Why would humans carry around live plants in a post-apocalyptic world?

Question

A while ago I saw this brilliant 3d render of a post-apocalyptic world in which a human carries around a live plant in a capsule, connected to the plant via a respirator:

(credit: Stephen Grimm via Youtube)

I would love to develop this idea for a post-apocalyptic world I'm building for a 2D pixel art game.

The idea is that some sort of chemical or nuclear warfare has made the atmosphere highly toxic for humans -- so they rely on certain special plants to create breathable air instead. Small settlements are centered around large trees with this special capability; humans traveling across the wastelands carry smaller plants in glass containers on their backs, breathing through a respirator much like in the photo above.

I don't mind a certain amount of handwaving -- that is, it's fine to invent some semi-magical compound that these plants create -- but here are a few questions/points to consider:

• perhaps the simplest scenario is that the plants simply create oxygen -- but how much quicker than real-world plants would they have to produce it so that a portable plant could sustain a human?

• Furthermore, if these plants create oxygen at such incredible rates, why wouldn't humans just bottle it up and carry the bottles rather than the live plants?

• if they create something other than oxygen, why would it have to be consumed straight away rather than be stored somewhere?

• an alternate idea that could be promising is that people are carrying or preserving the plants because the plants are at risk of extinction, not because that is the only way for the humans to breathe. but would people really carry them around in little boxes rather than building some kind of glass house, or seed vault, or some other more permanent structure?

All ideas appreciated :)

Answer 1

The Plants Filter Out Toxins

Part of the process of converting CO₂ to O₂ is storing the C (carbon), which is used to build the plant's biomass. So for a single plant to produce enough oxygen for a human, it must also grow incredibly fast and require a huge amount of water and nutrients. Besides, oxygen is about 21% of the atmosphere; it would have to be quite some disaster for an apocalyptic event to wipe out enough to threaten humans. A much more likely atmospheric catastrophe would involve toxic gases or particulates. You might have a toxin that is dangerous even at a few parts per billion. So I suggest that your survivors discover and breed a plant that is miraculously good at filtering out whatever has poisoned the atmosphere. This would allow for a realistic plant with reasonable metabolism, nutrient needs, weight, and size for a human to carry around. Because it efficiently removes something from the air instead of creating something, it would be more lightweight and last longer to bring the plant with you instead of lugging around a heavy pressurized tank of filtered air.

Also, this could give rise to interesting plot points — perhaps toxins are more concentrated in deep valleys, near disaster sites, or far from the wonder plant's biome, presenting a silent danger if the plant can't filter fast enough. Areas where the plant is flourishing would be natural refuges for large animal life to survive.

Answer 2

The average leaf produces 5 milliliters of air an hour. while a human uses around 16 liters of oxygen an hour, which makes for a need for around 3200 leaves. Assuming an average plant weighs 5 kilos and has 25 leaves, you need 600 kilos of plants. That's a lot to pull.

The plants need to be about one hundred times more efficient.

Assuming the plants are hundred times more efficient, just 6 kilos of plants will sustain a person. This means an average person can just carry around a house plant weighing 6-10 kilos and sustain themselves.

Oyxgen is too heavy for typical trip lengths.

Assuming an average trip takes 7 days, or extra if there's chaos, house plants are just more weight effective. An oxygen tube weighs 4 kilos for about 3 days of oxygen. You need 12 kilos to carry enough for a trip, and if something goes wrong you could quickly be in trouble. A plant lasts forever, is lighter, and can regrow itself if damage.

These post apocalyptic super plants also require much less maintenance. Even a very damaged society can produce them, while producing oxygen tubes requires a much better tech level.

Answer 3

Breathing OUT is dangerous

Imagine a world with some kind of insect that is attracted to the smell of human breath. If they smell you breathing, they will swarm you and tunnel through anything they have to to get into your nose and mouth where they will enter your lungs to lay thier eggs.

A normal oxygen tank or respirator wont help you because the air you take in is perfectly safe, its breathing out that you need to worry about. A face mask or respirator may filter your breath for a short while, but the scent of your breath still builds up on the filter itself which will attract the swarm.

That said, many plants that humans tolerate well are poisonous and have highly noxious smells to many insects (garlic, onions, mint, coffee, etc.) By mixing the smell of our breath with the natural scent of these plants we "sour" our scent such that is does not attract the swarm. While things like Garlic powder might offer a temporary filter, you never really know when the scene has faded enough to stop working until its too late; so, keeping a live plant to filter your breath through ensures that there is always a fresh scene to mask your breath with.

Answer 4

This plant needs to clear several major hurdles:

Rate of Oxygen Production

Option A: Ridiculous rate of oxygen production

Oxygen is "generated" by plants taking carbon dioxide and water and releasing purified O2 gas as a byproduct of sugar production. As in Nepene's answer, the efficiency of conversion must be significantly higher than that of ordinary green matter to sustain a human being. No matter; these plants have extremely high surface area through their fractal leaves and also displace high volumes of air through micro vibrations of tympanic structures throughout their leaves that act as high efficiency fans to allow this gas exchange to occur at an accelerated rate, allowing each plant to do the work of a hundred normal plants. This gives rise to a telltale whooshing sound as air is moved around and through the plant's "green lung" that is as audible and palpable as human breathing when reduced to a comparably sized opening.

Option B: Air purification

Alternatively, the plants don't need to produce all of the oxygen necessary to sustain the carrier, but produce earthly amounts of oxygen and instead very efficiently filter out chemical, biological and nuclear contaminants from the atmosphere. (The plants as you say are carried around as air purifiers. The air contains enough oxygen to sustain life but is not breathable due to high concentrations of toxins.)

Stupendous energy intake

All plants require an energy source. The more CO2 it processes or the more matter it filters from the atmosphere, the more energy it must consume. Most terrestrial plant life uses photosynthesis. The Sun is dim in this post-apocalyptic world, so photonic energy harvests are poor. Instead, these plants leverage a strange sort of symbiosis with their toxic environment by harnessing nuclear fission of radioactive particles, or by consuming chemical toxins from their environs as reagents to an energy-producing lifecycle that allows them to undertake the demanding work of purifying air and producing plant sugars. The downside is that once they are removed from this hostile world, they will wither almost to the point of dormancy or die from the shock.

Remaining Portable

In the case of producing unearthly amounts of oxygen, The plant would also have to be growing our outputting solid or liquid byproducts at an enormous rate. The plant (or the human) needs to be able to get rid of excess carbon. Trees are made of about 50% carbon, much of which is directly absorbed from the atmosphere during the air purification/sugar production process. A typical human being exhales over 1 kg of the stuff every day. Over the course of a week or two, the plant would double its mass if it were performing full duty producing O2 and sugars, making it unmovable within a matter of days. If it is purifying the putrid air from its surround, it would also accumulate nontrivial chemical deposits which would need to be purged regularly.

Through a biological miracle, this particular plant regularly extrudes and sheds its carbon reserves into sugar bubbles and/or shingles of a peat- or coal-like substance that break off easily and can also be consumed as fuel directly by burning--which unfortunately consumes oxygen, so the atmospheric oxygen content is decreasing with time, requiring either greater efficiency or a greater number of plants to balance the environment.

Staying hydrated

The plant also requires hydration. Water being too heavy to carry around in addition to the plant, either the plant or its containment vessel must do very well at hanging onto water. Cacti and other desert plants are designed with minimal surface area so as to retain moisture. Unfortunately this plant makes the opposite design choice by sacrificing water retention for rapid air filtration and so loses hydration ten times as rapidly as any ordinary plant. This makes for an exceedingly delicate balance--if the plant is ever exposed to raw atmosphere it will dry out rapidly unless it is kept at close to 100% humidity. For this reason it must be kept either fully or almost completely enclosed in a bubble that traps humidity on the inside, maintaining a jungle or rainforest climate inside the jar. You could get sophisticated here and say that it harvests moisture somehow from hydrogen gas vents through a complex reverse electrolysis process, or by humans feeding the plant chunks of some kind of fat, but I think the humidity control vessel offers a simpler and more credible explanation. Processing sugar and producing oxygen also consumes water--but maybe the plant, the human or a symbiotic machine or organism is able to liberate and recycle the water from the sugars, and produce pure carbon (hence the carbon shingles).

Of all these requirements, energy consumption might be the most challenging.

Answer 5

Alternate answer: It's cultural.

As other answers have already demonstrated, carrying a plant as a source of oxygen would not be practical, at least not with plants as they function right now. Instead, have the reason be social/cultural, rather than practical. After the apocalypse, it was observed that the more trees a community had in their local area, the less likely people were to experience respiratory infection, as trees help to filter air of contaminants. As a result, trees and plant life became somewhat revered, and in some community it became common practice to assign each person to a plant (likely a bonsai) that they carry around with them whenever they leave the community. This tree is a symbol of their reverence for nature, and is believed to bring good health.

Of course, you don't have to follow the specifics of the answer, but I think the idea of the plants being carried around for cultural or religious reasons is viable.

Answer 6

Do they have to be carrying the plants with them during daily life or constantly? I suggest a slightly different take:

It's a transport mission.

Perhaps the character has found a particularly useful or rare plant that's managed to survive the apocalypse and is transporting it back to the survivors. Or maybe it's being transported from one settlement to another. The apocalyptic wreckage makes using vehicles like cars either impossible or impractical, and the plants can't be exposed to the harsh conditions of smog/radiation/heat/etc that exist outside of the settlements, so the only way to transport them is to put them into a sort of plant-incubator and pull it there.

Plus, I think this provides a rich basis for plot hooks and a simple, easy-to-understand plot that's easy to make impactful. "These people need this valuable thing, so I'm going to transport it across dangerous terrain at great risk to myself" is an old standby of stories (c.f. The Last Of Us, Death Stranding, Book of Eli, etc), but that just means it's a solid trope the audience can grasp and you can use as a structure for character or worldbuilding.

Answer 7

Some thing, in the atmosphere, in the environment, is hazardous. Plants can produce a chemical to protect themselves that catalyzes its breakdown. It's a very fragile chemical so it can't be bottled.

Alternatively, it produces a fragile drug that will treat the injury caused by such a hazard.

This does not require enormous amounts of light, water, and growth, unlike oxygen.

Answer 8

They are Tobacco, and people are stressed

The only plants to survive (in the story's part of the world) were dear old Nicotinia.

The dream of protecting the plants and eventually having plant covered fields animates a small amount of the population.

Far more dream of taking the plants and benefiting from them in a more...straightforward way.

Leaving ones plants behind is unwise and just Not The Done Thing.

Answer 9

I'll not draw on the potential logistical problems regarding air or the long-term health of the plant may have since others have. I would personally think that if it's not part of a terrarium that the plant in the jar could be in trouble in the long term as it would slowly run out of nutrients in the jar.

But as for this person -- This person is on a quest for Pollen. Not just any pollen though, pollen from another of its kind to help a village tree.

The Village Tree, named for its ability to sustain a village, has evolved (or was bred to) suck the toxins out of an area so that the people surrounding it do not die from whatever the apocalypse was. However, there is a limit to the ability for this tree to draw in toxins without doing harm to itself. It's resilient, but not immune.

But there is a way to keep the tree healthy -- if it can be coaxed into bearing fruit, then the toxins get locked into the fruit which then can be dealt with. The tree, freed from its current load of toxins, can now happily absorb more of the stuff until the next year. But the issue is that the tree cannot pollinate itself and so somebody must set off into the wastes to find another Village Tree in order to get the pollen needed.

The traveller carries with them a smaller plant to provide a small area of pure enough air to travel through, possibly with the flowering part needed to pollinate or with pollinators. The travel is rough and the traveller uses the clean air provided by the plant as pure supplemental air as they are still travelling through a toxic environment.

While this plant is a hardy breed, people can't just carelessly handle it as it is the only thing allowing them to still live above-ground in this toxic environment. And they are limited in their ability to experiment as it's both their lifeline and a lack of equipment/knowledge/skills to do so.

My only thought is considering that a tree sustains a village, that I sense that it would travel in more of a covered wagon, maybe a greenhouse/wagon hybrid that the person moves with them within the sphere of the plants influence, the roof keeping the clean air a bit more trapped. Given the post-apocalyptic setting, it might not be a perfect seal, but it doe not have to be -- the plant purifies the air anyways so long as the trip is short enough.

If some of this sounds familiar, there was a video game with a similar premise though with a different execution -- Final Fantasy Crystal Chronicles for the Game Cube. It was a magic crystal instead of a plant, but it might be a launching point for some world building.

Answer 10

People have covered the best scientific approaches that come to mind, but I don't see a hard science tag, so... it's magic!

Magic

• The planet/satellite/asteroid/ship has a toxic or non-existent atmosphere and the plant creates a small area of Air Purification or Vacuum Breathing but people want to be able to have more freedom from it, so they use a mask setup as seen in the image.
• In cultivation terms: the plant could create Life Ki/Qi/Chi in an area where none exists. I've seen a number of stories where animals don't breathe air and instead consume elemental Life ki, which plants produce, and when they exhale, the air has less/no life ki in it.
• The plant creates a high or low mana zone that either enables a wizard/cultivator/whatever or - in the latter case - protects them from some creatures that rely on such things.
• This person in the suit is a druid and druids must share some atmosphere with their anchor tree/plant. This could explain why druids have some "home range" around their grove that they can't leave for long, but maybe this particular druid is one of a spacefaring kind and this is the solution they've found to meet that need.

Not necessarily magical

Some other less-obviously-magical ideas that popped into my head while typing the above:

• The plant is some part of the person's reproductive process, possibly as part of some fantastical or futuristic ex-vivo fetal development process. Basically, the plant is the suit-person's child and they have to lug it around with them for some amount of time as the child needs chemicals/pheromones/magic/ki from the parent to survive.
• The above, but in reverse: the plant is the parent and the suit-person is an ambulatory stage of life that the plant goes through.
• The plant produces an gas/chemical that the suit person is addicted to (whether they know or not!). This adaptation in the plant started as a way to get animals to stay around the plant to keep it safe from other predators and to provide fertilizers to it. Possibly it provides some benefit (longer life, disease resistance, better mood, whatever) that helps explain to addicts why they care so much for this plant.
• An alien species that doesn't like the suit-person's species is out to protect these plants for some reason. People carrying the plant around masks their life signs from the antagonistic aliens. Maybe this is because the sensors for lifesigns are carefully tuned to some emission from the plant and only pick up humans incidentally, so the people get lost in the noise when there's a plant nearby, but their suit needs to contain something the plant releases for it to work.
• This is a funerary rite. After someone dies, they have an Inheritor who carries around their remains with a plant that digests their remains over some period of time. This is thought to allow the Inheritor to gain some of their power, leftover lifespan, or disease resistance - and maybe it works! - or maybe it's just a legal requirement for inheritance.

Answer 11

Oxygen would not help

The idea that the plant would supply the needed oxygen has a weakness. Plants produce oxygen only if they have light. A traveller caught in the open at sunset would be in trouble. A traveller caught in the open when the sky is suddenly obscured by a thick cloud cover would be in trouble.

Symbiotic bacteria

The wasteland is full of dangerous bacteria or some kind of fungi. They settle in the lungs and form a mould that kills the infected person or animal. On the plant grow some symbiotic bacteria that counter the threat. They proliferate quickly on the plant and the air flow sucks some of them keeping the breather always populated with the good ones. Far from the plant their lifespan is not long enough though, the plant must be kept close at hand to have a continuous supply.