Data compilation and mycorrhizae classification
To examine the evolutionary changes in leaf and root morphology among seed plants, we compiled trait data from previously published databases. In the case of leaf vein density, we compiled information from Boyceet al. 2009 (257 species); Brodribb & Feild 2010 (272 species); Walls 2011 (87 species) and Li et al. 2015 (80 species, see Appendix 1 for a detailed list). Because “vein density” is defined as the average distance between veins (usually secondary and higher orders) within the mesophyll (Brodribb, Feild & Sack 2010), taxa with single-veined leaves like Gymnosperms or parallel veins without interconnections like most Monocots are lacking in this dataset. In the case of root traits, we used data described in Valverde-Barranteset al. (2017) for diameter (D, mm), SRL (g/m), and RTD *g/cm3) deposited in the Fine Root Ecological Database (Iversen et al. 2017). Mycorrhizal state was binned into four mycorrhizal groups based on the classification in Maherali et al.(2016): AM, ECM, transitional (AMNM) and non-mycorrhizal (NM) based on consensus classification from previous studies. Species reported as AM or NM in different studies were binned in the AMNM transitional stage (Maherali et al. 2016). In order to simplify classification, species that evolved to form associations with Dikarya fungi were all classified as ECM (including ericoid mycorrhizal, ectomycorrhizal, and dual AM and ectomycorrhizal species). However, considering the substantial differences in root morphology and anatomical adaptation differences between ECM and Ericoid species, all analysis in this study were repeated with and without Ericoid species to confirm trends.