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For the sake of the question, assume conditions are Earth-like in all manners other than those which would be required to spur the development of such a plant. I want the plant to gestate in the digestive tract and slowly kill the 'infected' by sucking the nutrients from them (like a tape worm). When said person dies the plant uses his/her body as compost to support growth and development.

  • How would such a plant evolve (is there any precedent)?

  • How would it work (besides the basics I laid out for you)

  • Is this realistic for a story?

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    $\begingroup$ This idea was partially explored in Orson Scott Card's Xenocide, with the Pequeninos. $\endgroup$ – Frostfyre Apr 27 '15 at 13:41
  • $\begingroup$ Related: worldbuilding.stackexchange.com/a/10829/7351 $\endgroup$ – apaul Apr 27 '15 at 14:18
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    $\begingroup$ @Frostfyre Not really. If I recall correctly, the Pequeninos just flayed open the bodies and planted a tree in their corpse so they could become part of the forest. Still, it's good, I decided to have an oak tree planted on my grave after reading that story. $\endgroup$ – Samuel Apr 27 '15 at 15:48
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    $\begingroup$ @Samuel The Pequeninos were adapted to a gene-altering disease that may have been artificially added to the ecosystem. The disease was responsible for the transformation. It was a good book. $\endgroup$ – Frostfyre Apr 27 '15 at 15:53
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    $\begingroup$ The Pequeninos did not plant a tree on the corpse. The flayed-while-alive individual generated the sapling. The Peqeninos would chew a painkilling for this. For confirmation, the human that they decided to honor in such a manner did not generate a sapling, which confused them. Also, their method of painkiller did not really work for humans which ended up giving the human they were honoring a horrifically painful death and almost resulted in a genocidal retaliation. $\endgroup$ – Michael Richardson Mar 29 '16 at 13:58
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The biggest problem is that you want the plant to thrive in two very different environments (relatively speaking):

                Body          Ground
Temperature     Hot           Cold
Moisture        Wet           Dry
Respiration     Liquid        Air 
Nutrients       Bloodborne    Soil/compost
Energy          Glucose       Photosynthesis

Therefore, for this to work, you really need to have a two stage lifespan. Similar to a caterpillar that meta-morphs into a butterfly, your plant will likely have to transition from thriving in one environment to another.

The seed, therefore must be inhaled, consumed, or embedded somehow. Natural processes suggest inhalation or ingestion as the most successful route. While I'd prefer inhalation (close to bloodstream and air, not as harsh as digestive), it appears you've already selected digestive.

So the seed has to not only survive the digestive tract, but the plant has to provide some enticement to being eaten. If it were inhalation, it wouldn't have to entice - it would merely send out spores or microscopic airborne seeds when jostled. The plant, therefore, provides significant satisfying nutrition, so as to get hosts to eat it.

Once consumed, the seed has to pass through the upper digestive tract, but get stuck in the lower digestive tract (lower acidity). This seems difficult, but perhaps it has a germination time of 1-2 hours once the outer acid protective surface is cleaned off. It then starts rooting in several directions, preventing it from moving further down the digestive tract. Small barbed roots that can absorb nutrients from the partially digested food would be best.

At this time it just absorbs energy that will be used during its transition, and builds into a tumor, with longer and longer roots. Eventually it will completely obstruct the digestive tract or the roots will pierce too many blood vessels or veins, and the host will die. If it's necessary that the host doesn't feel pain until near death, then it either synthesizes anesthetic which it emits through its roots, or it doesn't send out roots and instead merely becomes a digestive obstruction.

When the host dies, the process of decomposition sends chemical signals to the seedling and within a few hours it is expending energy sending a tendril upward (any way could be up at this point, but gravity leads the way) seeking light. Once light is obtained, the plant sends out leaves, roots itself more firmly to the host and ground below, and, eventually, flowers and fruits to catch another host for its offspring.

Pulling off the metamorphosis is the tricky part, but since we have animal analogs that do this, it shouldn't be too difficult to explain to the audience.

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    $\begingroup$ Different plants have different needs for their seeds to germinate. For example, corn loves hot moist environments for seed germination. $\endgroup$ – Jim2B Apr 27 '15 at 18:25
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    $\begingroup$ @Jim2B I should have considered that more carefully - I'm currently germinating a few hundred seeds for my garden, and a few dozen are on a heating mat. $\endgroup$ – Adam Davis Apr 27 '15 at 18:42
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    $\begingroup$ I was going to answer with something about a 2-stage process similar to the lifecycle of fungus but this does it beautifully. I definitely support the inhalation part though, any intelligent society is definitely going to recognize a deadly food no matter how long it takes to kill them. $\endgroup$ – thanby Apr 28 '15 at 14:03
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Well there are currently some plants that need their seeds to travel through the digestive track of animals before they can germinate. I was looking but couldn't find them right now. I think they got their coating so they could pass through a digestive system and be carried away.

So step that up a bit, and let it 'catch' in the digestive system, then start to sprout. Though making it more a symbiotic relationship (with at least one or two species) would make it more likely, maybe it helps process some other foods. And when the host dies, it will sprout out and use the body as a food source.

If it always killed it's host every time it is eaten most would learn to avoid it quickly.

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    $\begingroup$ If the host died several weeks or months later, without feeling sick immediately after ingestion, a vast majority of animals wouldn't associate that plant with the cause of death. $\endgroup$ – Samuel Apr 27 '15 at 15:59
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    $\begingroup$ @Samuel True, but if EVERY animal that swallowed those seeds, died, it would have to be enough time for it to procreate before death, or only animals that don't eat the seeds that will survive. It might be there is only a short period where it will work, or maybe a low chance of successfully rooting etc. $\endgroup$ – bowlturner Apr 27 '15 at 16:03
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    $\begingroup$ Sure, that'd be true about any parasite. Most likely animals would be scared off from eating it because it's growing out of their cousin who went missing last year. $\endgroup$ – Samuel Apr 27 '15 at 16:05
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    $\begingroup$ it's more like if animal that some don't like it they will survive longer and then it will evolve in a true aversion. $\endgroup$ – ratchet freak Apr 27 '15 at 17:02
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Plant Growing in the Lungs
This is a story ripped from the headlines. It is a plant growing in a living person's lung.

Ron Sveden, a retired teacher from Brewster, Massachusetts in the US was astonished to discover that what he thought was a tumor growing in his lung was actually a plant that had sprouted from an inhaled pea.

75-year old Sveden said he was told the pea seed had split and sprouted in his lung. It was about half an inch long (about 1.25 cm), which "is a pretty big thing", he said according to a news report from NBC.

Pea growing in lungs - example of plant growing in a living animal. Pea growing in lungs

Not the digestive tract but pretty close.

Fungus Growing in the Body
Another close match is fungal pneumonia (which I have). It is an example of fungus growing in a living animal's lungs. You inhale the spores and they take up residence, germinate, and begin growing.

My fungal pneumonia - example of fungus growing in living animals
My fungal pneumonia

After the fungus reaches a certain amount of growth, the fungal growth releases fungal cells into your blood system and spreads to other parts of the body. This enables the fungus to spread to any part of your body (intestines, brain, heart, liver, you name it) and begin a new fungus colony (this is the stage they caught mine at).

It is fatal if not treated.

Other Thoughts
The lungs are probably a more benign environment than the intestines.

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My thought would be that a plant couldn't do it. At least not by itself. Mostly because a plant needs to carry out photosynthesis in order to live.

HOWEVER there is an analog of a parasitic fungus that will kill its insect host then "bloom" out of the body as it continues to gather nutrients from its now deceased host.

ALSO there is an analog for a symbiotic relationship between fungi and plants. These are the lichens. Now these organisms still use photosynthesis, but, unlike a plant, which is defined by use of photosynthesis, nothing says that lichens necessarily require photosynthesis.

BY EXTENSION A particularly virulent strain of cordyceps mutates that can infect mammals. However, there is a bit of an issue. As diets change and mammals evolve stronger stomach acid, it is harder for the fungus to survive past the digestive tract. So it evolves a way of attaching itself to a plant ovum and hitching a ride into the digestive tract. The stomach acid erodes the tough outer shell of the seed and the cortyceps is released into the intestines and blooms with the added benefit of pushing the seed out of the mammal to so it can sprout, utilizing the decaying body for nutrients while getting that sweet sweet sunlight fix. Further evolution happens involving little hooks on the seed to adhere to the intestinal wall and cortyceps producing higher level of nitrogen locking the cortyceps and plant into a codependent symbiosis. Then the plant starts producing tasty fruit, humans start eating said tasty fruit, cortyceps-tree evolves to utilize humans, a new plague upon all of humanity is unleashed, and millions of grotesque mafia deaths happen in movies. All because you couldn't be bothered to core an apple.

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    $\begingroup$ +1: The OP is confused about the genus. If their primary nutrients source is not photosyntesis, it is not a plant (but there are photosyntetic bacteria and algae, too) $\endgroup$ – SJuan76 Apr 27 '15 at 22:31
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There is a virus or something that infects an ant and makes it climb a tree before sprouting or creating spores inside it's dying/dead body. Can't remember the ant or the plant/virus that does this but have heard this from a few sources,

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    $\begingroup$ You're thinking of the Cortyceps fungus. $\endgroup$ – Jake Apr 27 '15 at 15:40
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Is this realistic for a story?

In a modern society - no. We bury our dead in boxes (and those that we don't bury we burn), and don't grow anything near them.

The life-cycle of this plant makes sense for animals, but not for people. It would make sense for ancient, pre-historic cavemen, but not for any civilisation in the past few thousand years.

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