Fossil evidence has shown that bryophytes are the most primitive of the extant land plants. Bryophytes lack true xylem and phloem, although some mosses and liverworts have conducting tissues. Therefore, the absence of true xylem and phloem is a primitive feature. The presence of well-developed vascular tissue (xylem and phloem) in gymnosperms and angiosperms is a derived character.
Similarly, gymnosperms lack vessels in the xylem and angiosperms have vessels. The fossil record tells us that the gymnosperms came first, therefore, we know the vessels are a more recent, or derived, character. Often it is the fossil record that helps scientists polarize characters. For those plants for which we do not have an adequate fossil record, such as many of the green algae, polarizing characters and constructing a phylogeny becomes more difficult.
monophyletic a group that includes an ancestral species and all its descendants
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