I don't mean the obvious reason, they need a source of carbon. I mean, don't these plants live in the wild somewhere just fine, without co2 injection? If so, why do so many plants seem to require this in captivity? Obviously there are a vast range of things that are different, I'm not trying to say that we are really recreating nature in a glass box. I'm just wondering what it is in particular that is different in captivity that generates the need for co2 injection for some plants. Or are these plants so different from the plants in the wild now that they have different needs?