( ( The ( ( the head ) ( of ( the arrow ) ) ) ) ( ( indicates ( ( ( ( ( ( the end ) ( of ( the ( ( ( ( ( hour -LRB- ) 00 ) , ) ( 03 ( , ( 06 , ) ) ) ) . ) ) ) ) . ) etc ) -RRB- ) ( where ( ( ( as ( the tail ) ) ( of ( the arrow ) ) ) ( indicates ( ( the beginning ) ( of ( the period ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NP (NP (DT the) (NN head)) (PP (IN of) (NP (DT the) (NN arrow))))) (VP (VBZ indicates) (NP (NP (NP (DT the) (NN end)) (PP (IN of) (NP (DT the) (NP (NP (NN hour)) (-LRB- -LRB-) (NP (CD 00)) (, ,) (NP (NNP 03) (, ,) (NNP 06) (, ,)) (. .)))) (. .) (NP (FW etc)) (-RRB- -RRB-)) (SBAR (WHADVP (WRB where)) (S (NP (NP (RB as) (DT the) (NN tail)) (PP (IN of) (NP (DT the) (NN arrow)))) (VP (VBZ indicates) (NP (NP (DT the) (NN beginning)) (PP (IN of) (NP (DT the) (NN period))))))))) (. .)))
The the head of the arrow indicates the end of the hour(00, 03, 06, ..etc) where as the tail of the arrow indicates the beginning of the period.
(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.