( ( For ( ( quarks , ) ( which ( ( ( are confined ) and ) ( ( can not ) ( be ( experimentally ( observed ( as ( free particles ) ) ) ) ) ) ) ) ) ) ( , ( ( the situation ) ( ( is ( more complicated ) ) . ) ) ) )
(ROOT (S (PP (IN For) (NP (NP (NNS quarks)) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VP (VBP are) (VP (VBN confined))) (CC and) (VP (MD can) (RB not) (VP (VB be) (VP (ADVP (RB experimentally)) (VBN observed) (PP (IN as) (NP (JJ free) (NNS particles))))))))))) (, ,) (NP (DT the) (NN situation)) (VP (VBZ is) (ADJP (RBR more) (VBN complicated))) (. .)))
For quarks, which are confined and cannot be experimentally observed as free particles, the situation is more complicated.
(ROOT (S (S (NP (NP (DT The) (NNS quarks)) (PP (IN in) (NP (DT a) (NN particle)))) (VP (VBP are) (VP (VBN confined)))) (, ,) (NP (VBG meaning) (JJ individual) (NNS quarks)) (VP (MD can) (RB not) (VP (VB be) (ADJP (RB directly) (JJ observed)))) (. .)))