( ( During ( ( ( times ( of ( impending ( environmental stress ) ) ) ) , ) ( such ( as ( ( ( extended droughts ) or ) ( heat waves ) ) ) ) ) ) ( , ( Spirogyra ( ( ( switches ( from asexual ) ) ( to ( sexual reproduction ) ) ) . ) ) ) )
(ROOT (S (PP (IN During) (NP (NP (NNS times)) (PP (IN of) (NP (JJ impending) (JJ environmental) (NN stress))) (, ,) (PP (JJ such) (IN as) (NP (NP (JJ extended) (NNS droughts)) (CC or) (NP (NN heat) (NNS waves)))))) (, ,) (NP (NNP Spirogyra)) (VP (VBZ switches) (PP (IN from) (NP (NN asexual))) (PP (TO to) (NP (JJ sexual) (NN reproduction)))) (. .)))
During times of impending environmental stress, such as extended droughts or heat waves, Spirogyra switches from asexual to 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)))))) (. .)))