( ( Their ( main argument ) ) ( ( is ( that ( ( ( not ( all stems ) ) ( of ( Cooksonia-type plants ) ) ) ( have ( vascular tissue ) ) ) ) ) . ) )
(ROOT (S (NP (PRP$ Their) (JJ main) (NN argument)) (VP (VBZ is) (SBAR (IN that) (S (NP (NP (RB not) (DT all) (NN stems)) (PP (IN of) (NP (JJ Cooksonia-type) (NNS plants)))) (VP (VBP have) (NP (NN vascular) (NN tissue)))))) (. .)))
Their main argument is that not all stems of Cooksonia-type plants have vascular tissue.
(ROOT (S (NP (NP (JJ Present-day) (NNS plants)) (SBAR (WHNP (WDT that)) (S (VP (VBP have) (NP (DT a) (JJ complex) (NN vascular) (NN tissue) (NN system)))))) (VP (VBP are) (VP (VBN called) (NP (NN vasuluar) (NNS plants)))) (. .)))