Codes and supplemental data for 'Phloem evolution in ferns was shaped by phylogeny and records a trend of decreasing conductivity through geological time'

The phloem, the sugar-transporting tissue in vascular plants, is one of the most important but least well characterised plant tissues from an evolutionary perspective. Here we substantially extend the evidence for phloem evolution using ferns as a case study due to their long evolutionary history and famed vascular diversity. We carried out a comparative histological investigation of the conducting cells of the phloem, termed sieve elements, in 26 extant species, spanning 21 families and making over 18,000 measurements. We found that sieve element radius correlates with vascular bundle radius and leaf area, but that these correlations alone didn’t account for the observed diversity in sieve element radii. Instead, sieve element radius preserves a strong phylogenetic signal and is similar in closely related taxa despite differences in leaf and vascular characteristics. Next, by combining evidence from extinct and extant species, we recognised a trend of decreasing sieve element radius and total conducting area through geological time. We predict that this decrease resulted in reduced phloem conductivity especially in the eupolypod ferns that diversified into angiosperm-dominated ecosystems. Our findings demonstrate that phloem structure has changed over the long evolutionary history of ferns and that this likely had an important functional impact.