Final answer:
Additional oxygen does not directly promote plant growth in greenhouses; rather, it is increased levels of carbon dioxide that can enhance plant growth by acting as a 'fertilizer', promoting leaf cover and photosynthesis.
Step-by-step explanation:
The claim that plants growing in a greenhouse would grow faster if given additional O₂ (oxygen) is not supported by the primary process of photosynthesis. In photosynthesis, green plants utilise carbon dioxide (CO₂) and water (H₂O) to produce glucose (a sugar) and oxygen, with light as the energy source. The equation for photosynthesis is commonly represented as 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(s) + 6O₂(g).
It is actually carbon dioxide that can act as a 'fertilizer' for plants by increasing leaf cover and promoting growth, as seen in studies where plants grown under elevated CO₂ concentrations exhibit increased growth rates and stomatal densities. Also, the amount of leaf area greatly influences a plant's growth rate, because this determines the potential for photosynthesis. Therefore, while oxygen is a product of photosynthesis, adding extra oxygen does not directly promote plant growth in the way that increasing carbon dioxide concentration does.