Fig. 4

In contrast to the shoots (green), polar auxin transport (PAT; depicted by blue arrows) is acropetal in the roots of seed plants and the rhizophores of selaginellalean lycopsids, whose homologies are equivocal. Additionally, fingerprints for the directionality of PAT demonstrate that the rhizomorphs of lepidodendralean lycopsids, which are shoot homologs with rooting function, also had acropetal PAT [60]. This shared directionality of PAT implies that acropetal auxin flow transcends organ identity and is more tightly linked to the positively gravitropic response or rooting function of axes (gray), independent of their homology. In turn, this suggests that the positively gravitropic axes with rooting functions produced by K-branching in zosterophylls with simple body plan may also have had acropetal PAT (dashed blue arrows)