( it ( ( is ( indicated ( by ( ( the head ) ( of ( the arrow ) ) ) ) ) ) . ) )
(ROOT (S (NP (PRP it)) (VP (VBZ is) (VP (VBN indicated) (PP (IN by) (NP (NP (DT the) (NN head)) (PP (IN of) (NP (DT the) (NN arrow))))))) (. .)))
it is indicated by the head of the arrow.
(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.