There is a very good reason why the locals maintain such a strict perimeter around the ruins... too bad the protagonists don't learn why until it's way too late.
The plants themselves are clearly nightmare fuel for the locals. They're mobile, carnivorous, and infectious. If you touch them, well, you're boned. Anyone that touches them gets a prompt Mercy Kill - or is simply herded to the top of the ruins to wait for the plants to do their work.
The scene where they search for the "cell phone" by following the noise, before realizing it's the plant mimicking the noise to lure them closer into a trap. The sight of the plants making the imitation cell phone noise, and the sound itself, is genuinely disturbing.
When the plants start mimicking Stacy with all of them saying "I want to cut it" over and over and over again as she is filleting her own leg.
The plants being lightning fast when most plants are usually slow.
The amputation scene. It goes on for a very long time, with characters trying to smash and splinter gristly bone after the rest of the surrounding flesh has been hacked away - with no anesthetics whatsoever. To prevent bleeding out, they try cauterizing the wound with a pan that had been sitting on a fire. Matthias dies anyway, because they left him by the plants and were too busy arguing to notice what was going on.
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The sap the plants have in the novel is pretty terrifying. It burns skin on contact. Though it's never really explained in detail, one can only wonder what the sap actually does apart from burning skin.
The plants themselves have It Can Think growing all over them. While plants in real life can think, they usually leave sadism out of their thoughts. At one point, Jeff makes a "Danger" sign to warn any other people who may be stupid enough to go near this temple. A few pages later, the sign vanishes. Jeff goes to find it, and finds it covered in the damn vines to the point where he has to strain himself pulling it out. Quite the clever bastards aren't they?
The plants' way of thinking become even more disturbing before you even reach page 117. The plants mimicked the sound of a bird chirping, most likely indicating that the plants have attacked a bird before our main protagonists even arrived there.
The plants seem to have been drawn to Eric's sperm so much so, that a good amount of them clung to his fucking penis! Plants in real life aren't attracted or even drawn to human sperm. Ants on the other hand...
This is strange considering in the film, the plants are attracted to a variety of things their prey do. Blood is the main thing that attracts them while their victims' stupidity, unawareness, actions, arguments, and injuries attract them a lot less, though they'll prioritize injured victims since they're easier to go after and require less effort. They aren't attracted to sperm however.
You and your friends are driven to a secret ancient Mayan temple in Mexico. Your German friend is there to join his German friends who have already set up camp there way before you guys even got there. You guys get close to a temple surrounded by plants you've never encountered before and several people step out of the jungle and start yelling at you, waiving guns, rifles, and bows. Later on you find yourself trapped on a Mayan temple, with little knowledge as to why you were almost killed by a group of natives you don't even know. You hear what sounds like a bird and go to investigate it, then the rope snaps and you fall to what you think is your death, only to live, but suffer a broken leg. Now in severe pain, you're pulled back up to the surface of the temple and sleep your intense pain off, only to wake up the next day and find that same plant clinging to your leg and your penis. It's only at that moment that you begin to realize that the plants surrounding the temple are out to get you and anything else that sets foot on the temple. Shit can't get scarier than this people!
The plants themselves are this. They're more terrifying than the film even depicts them. They're smart as hell, feed on any living organisms they deem edible, have a sap that acts as their digestive fluids, can grow anywhere they feel like it without needing the sun and water, and worst of all, they're covering every inch of the temple, basically surrounding you and your friends. Then there's the part where they can mimic every sound they hear and have unnaturally fast reflexes... wait, they have reflexes....!?
