( ( Use ( ( ( ( ( a ( single ( headed arrow ) ) ) , ) ( , ( to ( indicate ( direction ( of ( ( ( ( ( a ( causal relationship ) ) , ) or ) ( a ( double ( headed arrow ) ) ) ) , ) ) ) ) ) ) ) , ) ( to ( indicate ( relationships ( of ( undetermined ( cause ( -LRB- ( or ( ( that ( ( are not ) causal ) ) -RRB- ) ) ) ) ) ) ) ) ) ) ) . )
(ROOT (S (VP (VB Use) (S (NP (NP (DT a) (JJ single) (JJ headed) (NN arrow)) (, ,) (VP (, ,) (S (VP (TO to) (VP (VB indicate) (NP (NP (NN direction)) (PP (IN of) (NP (NP (DT a) (JJ causal) (NN relationship)) (, ,) (CC or) (NP (DT a) (JJ double) (JJ headed) (NN arrow)) (, ,)))))))) (, ,)) (VP (TO to) (VP (VB indicate) (NP (NP (NNS relationships)) (PP (IN of) (NP (JJ undetermined) (NN cause) (PRN (-LRB- -LRB-) (CC or) (S (NP (DT that)) (VP (VBP are) (RB not) (ADJP (JJ causal)))) (-RRB- -RRB-))))))))) (. .)))
Use a single headed arrow, , to indicate direction of a causal relationship, or a double headed arrow, , to indicate relationships of undetermined cause (or  that are not causal).
(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.