( ( Plant cells ) ( ( are ( ( kept rigid ) ( by ( ( ( ( the pressure ) ( of ( their ( cell contents ) ) ) ) , ) ( which ( ( are mostly ) water ) ) ) ) ) ) . ) )
(ROOT (S (NP (NNP Plant) (NNS cells)) (VP (VBP are) (VP (VBN kept) (ADJP (JJ rigid)) (PP (IN by) (NP (NP (DT the) (NN pressure)) (PP (IN of) (NP (PRP$ their) (NN cell) (NNS contents))) (, ,) (SBAR (WHNP (WDT which)) (S (VP (VBP are) (ADVP (RB mostly)) (NP (NN water))))))))) (. .)))
Plant cells are kept rigid by the pressure of their cell contents, which are mostly water.
(ROOT (S (NP (RBS Most) (JJ growing) (NN plant) (NNS cells)) (VP (VBP expand) (PP (IN through) (NP (NN water) (NN uptake)))) (. .)))