On the origin of euphyllophyte roots: hypotheses from an Early Devonian <i>Psilophyton</i>

If the emergences of P. crenulatum had a rooting function, they are the oldest euphyllophyte rooting structures. They are also a novel, sui generis type of such structures among Early Devonian plants. They also provide the oldest direct evidence for gravity-induced morphological features in euphyllophytes. If they evolved from emergences like those of P. crenulatum, then euphyllophyte roots probably inherited from them apical growth and branching, and gene networks responsible for production of tip-growing filamentous cells. Progressive increase in size of such emergences could have increased auxin fluxes, leading to specification of vascular connections to subtending axes.