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

Phloem, the sugar-transporting tissue in vascular plants, is one of the most important but least characterized tissues from an evolutionary perspective. Here we substantially extend the evidence for phloem evolution by carrying out a histological investigation of phloem conducting cells, termed sieve elements, in 26 extant fern species spanning 21 families and making over 18,000 measurements. We report high diversity in fern phloem and that sieve element radius preserves a strong phylogenetic signal. We also detect clear scaling relationships between sieve element radius and both leaf and vasculature characteristics in the leptosporangiate ferns. By integrating fossils into our analysis, we recognized a trend of decreasing sieve element radius and total conducting area through geological time. We conclude that this decrease likely had a major functional impact by reducing phloem conductivity in the highly diverse polypod ferns that diversified into angiosperm-dominated ecosystems.