( This ( ( is ( indicated ( by ( ( ( ( an ( arrow head ) ) ( on one ) ) or ) ( both ( of ( ( the ends ) ( of ( the arc ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT This)) (VP (VBZ is) (VP (VBN indicated) (PP (IN by) (NP (NP (NP (DT an) (NN arrow) (NN head)) (PP (IN on) (NP (CD one)))) (CC or) (NP (NP (DT both)) (PP (IN of) (NP (NP (DT the) (NNS ends)) (PP (IN of) (NP (DT the) (NN arc)))))))))) (. .)))
This is indicated by an arrow head on one or both of the ends of the arc.
(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.