Might seem like a stupid question at first, but let's think about this for a minute.
Obviously, the living tissues contain some water, but it seems negligible compared to the quantities that plants can seem to require.
Ask practically any child, or non-expert, "What do plants need?" And you'll probably get some variation of the same answer, "Sunlight and water", every now and then, you might even get "Carbon-dioxide".
Light is understandable, right?
Dihydrogen-monoxide (water) and carbon-dioxide (exhalation) are also required for oxygenic photosynthesis:
CO2 + H2O + photons→ [CH2O] + O2
Or more specifically:
6CO2 + 6H2O + photons→ [C6H12O6] + 6O2
But still, I feel like this doesn't really account for the entirety of water spent (even considering evaporation, drainage, etc). Otherwise (it seems), we could flood LED lit, hydroponic greenhouses with pure carbon-dioxide, and watch the plants grow, science-fiction style.
Am I missing something here, or am I just incorrect?