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We all know of plants that draw in animals with sweet nectar or even just sit around hoping an insect will sit on it and get caught, but I'm imagining a forest dimly lit by the flickering flames of a plant that tried to attract moths for consumption. These plants are then harvested by those who dwell in the forest for their fuel.
How would a plant produce a small flame (what chemical process would need to occur) and how would it prevent itself from being burned by it?
First lets look at another reason that a plant might want to produce fire:
A great many parasitic insects are attracted to CO2 and most organic substances release some amount of CO2 when burned. So producing CO2 may create a small advantage in attracting insects for pollination and/or consumption.
Next how might a plant self ignite:
According to several sources; cotton rags when soaked in linseed oil can spontaneously combust, under the right conditions.
The oxidation of linseed oil is an exothermic reaction, which accelerates as the temperature of the rags increases. When heat accumulation exceeds the rate of heat dissipation into the environment, the temperature increases and may eventually become hot enough to make the rags spontaneously combust. - Wikipedia
We could extrapolate on that, and picture a plant that could produce a bulb of fibrous material that it would soak in a oil/resin that had similar oxidizing properties. ta-da... Fire!
Now on to sustaining the fire:
The above mentioned bulb could and probably would burn rather quickly. For a sustained fire we would need the stem to act as a tube to continue to feed the, now burnt, bulb with oil without the stem itself burning away.
If the stem was kept sufficiently moist it should hold up for a good long time. (have you ever boiled water in a banana leaf?) So that shouldn't be a huge hurdle.
Continuing to feed the bulb with oil should be fairly straight forward as well, the heat from the flame should create enough suction to keep the oil flowing, or you could look for a more wick like solution.
After the initial combustion the plant would only really need to continue to produce enough oil to keep a small flame going, I doubt a major reservoir would really be needed.
Now what happens when if the flame goes out?
A simple solution here would be that the oil dries and hardens at the end of the stem forming a clot, this would be the signal to the plant to start producing a new bulb.
Fire requires a lot of energy, most of which does not make light. Bioluminescence is how a plant would produce light.
But, you want fire. Ok.
Plants already produce oxygen and hydrogen via photosynthesis. If the plant stores one or both of these gasses it could use them to keep a small flame lit for short periods.
A safer source of fuel would probably be a wax or oil produced naturally by the plant.
The difficulty is in getting the fuel ignited. This might be done with a lens made from water to focus direct sunlight. Once the flame is going, it could burn into the night and attract moths.
By the way, the reason moths are "attracted" to light sources is a pretty interesting aside. Moths aren't necessarily attracted to lights, their navigation is just getting messed up. Moths are expecting the light they see to be parallel light beams, since they evolved when all they got were parallel rays from the sun or moon. This means if they keep incoming light rays at the same angle, they'll flight in a straight line. Point sources of light like lamps or candles have very divergent light rays. So, moths spiral into a point light source because a spiral on a diverging light source looks like a straight line to a moth. Neat, right?
Samuel's answer shows that moths are only attracted to local lights because their current evolved behavior is relative to a world with a single, distant light source. So, in a world were plants evolved flame production, it would likely not be to consume bewildered moths because moths would equally evolve to simply avoid the flames.
There would have to be another selective pressure for this to be convincing. I may be wrong, but I believe that bioluminescence is almost always a method of communication, so these plants would be more believable if the flames served a communication purpose that aided survival. But why would a plant need to communicate?
Perhaps the communicating might be necessary for reproduction. Many plants have male and female counterparts, rather than having both parts, as most plants to. Sporing plants also have an interesting reproductive cycle. Some odd ones have very, very odd reproductive cycles. In your world, the flame could be some kind of signal that involves other plants in the reproductive cycle. I'm not going to bother with details on how the reproduction might work, because there really is a plethora of real example you could modify. If you toss in a little symbiosis you can get really creative. One species can play off of the other and they both benefit from the flame adaptation.
But now the question, why flame instead of bioluminescence? We already know bioluminescence can evolve and it is quite effective in aiding survival. Why would flame be any different? The first thing I can think of is that flame fills the air with exhaust. Yes, it makes light too, but perhaps the light in combination with a particular gas that comes from the burning is what triggers the other plants to do their part of the reproductive cycle.
With all this in mind, I envision that the plants grow in close quarters because the light and the exhaust would thin quickly as more distance was given between individuals. I also envision that the plants only light once or twice a year. Making flame is far more consumptive than bioluminescence, so the plants need time to fill a fuel reservoir.
And now the final questions: What is the fuel and what would ignite it? Ignition is probably best solved with electric spark. A simple moving part on the plant can build up static and release a spark at the right moment. An explosive chemical reaction might be possible, but at was mentioned in pmcoltrane's answer even the bombardier beetle's explosive reaction tops out at under 100C. The real problem I think is the fuel. We don't really know of any chemicals that burn in low enough heat so they would not damage the plant, plus most combustion reactions need forced air (or compression) to start up. Maybe a wind channel structure could handle that. Perhaps your audience would be willing to suspend their disbelief on these points. I probably would be if the rest of the story was interesting and they way the people used the fuel was a central point in the plot.
There are plants that evolved to create wildfires to kill the competition. Of course, native to Australia.
Adaptations that promote fire include: a high content of volatile oils in the leaves and litter; litter that breaks down extremely slowly; an open canopy; long strands of bark that hang from limbs after peeling and which can be carried alight for many kilometres to start new spot' fires well ahead of the fire front.
http://www.australia.gov.au/about-australia/australian-story/eucalypts
There are definitely going to be some hypothetical's in my answer; but here we go.
It sounds like your ideal plant is something akin to the venus fly trap, pitcher plant, or even the carrion flowers (which attract insects for pollination as opposed to food) in terms of behavior; however rather then sweet or rotten smells you want fire to be the primary attractant.
Surviving Fire
Lets start with a plants ability to survive fires. Natural fires are typical of various conditions like lightning and dry plant matter (Fire Ecology for more on the subject of natural areas with fires). Plants survive these conditions in a number of ways, Ponderosa Pine trees lose lower branches and have tough trunks, thus they keep their living branches and leaves (needles) above the reach of fire. Although if you're aiming for a smaller plant then your particular plant will be one that stores energy in the roots and regrows out of the ground following a fire.
So that's an overview of how nature does it now. What about some ways it could handle it? For one you could have the cellulose of your plant pull silicate and other minerals out of the ground and embed them in the bark creating a stone-like layer to protect from flame on the outside of your plant. The leaves could also produce a flame resistant oil/coating that protects from heat and open flame. Fire-retardant gel absorbs large amounts of water to form a protective blanket.
Making fire
So now we need to make a fire. The main way I see this working is having your flower be a swamp denizen. The large amount of rotting materials combined with a symbiotic relationship to certain bacteria could allow your plant to create and store methane gas for burning. But what about the heat? Well nature even has you covered here; Thermogenic Plants create excess heat; although you'd have to scale this up I would say that it's at least possible. So your plant could have a stalk that releases a stream of methane and is lit by thermogenic action. It could operate on photosynthesis during the day and come nightfall the fire could start.
My immediate thought was something like a pitcher plant, which is already carnivorous, and lures its prey with nectar before trapping it in a pitfall.
What if the plant were something like a pitcher plant, but secreted liquid wax into its pitcher, like a natural candle? A specialized leaf, stem, or flower parts could evolve to be naturally porous, to become a sacrificial wick when dried out.
Once ignited, it would lure moths and other insects, which would fall into the liquid wax. The wick would burn until all the wax was consumed, at which point the flame needs to die before burning the plant. (The plant may have a high water content, or line itself with a nonflammable buffer.)
That leaves the problem of ignition: the bombardier beetle's chemical reaction doesn't even reach 100Ā°C. This list of hypergolic reactants looks pretty toxic. Maybe the wick naturally produces something like nitrocellulose, and ignites with a brief spark once the wick dries out sufficiently? Plenty of animals harness electricity: I'm not sure about plants.
Alcohol and acid can be combined to produce fire (though the alcohol should be of a certain concentration and the acid should be of a certain nature -- details, details...).
There are several instances of fruit naturally fermenting in nature, instances of plants with reservoir systems, and instances of naturally occurring acidic compounds of the correct nature (though mostly in animals, not plants).
It is plausible that a plant could maintain a reservoir-segment base that catches whatever seeded fruit is not eaten outright (or even more interestingly, a plant that has a reservoir catch and lives symbiotically with another, taller plant that produces the fruit and drops it, some entering the reservoir). This reservoir would be a good place to ferment without disturbance, and either the shape of it could encourage evaporative distillation or some sort of root-like leeching mechanism could grow as a mesh through the fruit to cause alcohol to be drawn to a secondary reservoir or leaf-like blivet due to viscosity differential or whatever.
The same plant, if it produced an acid of the right form could combine these two to create fire.
The real issue is, why would the plant do this? While the complexity of the relationship and mechanics of a symbiotic plant system like a fruit-tree/acid-making flame thrower are actually not that far out considering how many intensely complex (and at first glance downright unlikely!) symbiotic systems exist in real life, the odds that this would be a beneficial capability for a plant that lacks the cognitive ability to decide when to shoot fire are pretty long.
If you can resolve the issue of how the plant decides when to do this (unless you just invent intelligent plant life in your world, then the problem is solved, but you have a new problem of making plant-monsters acceptable to the audience without going completely campy 1950's sci-fi style) then you have a sound basis for a fire-breathing or flame-throwing plant system in the form of a fruit-to-alcohol and acid-excretion system that combines wherever the fire is supposed to happen. The moth thing is not, I believe, sufficient basis for this -- you need something else. Something like self-destructive perimeter defense which is individually sacrificial but overwhelmingly effective for the greater system, perhaps (similar to how war fever has beneficial group survival outcomes for humans and many other mammals).
Plants can & do produce highly flammable substances. Consider the gas plant: https://en.wikipedia.org/wiki/Dictamnus Or have you ever squeezed an orange peel into a candle flame? So the problems are 1) an ignition mechanism; and 2) an evolutionary reason/pathway for spitting flame.
The plant could hang a small sphere of dried sugar above a wick of fibrous material, which would be partially submerged inside a constantly replenishing supply of oil. The wick would double as an extremely efficient nutrient absorber, and the moth would fly into the flame, burn up, and have its remains absorbed.
