( ( Arrows ( heading down ) ) ( ( indicate ( a ( parent version ) ) ) . ) )
(ROOT (S (NP (NP (NNS Arrows)) (VP (VBG heading) (PRT (RP down)))) (VP (VBP indicate) (NP (DT a) (NN parent) (NN version))) (. .)))
Arrows heading down indicate a parent version.
(ROOT (S (NP (JJ Reversible) (NNS reactions)) (VP (VBP are) (VP (VBN indicated) (PP (IN by) (NP (NP (JJ opposite-headed) (NNS kinds)) (PP (IN of) (NP (NNS arrows))))))) (. .)))
Reversible reactions are indicated by opposite-headed kinds of arrows.