( Usually ( ( sexual reproduction ) . ) )
(ROOT (FRAG (ADVP (RB Usually)) (NP (JJ sexual) (NN reproduction)) (. .)))
Usually sexual reproduction.
(ROOT (S (NP (JJ Sexual) (NN reproduction)) (ADVP (RB usually)) (VP (VBP occur) (PP (IN during) (NP (NP (NNS times)) (PP (IN of) (NP (JJ environmental) (NN stress)))))) (. .)))