( ( This gas ) ( ( is oxygen ) . ) )
(ROOT (S (NP (DT This) (NN gas)) (VP (VBZ is) (NP (NN oxygen))) (. .)))
This gas is oxygen.
(ROOT (S (NP (NN Oxygen) (NN gas)) (VP (VBZ is) (VP (VBN given) (PRT (RP off)) (PP (IN by) (NP (NNS plants))))) (. .)))