Fig. 1

Root cross sections from three pairs of species, where one member of the species pair forms storage roots and the other does not. To the left is a phylogeny depicting the evolutionary relationships among the six species with arrows denoting the two tribes, Ipomoeeae and Merremieae. The left-most three columns are root sections stained with Lugol’s iodine, which indicates starch a dark blue to black color. The right-most three columns are root sections stained with phloroglucinol-HCl, which stains lignin orange to pink. Scale bars are included with each section. Black bars are 1 mm, and blue bars are 0.5 mm in length. White arrows indicate starch-storage tissue