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On this planet, volcanoes are constantly spewing ash, polluting the already dense atmosphere and darkening the world. In addition, the star which this planet orbits is dimmer, resulting in a very dark planet. Animals have evolved here to rely more on hearing and smell, and plants are black in color to maximize light absorption for photosynthesis. However, these plants have eerie, glowing buds. This doesn't make sense to me; why would these plants glow and use energy to do so when such energy is precious?

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Whenever there is a puzzling, wasteful, obvious excess in an organism's biology or behaviour, the answer is almost always SEX!

So, this glow would be a sexual attractant for pollinators or seed distributors. The plant probably has some sort of audible and/or olfactory signal, but sight must still be useful at short range, and the plant glows to attract the relevant animal at short range. The glowiest plant would be the most successful at attracting the pollinator/seed distributor, and would hence have more offspring.

Additionally, if some glowing plants provide a food reward to these reproductive facilitator animals, then some other plants may well mimic these attractants in order to attract their prey, which might then be killed by poison, some mechanical means, or may end up being implanted with the plant's offspring, which might grow on the living animal, perhaps to eventually kill it.

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    $\begingroup$ If producing the chemicals for bioluminescence is less costly to "advertise" than the energy requirements of other methods, glowing plants are very believable. $\endgroup$
    – Gillgamesh
    Feb 27 at 13:32
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    $\begingroup$ Another thing is that in a volcanic area smells, which a lot of plants use for seeexxx, wouldn't be super duper useful! (I think at least.) $\endgroup$
    – Idan
    Feb 27 at 18:37
  • $\begingroup$ Or if the primary target for the fruit being eaten are nocturnal, glowing fruit when ripe would be a big advantage. $\endgroup$
    – John
    Feb 27 at 20:45
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    $\begingroup$ It's like flowers use bright colors so that the bees can see them. So too, these plants have glowing flowers for the bees. OP has stated that animals have evolved to rely more on hearing and smell. But didn't assume anything about the bees. $\endgroup$
    – Galaxy
    Feb 27 at 21:30
  • $\begingroup$ @Galaxy it's actually perfect; they glow to sort of entrance the animals' visual senses, because there isn'tuch to look at other that these glowing plants $\endgroup$
    – alkahest
    Mar 20 at 18:25
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Scaring away herbivores

Imagine you are a prey animal in a world with very little light. There's still some - and so, just like in the abyssal zones of Earth's oceans, your predators still have eyes of some sort. And so do you. But since you operate mostly on hearing and smell, environmental light is not an advantage to you; rather it is a danger.

You see a field of tasty plants fully illuminated by big glowy balls. Approaching them will light you up to any predators in the area, and you'll be tiger chow long before you can graze your fill.

Fluorescence

Not all glowing light is produced biochemically.

Visible light is scarce in an ash-choked world like yours. But higher-energy radiation can still get through. The plant absorbs those high energy particles and releases slightly lower energy particles, shifting invisible light into visible light without a "cost" to itself, for the same variety of purposes that coral does in our world: producing usable wavelengths of light for symbiotes, photoacclimation (fluctuating in response to more or less light becoming available), or eliminating radicals produced by its photosynthesis processes.

Heck, the plant may even grow this specialized organ for itself - stepping down extremely high-energy radiation into visible light that the rest of its leaves can consume via ordinary photosynthesis.

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To spread their seeds

The pods are full of seeds and are a tasty treat for animals to eat, AKA fruit

They glow to make them easy to see when they're ripe just like fruit change colour.

enter image description here

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    $\begingroup$ it could also be caused by symbiotic bacteria, there are a lot of bioluminescent symbiotic bacteria. they do it to increase their own dispersal or that of their host. If the primary target animal is nocturnal glowing would be a big benefit. $\endgroup$
    – John
    Feb 27 at 1:28
  • $\begingroup$ @John except it's just the bulbs glowing, not the whole plant $\endgroup$
    – Thorne
    Feb 27 at 2:02
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    $\begingroup$ that's fine a lot of bioluminescent bacteria only live in certain parts of their symbiotes. $\endgroup$
    – John
    Feb 27 at 20:43
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Geothermal Plants

You could have some plants that live primarily in very hot areas, right next to the lava rivers or volcanic lakes. In order to cool themselves down the plants run a vine up the rocky hills where it is cooler. The plants grow "radiators" at the end of these vines, which glow with the volcanic heat they dissipate.

The plants may in fact only use photosynthesis as a backup, they are primarily geothermal. They run on the heat difference between the parts of them near the lava and the parts further away. The "vines" connecting the two parts could facilitate this either with fluid/gas flow or electrically.

Short version: They glow because they are hot. They are hot because they are drawing energy from the volcanoes.

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  • $\begingroup$ The amount of heat you need to cause glowing (without combustion) is way outside of the realm of what we would consider to be "usual" plant matter. That's not to say this answer is incorrect or bad, but it would require some further explanation of the plant materials in question. I'm no materials engineer but in terms of hardness of such a material you'd expect this to be close to an ore/mineral than an soft material, and the amount of energy on display would make it a very bountiful power source (for that last point I'm already assuming the glowy bits are not near the glowy lava) $\endgroup$
    – Flater
    Mar 1 at 4:43
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Its was a gene-drive accident. What started out as a little viral experiment, to convert all examples of a fish species in a botanical garden into glowing "fire-fish", turned via a predator into a freely propagating disease, turning everything living into a glowstick. Part of natural selection rewarded, to hide yourself when the glow was most visible - aka at night and get your buisness done during the day. Thus at night, only plants are visible, while most animals hide into shelter to avoid dangers.

PS: The virus mingled with other viruses of the long lost civilization, so animals with subway plans on the skin or advertisements, are comon. Oh, and the plants of course have that too. Do not venture into the CocaHola forrest..

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Let's generalize:

Why does [organism] do [specific thing that requires non-zero effort]?

The answer is always "because they had better odds of survival compared to not doing it". So the task is to come up with a benefit to bioluminescence, as that justifies being bioluminescent.

Why are creatures bioluminescent in real life? The reasons are usually one of three categories:

Food: it attracts prey. For your plants, this might not be relevant unless you're happy making them carnivorous. You could apply this laterally: animals like sleeping in a lit up area, therefore they defecate near the plant, which provides nutrition for the plant's soil.

Procreation: for animals, this is expressed as a mating ritual. For plants, we could argue that it attracts pollinators, so that they can carry the pollen and ensure the plant species' continued existence. This could be combined with the previous point (fertilization) for a double whammy, although pollinators are usually not animals that excrete substantial amounts of fertilizer.

Defense: it hinders predators. For sea creatures this manifests as a way to hide your shadow or shadowy silhouette to help camouflage you. For plants, it could be that animals avoid light sources (because they're not used to it) and therefore it acts as a deterrent.

There are more possible solutions to this question than I listed, but it hinges on the other specifics of your world. Never forget that no organism stands alone. They are part of an ecosystem, so their existence must be supportable by the ecosystem.

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Escaped cultivars

Long time ago, the planet was inhabited by intelligent species. Species which admired beauty, and breed animals and plants for their beauty. Initially, a few plants had tiny glowing patches due to some freak mutations. This was cross breed until they had plants with fully glowing bulbs. The plants were popular, and kept in many gardens. They were easy to keep.

Once the species left the planet, the plants spread over the world as there were no gardeners left to maintain the gardens.

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Maybe it's a side effect of the enzymes use in some chemosynthetic process. It wouldn't evolve out because one of the above reasons gave it a mild advantage.

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Many bioluminescent organisms do not produce their own light. They contain bacteria that does, which responds to their chemical signals. So the bacteria's primary function might be some other beneficial factor, like removing toxins or breaking down food materials, but the secondary advantage is the bioluminescence.

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    $\begingroup$ Doesn't this just bump the question of "why waste energy on glowing" to be about the bacteria rather than the plant? $\endgroup$
    – SPavel
    Feb 27 at 15:49
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    $\begingroup$ This answer nitpicks a detail to justify avoiding answering the actual question, passing the buck to the proverbially picked nit. $\endgroup$
    – Flater
    Mar 1 at 4:39

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