Fascinating stuff...
At the risk of starting a climate change discussion


A starter: "Plants can grow up to 50 percent faster in concentrations of 1,000 ppm CO2 when compared with ambient conditions"..."Atmospheric concentrations of carbon dioxide fluctuate slightly with the change of the seasons, driven primarily by seasonal plant growth in the Northern Hemisphere."
from here: http://en.wikipedia.org/wiki/Carbon_dio ... n_fixation
A physics forum asks the same Q: http://www.physicsforums.com/showthread.php?t=378276
I recall my Mom saying that 30 years ago, nurses would remove flowers at night from patients' rooms, because it was believed plants would remove oxygen from the air!
Barb