Liverworts, mosses, and hornworts diverged before the evolution of xylem and phloem (Fig. 33.1). The hypothesis shown in Fig. 33.1 implies that liverworts, mosses, and hornworts evolved independently for hundreds of millions of years and that they do not form a monophyletic group. Recall from Chapter 23 that a monophyletic group includes a common ancestor and all of its descendants, whereas a paraphyletic group includes a common ancestor and some of its descendants. However, it is important to recognize that the phylogenetic relationship between liverworts, mosses, and hornworts remains uncertain. Because these three groups share many features, we refer to them collectively as bryophytes.

As the living representatives of the first plant groups to diverge after plants moved onto land, bryophytes provide us with insights into how plants gained a foothold in the terrestrial environment. The insights from bryophytes are particularly welcome because the first plants are poorly represented in the fossil record. Only tiny spores and fragments of a cuticle-like covering record these early events. Of course, we must not lose sight of the fact that bryophytes have continued to evolve as the conditions for life on land have changed over the past 400+ million years. Where bryophytes grow today and what they look like have been shaped by their long coexistence with vascular plants.

Quick Check 1 Look at Fig. 33.1 and determine whether green algae, bryophytes, and vascular plants are monophyletic or paraphyletic.