( ( The ( arrow heads ) ) ( ( were ( of ( two ( kinds viz ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN arrow) (NNS heads)) (VP (VBD were) (PP (IN of) (NP (CD two) (NNS kinds) (NN viz)))) (. .)))
The arrow heads were of two kinds viz.
(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.