i think we're making this out to be too complicated. if we exhale (weed) smoke onto a plant, we are giving it co2, some o2, buncha nitrogen, and broken down canabidols (sp?). the worst this could do to a plant is maybe make a layer of resin on it, which may hurt its growth by preventing proper gas exchange and transpiration, but overall, unless you exhale on one plant every time, this shouldnt be a problem. the co2 would help the plant, just like regular breathing would do co (carbon monoxide, created during the combustion of just about everything that burns) does nothing for the plant, neither good nor bad, and will dissipate fast enough unless we're talking about some SERIOUS chain smoking by many people in a closed area, and nitrogen in the air does nothing for us or plants. so basically smoking with plants does nothing, unless they truly have a psychological aspect we have yet to discover, in which case the effects of smoking on plants would still be up for grabs, but i highly doubt the existence of such an element.