( ( Groups ( of ( tissues ( ( working together ) ( to ( perform ( a ( common function ) ) ) ) ) ) ) ) ( ( are ( called organs ) ) . ) )
(ROOT (S (NP (NP (NNS Groups)) (PP (IN of) (NP (NP (NNS tissues)) (VP (VBG working) (ADVP (RB together)) (S (VP (TO to) (VP (VB perform) (NP (DT a) (JJ common) (NN function))))))))) (VP (VBP are) (VP (VBN called) (NP (NNS organs)))) (. .)))
Groups of tissues working together to perform a common                    function are called organs.
(ROOT (S (SBAR (WHADVP (WRB When)) (S (NP (NP (JJ different) (NNS types)) (PP (IN of) (NP (NNS tissues)))) (VP (VBP work) (ADVP (RB together)) (S (VP (TO to) (VP (VB perform) (NP (DT a) (JJ unique) (NN function)))))))) (, ,) (NP (PRP they)) (VP (VBP form) (NP (NN organ))) (. .)))