Fig. 5

Euphyllophyte leaves are thought to have evolved from lateral branching systems like those seen in early representatives of the clade (e.g., Psilophyton). Structural fingerprints for adaxial–abaxial polarity (dorsiventral polarity) observed in fossils indicate that whereas seed plants evolved determinate growth before adaxial–abaxial polarity in the leaves, in filicalean fern leaves evolution of adaxial–abaxial polarity preceded determinacy. The early fern Psalixochlaena exhibits adaxial–abaxial polarity in its leaves (i.e., protoxylem on the adaxial side and phloem on the abaxial side of the leaf vascular bundle cross-sectioned in the figure), which had indeterminate growth; in contrast, the leaves of the early seed plant Elkinsia had determinate growth but their vascularization had radial symmetry (protoxylem surrounded by metaxylem in the vascular bundle cross-sectioned in the figure), at least in their terminal segments. This observation provides one of the lines of evidence supporting independent evolution of leaves in ferns and seed plants