A North American relative of the Persian walnut, the black walnut (J. nigra) is smaller, with a harder shell and a stronger, distinctive flavor. It was once commonly used to make breads, confections, and ice creams, but it’s difficult to extract from the shell in large pieces and has been largely ignored. Most still come from wild trees in Missouri. Another American species, the butternut (J. cinerea), is even less known, but remarkable for its high protein content—near 30%—and esteemed by enthusiasts as among the tastiest nuts. The Japanese have an indigenous walnut, J. ailantifolia, one of whose varieties is the distinctively heart-shaped heartnut.