( ( The oxygen ) ( ( ( contributes ( only ( a ( minor fraction ) ) ) ) ( to ( the intensities ) ) ) . ) )
(ROOT (S (NP (DT The) (NN oxygen)) (VP (VBZ contributes) (NP (RB only) (DT a) (JJ minor) (NN fraction)) (PP (TO to) (NP (DT the) (NNS intensities)))) (. .)))
The oxygen contributes only a minor fraction to the intensities.
(ROOT (S (NP (NNS Plants)) (VP (CONJP (RB not) (JJ only)) (VP (VBP contribute) (NP (NN food))) (CC but) (VP (VBD oxygen) (PP (IN for) (NP (NNS organisms))))) (. .)))