( ( The ( index leaf ) ) ( ( is ( ( ( ( the ( four leaf ) ) ( in ( ( a whorl ) ( of ( leaves ( of ( an ( ( actively growing ) ( major terminal ) ) ) ) ) ) ) ) ) , ) ( counting ( ( the ( first ( expanding ( young leaf ) ) ) ) ( in ( ( the whorl ) ( as ( No. 1 ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT The) (NN index) (NN leaf)) (VP (VBZ is) (NP (NP (DT the) (CD four) (NN leaf)) (PP (IN in) (NP (NP (DT a) (NN whorl)) (PP (IN of) (NP (NP (NNS leaves)) (PP (IN of) (NP (DT an) (ADJP (RB actively) (VBG growing)) (JJ major) (NN terminal))))))) (, ,) (VP (VBG counting) (NP (NP (DT the) (JJ first) (VBG expanding) (JJ young) (NN leaf)) (PP (IN in) (NP (NP (DT the) (NN whorl)) (PP (IN as) (NP (NNP No.) (CD 1))))))))) (. .)))
The index leaf is the four leaf in a whorl of leaves of an actively growing major terminal, counting the first expanding young leaf in the whorl as No. 1.
(ROOT (S (NP (JJ Young) (NNS roots) (CC and) (NNS leaves)) (VP (VBP are) (NP (NP (DT the) (JJ major) (NNS sites)) (PP (IN of) (NP (NN gibberellin) (NN production))))) (. .)))