Question: In my alternate history (1800s), I have troop hiking through tropical sub-Saharan Africa. For plot reasons, I have man-killing mosquitos, but I also have a plant that exudes CO2 and other attractants. These flowering plants are placed in the room at night to attract the mosquitos away from people. 

Evolution-wise, however, 'consuming' CO2 and giving off O2 is so deeply ingrained in a plant's DNA, it is hardly believable to be the other way around.

I'd easily hand-wave it away as some special scent, but the CO2 is critical to the plot. **Can a plant evolve to give off CO2?** Or rather, how difficult would that be to explain.

Answer: 
Plants give off CO2 at night, when they start to respirate using the 'fuel' (glucose) synthesised by photosynthesis in the day. Check out the [Calvin-Benson cycle](https://en.wikipedia.org/wiki/Light-independent_reactions#Coupling_to_other_metabolic_pathways)
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> Can a plant evolve to give off CO2?
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Look at the definition of the plant. The plant is a creature that grows by $H\_2O + CO\_2 +light \to C\_nH\_m + O\_2$. The $C\_nH\_m$ part, called *biomass*, *hydrocarbons*, *glucose* or *oil* in plain English, is what constituates the body of the plant. The only way you can produce $CO\_2$ is to turn the reaction backwards. But, it will destroy the plant since the opposite reaction, $C\_nH\_m + O\_2 \to H\_2O + CO\_2 +heat$, is known as "burning" or "decay". This is what mushrooms and animals do: they decay the biomass and use the sun energy to move around.

That is, white can become black but it won't be white after that "transformation". Ok? Plant, which produces CO2 denies its "plant feature".

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> I also have a plant that exudes CO2 and other attractants. These flowering plants are placed in the room at night to attract the mosquitos away from people.
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The "plants" that exhale CO2 are called "mushrooms" or "animals". Mosquitoes hunt for the latter, I suppose. You can easily use such animals as such "plants" to attract mosquitoes away from people (I am not sure why animals would be more attractive than human though).

Ok, diverting mascitos from human to animals may be a bad idea since maskitos will proliferate on the blood. You probably could have plants that grow up to certain size and then stop growing. They will accumulate CH during the day sunbath and release all the CO2+H2O they accumulated during the day at night. But you could just cut some plants and burn them at night by feeding to the animals and mushrooms, for instance.

Hope that now my point that plant that releases CO2 is nonsense is clear. You would write about this and literate other people (I guess that majority are as illiterate not to understand this basic fact of life) because it is really important for sustainability we are talking about when build our worlds. Being illiterate, we move into the false direction. That is a problem. We build the better world by having false notion of beauty.