it is pretty trivial to restrain humans yet be mostly harmless. $\endgroup$ Nov 23 at 9:37
  • $\begingroup$ I am unsure: Does the answer need vines, or is that an example? Also, what are the humans expected to carry by ways of tools, and: how long do they have to be restrained to count as captured? $\endgroup$
    – bukwyrm
    Nov 23 at 10:57
  • $\begingroup$ I think some tangled vines with thorns would already do the trick, no? They would not be hunting humans, and they would not capture everyone, but they might still hold the ocasional unlucky wanderer. $\endgroup$
    – Burki
    Nov 23 at 11:04
  • $\begingroup$ Answer in a comment: ...as long as it is named Audrey II $\endgroup$
    – CGCampbell
    Nov 23 at 16:47
  • $\begingroup$ Just ask anyone who has tried to prune a large rose bush :-) $\endgroup$
    – jamesqf
    Nov 23 at 17:16
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Trap and Poison:

If you want to catch something the size and skill of a human, then NOT harm them you've got logic issues, since some benefit needs to come to a plant. You'll need to explain why a plant would have a system for capturing humans unharmed. But once you get past that, I'd suggest a few mechanisms that work well by themselves or in combination.

First, your plant produces a fruit that is a powerful tranquilizer - possibly a pleasurable or addictive one. Now, depending on how you define capture and the timeframe over which you want to capture them, they are captured short-term (knocked out) or long-term (they wake up and REALLY want more of the fruit, despite it knocking them out).the person spends all their time passed out under the tree. The justification here is that perhaps the tranquilizer is fatal to other species that die and decompose under the plant, OR that the tranquilized animal is killed by predators (who are possibly ALSO tranquilized and killed by yet other predators, leading to a La Brea-type situation with lots of decaying animals.

  • If you want a vine-type mechanism, then the vine is coated in the tranquilizer, and the tranquilizer induces a craving to wrap the vines around themselves or cuddle up into the vines. The person is then self-capturing!
  • Another variation on this theme is that the plant relies on monkeys to carry pollen from tree to tree, and humans are close enough to monkeys to be susceptible to the tree's lure for the monkeys. The monkeys eat fruit from one plant, then are compelled to look for more in another tree. The tree protects it's addictive fruit inside a net of vines to keep other less driven animals out (and the monkeys know how to get in and out). Humans, however, force their way frantically into the net to get the fruit (once they have tasted it) and are trapped. This still works for the plant - the human carefully gets fruit from plant A, along with pollen, is instantly addicted, and seeks out tree B where they get more fruit, deposit pollen, and become hopelessly trapped.

Another effective way a plant could capture a human-sized animal is if the root growth created a shaft down into the ground or into a grown structure that functions as a pit. When big enough, a thin covering over the "pit" is thinned enough to allow a human to be lured to the pit (with fruit, fake treasure, or whatever) and the funnel-shaped pit then wedges the human into an ever-narrowing hole. Directional spikes wedge the human in so they can't extract themselves.

  • Ideally, the pit is lined with drugged thorns so the human that falls down it is tranquilized to fall into a deep slumber and not resist. Then the plant can hold the person there indefinitely for whatever nefarious reason an author can have (since plants obviously don't think... Or do they?) If it has to be vines, then the pit is lined with vines that swell with fluids when touched. The vines swell and pressure-tighten, with more swelling the more the human resists.

  • A spring-loaded (the spring being a branch) loop snare could be grown, with a lowering branch trailing a network of roots to hold it in place, then an appropriate trigger causing the loop snare to pull tight around a human's limb. The tranquilizer then numbs the person unconscious, allowing them to be held indefinitely. tree loop snare

grown pit trap

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    $\begingroup$ Very good point about the need for evolutionary reasons behind it - OP wants the plant to be "bound by real rules of biology" and evolution the most important. --- Though it does not need a reason to for capturing humans unharmed, it just needs a reason for the apparatus to do so, capture might be incidental $\endgroup$
    – bukwyrm
    Nov 23 at 10:45
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    $\begingroup$ "If you want to catch something the size and skill of a human, then NOT harm them you've got logic issues, since some benefit needs to come to a plant" not necessarily, and perhaps not within the OPS vision. The flora could be alien in origin, or the humans crash landed/colonizing another planet. The flora could have evolved to trap and digest native fauna, and in doing so would perhaps use enzymes, etc, not harmful to humans. A trapped human might be indirectly killed via starvation, but not necessarily directly killed through digestion. $\endgroup$
    – CGCampbell
    Nov 23 at 17:03
  • $\begingroup$ @CGCampbell I'm saying the causation for evolving such mechanisms is highly relevant to how it should be carried out. Systems evolve together to deliver outcomes. I need to follow the logic for it to make sense. $\endgroup$
    – DWKraus
    Nov 23 at 23:29
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The plants would be bound by real rules of biology, and couldn't use muscles or other traits exclusive to groups other than plants.

Direct attack is less likely at human size and reaction speed, but you can use the idea of traps.

  • I remember I saw something like this in that documentary... what's it name?... something like Harry Potter - except they stepped onto a tangle of vines or roots that shifted beneath them. I can't remember if that happened before or after they played some kind of weird chess.

  • with "gravity assist" the things can go quite fast - like a mass of vines clinging up from the lower branches of the trees by tendrils, tendrils that give way when the prey is well under and so bury the pray in the tangle of vines dropping from above.

  • then World's Biggest Carnivorous Plant Catches Whole Sheep! - (well, not quite, it's a sheep that got its wool tangled in brumbles. But a plant doing this way will have chances at the nitrogen locked into those unfortunate proteins). You can use the idea having a plant reorient its thorns (or spawning then over minutes) to make the escape impossible for any prey that got far enough inside of what looked like an innocuous bush.

Once the prey is immobilized, vine coiling and the other horrors can take their natural course at leisure.

Note the first two potentially match the requirement of "capture a human without harming them"

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    $\begingroup$ The plant you're thinking of from Harry Potter was called "Devil's Snare" - it appears at the end of the first book (Philospher's Stone) before the giant chess set. $\endgroup$
    – Gh0stFish
    Nov 23 at 14:22
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  • Possibly stretching the definition of capture a bit here, but maybe the plant has absolutely no interest in capturing humans, it just happens to do so, by constraining them to be within a certain radius. The plant produces a toxin that knocks out some endocrinal capability of the humans, and at the same time produces a substance that is metabolized into a working analogue of the, now missing, endocrinally produced substance. The humans will have to stay near the plant (the substance breaks down easily, and has to be ingested from living plant tissue), or they die. Depending on the substance in question, this might be something that is needed every hour, or once a month, thus giving a wide range of radii to choose from. - If the answer needs vines, the human might need to wrap themselves in living vines to take up the vital substance transdermally.

  • A plant may have evolved to quickly cover and absorb carcasses, masking the smell, deterring scavengers, perhaps working in concert with a fungus? The plant would grow by spreading their roots, being visible topside only by a few blades of grasslike leaves. As soon as carrion is detected by the leaves, the plant digs into its potato-like subterranean stores and kicks into overdrive, covering the carrion in vines and thistles, releasing substances to help the fungus grow (the fungus will help break down the carrion for ease of uptake). The vines harden quickly, effectively encasing the carrion in a cage (So no big scavengers can rob the plant of its prize). All the tasty nutrients get distributed and stored in the root network. Now, if a blind-drunk human should happen to sleep off their intoxication, they might find themselves in a pickle (literally) the next day. Their immune system will fight the fungus, resulting in nothing more than a rash and a cough, but the vines will lock them in place regardless.

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Very strong vines with an exceptionally sticky adhesive could work. Something similar to sun dew plants.

For some reason I am reminded of The Day of the Triffids .

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How "unharmed" do you want? I'm going for "alive".

Thorns.

Brambles are extremely dense, and at the right stage in their growth very strong. As anyone who has gone blackberrying can attest, the thorns are also lethally sharp and can feel strong enough to pull your skin off.

A bush can immobilise a grown sheep when the wool gets tangled, leading this farmer to theorise that they are actually carnivorous and fertilising themselves with decaying sheep: https://www.youtube.com/watch?v=RuzLXxbGc4c

So, a large tough dense bramble, equipped with some kind of Venus Fly Trap style motion trigger to close the branches; the more you struggle the tighter they close.

They are tough to break anyway, your motion would be restricted, and every move you make would be extremely painful, but would probably not kill you.

EDIT: Thinking on this a bit more, the trigger shoots could be away from the thorny branches, on suckers that spread over the ground. Pushing through a dense forest, your hero finds a sort of tunnel where the going is a bit easier. As he walks on through it, he fails to notice the surrounding undergrowth closing in, until SHOOMP! like a Venus Fly Trap, it all closes up.

This behaviour could evolve quite nicely over the normal millions of years as a means of catching any large animal; however, with humans' greater intelligence, very few of them are then digested.

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You might consider Drosera capensis, also known as Cape sundew. This plant has long and thin leaves, covered with small sticky tentacles. The leaves curl up to entrap anything that gets stuck to them.

Sundew leaf wrapped around a fly.

A larger version could have a burr-like surface to further entangle the human's clothing, delaying extraction and giving time for the leaves to curl.

As someone that recently hiked through some brush, I can testify that it is easy to get trapped in vines, even without any sticky mechanism. It was easy enough to get out again, but it took significant time, and time is what is needed here.

The plant wouldn't even need to be huge, using a single leaf to make the capture. A struggling human could be trapped by multiple leaves, each of which need be only large enough to wrap around an arm or leg. Once the hands are immobilized, it's game over.

See: Best of Sundew Timelapses Compilation - YouTube

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