( ( Because ( ( of ( their ( vascular tissues ) ) ) ( , ( stems ( keep ( ( even tall ) plants ) ) ) ) ) ) ( ( ( supplied ( with water ) ) ( so ( they ( ( do n't ) ( ( dry out ) ( in ( the air ) ) ) ) ) ) ) . ) )
(ROOT (S (SBAR (IN Because) (S (PP (IN of) (NP (PRP$ their) (NN vascular) (NNS tissues))) (, ,) (VP (VBZ stems) (VP (VB keep) (NP (ADJP (RB even) (JJ tall)) (NNS plants)))))) (VP (VBD supplied) (PP (IN with) (NP (NN water))) (SBAR (IN so) (S (NP (PRP they)) (VP (VBP do) (RB n't) (VP (VB dry) (PRT (RP out)) (PP (IN in) (NP (DT the) (NN air)))))))) (. .)))
Because of their  vascular tissues, stems keep even tall plants supplied with water so they don't dry out in the air.
(ROOT (S (NP (JJ Vascular) (NN tissue)) (VP (VBD allowed) (NP (DT these) (NNS plants)) (S (VP (TO to) (VP (VB grow) (NP (JJR taller)))))) (. .)))