Figure 4. Stepwise and independent origins of roots among vascular plants.

The roots of extant lycophytes evolved in a stepwise manner. There was a stage, represented by Asteroxylon mackiei, characterised by the presence of radially symmetric, positively gravitropic, isotomously branching axes. Subsequently, a root cap, root hairs, endogenous development, and an endodermis evolved. This sequence of events is consistent with the hypothesis that the common ancestor of all extant vascular plants was rootless, and roots with caps had at least two independent origins among lycophytes and euphyllophytes. The extensive similarities between roots of extant lycophytes and euphyllophytes therefore represent examples of convergent evolution.