Evaluation location: Illinois, United States
Ulmus taxa leaves were randomly collected from ground level from all four cardinal directions and held in cold storage in plastic bags at 5 C (41 F) for a maximum of 2 d. Ten leaves for each taxon were measured for leaf thickness, and inner and outer leaf toughness. Leaf thickness was determined by using a Vernier caliper to measure the thickness of each leaf (nearest mm) approximately one-half the distance from the leaf margin to the mid-rib. Inner and outer leaf toughness was determined to the nearest gram using a Chatillon™ digital force meter (pentrometer) (Greensboro, N.C.) applied to within 0.5 cm from the edge of the leaf for measuring outer toughness, and in the center of the leaf adjoining the mid-rib for inner toughness, respectively.