( ( The ( arrow heads ) ) ( ( indicate ( ( ( the directions ) ( of update ) ) ( that ( are permissible ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN arrow) (NNS heads)) (VP (VBP indicate) (NP (NP (DT the) (NNS directions)) (PP (IN of) (NP (NN update))) (SBAR (WHNP (WDT that)) (S (VP (VBP are) (ADJP (JJ permissible))))))) (. .)))
The arrow heads indicate the directions of update that are permissible.
(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.