Fern phloem evolution records a trend of decreasing conductivity through geological time.

Sieve element radius and total conducting area in ferns show a geological-time trend of decreasing size, based on 18,000+ histological measurements across 26 extant species.

• Why it matters: Phloem evolution is poorly characterized, and linking tissue structure to evolutionary history is needed to understand how fern phloem may have changed over time.
• What they did: The study histologically examined sieve elements in 26 extant fern species (21 families) with 18,000+ measurements, finding strong phylogenetic signal and scaling with leaf and vasculature traits in leptosporangiate ferns.
• The result: Fossil integration reveals decreasing sieve element radius and total conducting area through geological time, implying reduced phloem conductivity in polypod ferns that diversified into angiosperm-dominated ecosystems.